CALIFORNIA AGRICULTURAL EXPERIMENT STATION

FRUIT DEHYDRATION

I. Principles and Equipment

R. L. PERRY, E. M. MRAK, H. J. PHAFF,
G. L. MARSH, and C. D. FISHER

December, 1946

Bulletin 698

THE COLLEGE OF AGRICULTURE
UN! V E R S I T Y OF C A L I F O R N 1 A BE R K E L E Y

CONTENTS

PAGE

Introduction 3

History of dehydration 3

Statistics 5

Advantages 8

Limitations 8

Principles of pretreatment 10

Condition of raw material 10

Harvesting 11

Storage of fresh fruit 11

Washing fruit prior to drying 12

Spray-residue removal 12

Sorting and trimming 13

Cutting, pitting, and peeling 14

Treatment of pits and waste materials 14

Dipping 16

Blanching 17

Sulfuring 22

Preparation equipment 23

Plant layout 23

Fruit boxes 26

Cutting tables 26

Blanchers 26

Boilers 28

Fruit washers 29

Washers and materials for spray-residue removal 29

Peeling, pitting, and cutting machines 31

Miscellaneous small equipment 31

Tray and box washers 32

Skin-checking equipment 32

Sulfuring houses and burners 33

Principles relating to dehydration 35

Process of evaporation 35

Moisture in fruit 37

Properties of air-vapor mixtures 38

Measuring air conditions 39

The psychrometric chart 40

Eecirculation 43

Drying rates and drying times 44

Fruit temperatures during dehydration 47

Air flow 47

Air pressures and friction losses 48

Power required to move air 51

Drying equipment 51

Evaporaters 51

Dehydraters 52

Countercurrent dehydraters 53

Cross-flow dehydraters 54

Center-inlet dehydraters 56

Center-exhaust or two-stage dehydraters 56

Cabinet dehydraters 56

Conveyor dehydraters 56

Vacuum dehydraters 57

Drum driers 57

Furnaces 57

Fans 60

Tunnel dehydrater operation 62

FRUIT DEHYDRATION: I. PRINCIPLES AND
EQUIPMENT'

R. L. PERRY,3 E. M. MRAK/ H. J. PHAFF/' K. L. MARSH,"
and C. D. FISHER7

INTRODUCTION

History of Dehydration. — Fruits have been preserved by drying since the
dawn of history, but the use of dehydration8 for this purpose is a recent devel-
opment. In California, mechanical driers made their first appearance in the
Nineteenth Century. At the beginning of the fruit-drying industry mostly
natural-draft evaporaters9 were used; later, forced-draft evaporaters were
introduced. The use of forced draft was an improvement, although all of
the early driers were inefficient and frequently difficult to operate satisfac-
torily. The characteristics and operating procedures of these driers are de-
scribed in publications by Cruess (1919a),10 Cruess, Christie, and Flossfeder
(1920), Caldwell (1923), and Wiegand (1924).

Except for the drying of apples and hops, natural-draft evaporaters are
not now used extensively in California. Until 1919, sun-drying was favored
for prunes and other fruits. In the prune-drying season of September, 1918,
however, heavy rains caused severe losses to many growers. Most of the trays
of fruit were stacked, but many were wet before they were stacked. After five
days of rainy weather, many of the prunes became so moldy that little could
be done to save them. Sulfuring was suggested as a means of retarding spoil-
age ; this procedure was rejected, however, because there is no trade demand
for sulfured prunes. Cruess (1919a, 1921) stressed the importance of dehy-
dration and advised its use as an insurance against rain damage. Fortunately,
the forced-draft dehydrater was developed and marketed in ample time to
satisfy the subsequent demand for an efficient drier. According to Christie

1 Received for publication August 9, 1945.

- This bulletin presents the principles and operations of dehydration ; specific directions
for the dehydration of cut fruits and of whole fruits will be covered in separate publications.

- Associate Professor of Agricultural Engineering and Agricultural Engineer in the
Experiment Station.

4 Associate Professor of Food Technology and Associate Mycologist in the Experiment
Station.

"' Instructor of Food Technology and Assistant Microbiologist in the Experiment Station.

u Assistant Professor of Food Technology and Assistant Chemist in the Experiment
Station.

7 Chief Chemist, Dried Fruit Association of California.

H The term dehydration, as used in this publication, refers to the process of drying in a
dehydrater. A dehydrater is a mechanical drier equipped to control temperature, air flow,
and humidity. The operator of a dehydrater is called a dehydrator. Fruit dried in a dehy-
drater is termed dehydrated fruit, in contrast to sun-dried fruit, which is dried in the sun,
and evaporated fruit, which is dried in an evaporater.

9 An evaporater is a natural-draft drier equipped with artificial heat. Evaporaters which
are equipped with forced draft, but which do not offer recirculation are termed air-blast
evaporaters.

10 See "Literature Cited," at the end of this bulletin, for complete data on citations, which
are referred to in the text by author and date.

[3]

4 California Experiment Station Bulletin 698

(1923), the use of dehydraters for the drying of prunes increased rapidly
after 1918. At present over three quarters of the California prune crop is
dehydrated.

Grape dehydration began as a means of salvaging raisins when sun-drying
conditions were unfavorable, but it assumed new importance as a means of
preserving wine grapes in the years following the adoption of the Eighteenth
Amendment. Wine grapes were later handled by other methods, and interest
in grape dehydration declined until about 1925. In that year, a new type of
raisin — the golden-bleached Thompson — was introduced as a competitor of
the commercially known, light-colored Smyrna raisin. Prior to World War II,
approximately 25,000 tons of golden-bleached raisins were produced per year,
but, because of loss of export markets, production was curtailed in 1940 and
1941. In 1942, however, production was again increased, this time to meet the
need of converting a greater percentage of grapes into raisins. In 1943 the War
Food Administration required the drying of practically all raisin grapes. To
aid producers in meeting this requirement the Administration sponsored the
construction of a number of dehydraters. This spurred the industry to a
record production in 1943 of 40,000 tons of golden-bleached and 10,000 tons
of Valencia-type Muscat raisins. In 1944 and 1945, however, golden-bleached
fell to about 28,000 tons and Valencias, because of almost no demand, to about
800 tons.

In 1931, low prices for canning clingstone peaches stimulated a demand for
their dehydration. Maximum production — about 50,000 tons of fresh peaches,
or about 6,500 tons of dry — was reached in 1936. Since then, production has
varied with cannery demands and with prices for the fresh fruits. Phaff, Mrak,
et at. (1945) have described the manufacture of blanched dehydrated cling-
stone peaches. This new product, first made on a small scale in 1943, was
produced later for use primarily by the armed forces.

A limited tonnage of dehydrated, unsulfured cut fruits has been prepared
for sale in "health-food" stores and for use in the manufacture of baby foods.
This fruit, however, because of its undesirable appearance and flavor, has not
been widely accepted. In California in the past, practically all sulfured cut
fruits, with the exception of apples, were dried in the sun. Occasionally, the
drying of partially sun-dried fruit has been completed in a dehydrater. At
present, however, there is considerable interest in drying cut fruits entirely
in dehydraters.

During the past few years, there has been an increasing tendency to dehy-
drate figs as a means of eliminating the hazard of rain damage and of releasing
drying-yard space for other purposes. A large proportion of the crop, how-
ever, while on the trees or the ground, prior to harvesting, and while on trays
in the drying yard, is still dried in the sun.

Since the beginning of World War II, there has been an increasing produc-
tion of dehydrated cranberries and blueberries for use as sauce and in bakery
products. Other small fruits, such as huckleberries, have been dried on only
an experimental scale, in most instances ; this production has been described
by Friar and Mrak (1943). Consideration has been given to the possibility of
drying guavas for later use in jams and jellies. Dehydrated sulfured bananas
from Mexico have made their appearance on American markets.

Fruit Dehydration : I. Principles and Equipment 5

Statistics. — In California, fruit drying is an industry of considerable im-
portance; indeed, a large proportion of the many kinds of fruits in this state
is grown for this specific purpose. The principal important exceptions are
pears and clingstone peaches. Some fruits, notably Royal or Blenheim apricots,
can be used interchangeably for fresh shipment, canning, freezing, and dry-
ing. In some districts the selling of unblemished and well-formed apricots to
canneries, and drying of the remainder of the crop has become common prac-
tice. Certain varieties of apricots, especially Moorpark and Hemskirke, are
seldom canned or sold fresh, but are grown almost exclusively for drying.
Tiltons are usually dried, but are also canned or sold fresh in some regions
of the state.

Muir and Lovell peaches are used largely for drying, whereas the Elberta
and J. H. Hale varieties, although sometimes dried, are planted chiefly for
fresh shipment. During the last few years there has been an increase in the
canning of Lovell, Elberta, and J. H. Hale freestone peaches. In addition a
small tonnage is frozen each year. Raisin grapes may be dried, sold to wineries,
shipped fresh, or, less frequently, canned. Figs, for the most part, are dried
or shipped fresh; only one variety — the Kadota — is canned. In California
prunes are grown almost entirely for drying, although in some districts
Sugar prunes are shipped fresh.

Cherries are seldom dried ; they are produced primarily for fresh shipment,
canning, freezing, and sulfiting. In California the Black Tartarian, Bing,
Lambert, and Black Republican varieties are produced almost entirely for
shipping. The Royal Anne (Napoleon) variety, on the contrary, is used for
canning and sulfiting, as well as for shipping fresh. Sour cherries, which are
not commercially important in California, are almost exclusively canned or
frozen for subsequent use by bakeries. About 50 to 60 per cent of the Cali-
fornia apple crop is utilized fresh. The remainder, especially the lower grades,
is canned, dried, or made into juice. Other fruits, such as berries, persimmons,
and plums are seldom dried.

The principal drying localities, seasons, varieties, yields, and over-all dry-
ing ratios11 of fruits dried commercially are given in table 1. The yields and
over-all drying ratios vary with the variety and locality, as well as from year
to year. These variations make extremely difficult the deriving of reliable
average drying ratios for the various fruits ; therefore, only the ranges are
given in this bulletin.

The tonnages of the most important dried fruits produced in California,
as compiled by Shear (1943) , are given in table 2. More than half of this fruit
is still dried in the sun. Although the dehydration of fruits has been increasing
from year to year, this increase has been restricted mostly to large producers
of dried prunes, raisins, or figs. Small producers, because of the high initial
cost of plant construction, did not find dehydration economically feasible in
the past, for, until recently, there have been no dehydrater designs suitable

11 The term over-all drying ratio refers to the number of pounds of fresh fruit required to
produce 1 lb. of dried fruit. For example, if 6 lbs. of fresh apricots yield 1 lb. of dried
apricots, the over-all drying ratio is 6 to 1 (often expressed 6 : 1). In contrast to the over-all
drving ratio, the actual drying ratio refers to the pounds of prepared fresh fruit to vield
1 lb. of dried fruit. For example, if 4 lbs. of apricot halves are required to produce 1 lb. of
dried apricots, the actual drying ratio is 4 :1.

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Fruit Dehydration : I. Principles and Equipment

for use on small farms. Farm dehydraters are described iu this bulletin in
the section on dehydration equipment.

Table 3 shows the percentages by uses of harvested California fruits for
the years 1934-1938, as compiled by Shear (1943). Slightly over half the
deciduous-tree fruits produced in California are dried, and the rest are either
canned, frozen, or used fresh.

TABLE 2

Tonnages of Dried-Fruit Production in California, Five-Year Averages,

1894-1938, and Annual, 1932-1945*

(Unprocessed dry weight)

Crop year

1899-1903..
1904-1908..
1909-1913..
1914-1918. .
1919-1923. .
1924-1928..
1929-1933. .
1934-1938..
Annual:

1932

1933

1934

1935

1936

1937

1938

1939

1940

1941

1942

1943

1944

19458

Apples

tons
2,200
3,200
2,600
3,000
5,600
9,000
9,800
10,700
10,500

9,800
13,300
8,800
12.500
11,700
12,000
7,500
10,900
4,800
8.500
6.700
8,900
5,200
8,800

Apricots

tons
8,400
11,400
6,700
14,100
16,300
16,500
20,000
31,200
26.200

35,300
37,500
16,800
25,800
32,200
34,400
21 . 500
41,000
11,300
19,700
20.800

6,600
25,700

8.200

Figsf

Peaches

tons

1,500

3,100

3,300

4,700

8,500

10,900

10,600

19,100

25.500

19.000
21,500
23,500
24,000
20,000
28,700
31,500
26,900
32,000
33,500
28,200
36.700
35.200
31.700

tons
10,900
18,500
15,500
23,100
31,700
27,000
22,900
21,700
23,400

22,200
23,400
25,800
19,500
26,400
23.000
22.200
24,900
24,400
14,900
23.400
16,400
26,700
22,500

Pears

tons

3,500

3,700

1,600

1,400

1,100

3,100

4,000

5,100

5,800

Prunes

5,500
7,000
4,900
6,100
8,100
3,500
6,500
8,100
3,100
3,600
2,600
3,700
3,300
4,900

tons

35,300

73,200

54,600

72, 100

76,200

115,500

176,400

185,600

212,200

168,000
182,000
171,000
258,000
159,000
249,000
224,000
185,000
175,000
178,000
171,000
196,000
159,000
231,000

Raisins and

other
dried grapes }

tons

43,700

46,800

55,000

71,000

136,200

206,300

242,900

211,900

220.400

265,500
198,200
173,800
204,000
183,400
248.000
292,000
247,020
172.200
210,000
255,000
401,500
309,500
249,000

* After Shear, S. W. Deciduous fruit statistics as of January, 1943. Contribution from the Giannini Founda-
tion of Agric. Econ. Rept. 83. (Mimeo.)

t Includes merchantable and nonmerchantable figs.

j Dried grapes other than raisins are included from 1936 to date.

§ Preliminary estimates.

Preservation by drying effects a great saving in shipping weight and volume,
as well as in manpower. These factors are important. Table 4 shows compara-
tive equivalent weights and volumes of fruit packed for shipment in the fresh,
canned, and dried form, based on 1 ton of fresh fruit.

Most of the dehydraters in California are located in the prune and seedless-
grape areas, particularly in the San Joaquin, Sacramento, and Santa Clara
valleys and in Napa and Sonoma counties. Because a large proportion of the
deciduous fruits is grown in these areas, it is possible in most instances to
use existing dehydration plants for the drying of other fruits. Maps of the
distribution of various fruit crops in California are given in Crawford and
Hurd (1941). Evaporaters are located principally in Napa, Sonoma, and
Santa Cruz counties — the important apple-producing areas. These evapora-

8 California Experiment Station Bulletin 698

ters are not suitable for other fruits, although floor-kiln evaporaters can be
used for hops.

In the Pacific Northwest, prunes are dried by dehydration, and apples
largely by evaporation, although all new plants are dehydraters. Evaporaters
are also used for the drying of apples in New York and in other eastern states.
Sun-drying, however, is primarily a California practice.

Advantages. — Several advantages favor dehydration as a method of drying.
Fruit drying on trays in the sun is susceptible to insect infestation, contami-
nation with microorganisms which may cause molding and fermentation,
contamination with dust and dirt, and damage caused by rain and by ani-
mals. This type of drying is usually slow, causing great losses in sulfur
dioxide in sulfured fruits. Dehydration, if properly controlled, not only pro-
tects against all of these hazards but also produces clean fruit of higher
quality. The percentage of sulfur dioxide retained during dehydration is, on
an average, three to four times as high as that during sun-drying. Dehydration
is preferable to evaporation because it more accurately controls temperature,
air flow, and humidity. Some evidence indicates that a slightly better yield of
prunes can be obtained by dehydrating than by sun-drying. Dehydration
releases the sun-drying field for the growing of crops or for other uses, in-
creases the turnover of trays so that fewer are needed, and saves in drying-
yard labor.

Limitations. — Certain fruits cannot be dehydrated very successfully. Im-
perial prunes and some plums tend to crack and bleed; in this way they lose
weight and become sticky. Until recently, cut fruits, with the exception of
apples, when dried entirely in a dehydrater, were off-color and unacceptable
to the trade. In order to avoid off-color in the fruit, it was necessary to expose
the freshly sulfured fruit to the sun for several hours before dehydration.
Not only is this procedure expensive and troublesome, but, in addition, the
advantages of cleanliness and freedom from insect infestation offered by
dehydration are partially lost. The pretreatment of cut fruits before dehydra-
tion requires the services of more workers than may be available.

The inefficient or improper operation of dehydraters may be costly, or may
give rise to an inferior product through case hardening, bleeding, scorching,
or smudging.

In certain years, variation in the fruit or rapid drying at low humidity may
cause the outside of the fruit to dry more rapidly than the moisture can diffuse
to the surface from the interior flesh. This condition results in the formation
of a hard, overdried layer on the surface, although the interior may be under-
dried. This type of uneven drying is termed "case hardening." In storage, case-
hardened fruit is likely to undergo fermentation when the entrapped moisture
diffuses to the surface. In 1939, because of case hardening caused by im-
proper dehydrater operation, certain growers were compelled to dehydrate
their prunes two or even three times. Failure to control humidities, a tendency
to use excessively high temperatures, and periodic variations in the composi-
tion and structure of the fruit are the main causes of case hardening.

The use of excessively high temperatures to accelerate drying may cause
scorching, with an accompanying decrease in quality and, possibly, a loss in
weight, The use of low-grade fuels, or inefficient burners, or direct heat from

Fruit Dehydration : I. Principles and Equipment

TABLE 3
Uses of California Fruits, Average Crop Years, 1934-1938*

Kind of fruit

Total, listed

Deciduous tree. . . .

Grapes

Citrus

Deciduous tree, total

Apples

Peaches

Pears

Prunes

Apricots

Plums

Cherries

Figs

Grapes, total

Wine

Table

Kaisin

Citrus, total

Oranges

Grapefruit

Lemons

Dried

Canned

Otherwise
processed

Used fresh

per cent

per cent

per cent

per cent

33.0

1.1

20.5

38.8

54.6

21.7

1.1

22.6

41.3

0.2

45.0

13.5

0.0

2.0

11.0

87.0

54.6

21.7

1.1

22.6

35.2

0.2

7.2

57.4

28.9

55.1

0.0

16.0

14.6

29.9

0.0

55.5

1)0.0

0.0

0.0

66.5

25.2

0.0

8.3

0.0

3.0

0.0

07.0

0.0

16.7

28.9

54.4

87.9

5.8

0.0

6.3

41.3

0.2

45.0

13.5

o.ot

0.0

100. Of

0.0

o.ot

0.0

46. 3f

53.7

73.0

0.3

18.8

7.3

0.0

2.0

11.0

87.0

0.0

l.S

0.4

88.8

0.0

t

6 . 3

93.7

0 0

3.2

17.8

79.0

* Compiled from data by: S. W. Shear. Deciduous fruit statistics as of January, 1943. Contribution from the
Giannini Foundation of Agric. Econ. Reot. 83. (Mimeo.)

t Dried table and wine varieties included in "otherwise processed" as mostly used for wine and juice ulti-
mately.

\ Assumed to be zero.

TABLE 4

Comparative Equivalent Weights and Volumes of Fruit Packed for Shipment, in
Fresh, Canned, and Dried Form, Based on One Ton of Fresh Fruit

Fruit

Fresl

fruit

Canned fruit*

Dried fruit, t bulk pack

Weight

Volume

Weight

Volume

Weight

Volume

Apples

pounds

2,240*
2.380§
2,310§
2,240§
2,420§
2,320§

cu.ft.
61.5
51.0
56.0
46.0
45.0
45.0

pounds
2,460
3,300
2,400
2,400

3,300

cu. ft.
49.0
65.5
48.0
48.0

60.0

pounds
232
408
373
373
800
580

cu. ft.
6.9
9 0

Freestone peaches

Pears

8.3
9.5
15.9

14.1

* Weights and volumes given for cases of No. 2} 2 cans.

t Weights and volumes given for 25-pound bulk-pack boxes.

t Standard apple boxes.

§ Standard lugs for the fruit.

oil burners may result in a deposit of soot and smoke particles on the fruit.
Although this deposit is not readily observable on prunes, it does have an
adverse effect on their quality — especially on their flavor.

In order to secure the best results, conscientious and well-trained operators
are needed. Readings of wet- and dry-bulb temperatures should be taken, and

10 California Experiment Station Bulletin 698

examination of the drying fruit made at frequent intervals. Unfortunately,
dehydraters are sometimes operated in a manner so careless that damage to
the quality of the product results.

Neither sun-dried nor dehydrated fruits reconstitute, or rehydrate, in the
ratio to which they dry. This is unfortunate, since the consumer usually does
not consider the losses in preparation, nor realize the actual volume of fresh
fruit represented by the reconstituted dried product. In some instances pro-
longed periods of soaking are required in reconstituting the dried fruit to the
maximum and desired volume. As an example, dehydrated clingstone peaches
should be soaked 12 to 16 hours before cooking. If the peaches are cooked
without soaking, their volume may be less than half that obtained by cooking
after soaking. The time required for soaking and cooking is considerably
reduced if, in the drying process, the fruit is blanched before sulf uring.

In many instances the term "dehydrated" to the consumer implies that
dehydrated food is actually imperishable. This is an unfortunate misconcep-
tion, because dehydrated foods are perishable and must be handled accord-
ingly. During storage, dehydrated fruits are susceptible to insect infestation,
microbiological deterioration, and chemical changes. These destructive proc-
esses not only affect the appearance of the fruit, but cause changes in taste and
nutritive values.

PRINCIPLES OF PRETREATMENT

The preparation procedures to which the fruit is subjected before dehydra-
tion are known collectively as pretreatmenf.

Condition of Raw Material. — The quality of the final product reflects the
condition of the fruit at the time of dehydration. What is accomplished in
washing, spray-residue removal, and trimming may well determine the final
grade and whether the dried product conforms with the requirements of the
Food and Drug Administration. Peeling, cutting, lye-dipping, sulfuring, and
blanching affect the drying characteristics, vitamin retention, cooking quali-
ties, edibility, and storage characteristics, as well as the quality grade of the
finished product. Inadequate or faulty preparation may mean the difference
between profit and loss.

The production of dried fruits of high quality necessitates the use of proper
cultural practices, maturity standards, and harvesting methods. Cultural
practices are discussed fully in other publications by Allen (1937), Condit
(1941), Davis and Tufts (1941), Hendrickson (1937)", Howard (1943), Jacob
(1940), Philp and Davis (1946), Veihmeyer and Hendrickson (1943), and
others.

As a rule, fruits to be dehydrated should be fully mature and in firm-ripe
condition. Immature fruits should not be dehydrated ; they result in a product
poor in color and flavor, and have a high shrinkage ratio. Great care should
be taken to avoid injury to the fresh fruit during packing and subsequent
handling.

Apricots or peaches to be blanched before dehydration must be firm — never
mushy ripe — in order to avoid slabbing and to prevent excessive losses from
bleeding. Bartlett pears are picked while hard ripe. The pears are then allowed
to ripen in boxes while in storage. Ripening may be hastened by the use of

Fruit Dehydration : I. Principles and Equipment 11

ethylene gas or by storing mature pears near immature ones. Maturity can be
determined by a pressure tester, as described by Davis and Tufts (1941), with
the pressure test reading not over 19 pounds.

For best quality and greater yield, raisin grapes should not be picked for
drying until the Balling, or Brix degree (approximate sugar percentage de-
termined by the use of a hydrometer) of the juice reaches 23; grapes allowed
to attain 24 to 26° Balling yield even better raisins; (see Jacob, 1942 and
1944). In rainy and cool seasons, however, this range is sometimes difficult
or even impossible to attain. In event of rain, grapes comparatively low in
sugar may have to be dehydrated in order to salvage the crop, but they should
attain at least a 21° Balling before drying.

Cherries and berries should be picked when table ripe. Plums can be handled
more easily if they are on the firm-ripe side of maturity and are allowed to
finish ripening in boxes. Figs usually dry to a considerable extent on the tree
before they drop and harden. Prunes, too, late in the season in certain areas,
often dry partially on the tree or on the ground. Persimmons may be firm
when harvested, but should be fully ripened before drying to avoid excessive
puckery taste.

Harvesting. — As a rule, figs and prunes drop to the ground when they are
ready for drying. In the Sacramento and San Joaquin valleys, however, prunes
often remain on the tree even after they are fully mature, and it is necessary
to knock them from the trees with poles. Although the harvesting of apricots
and peaches can be done at lower cost per ton by shaking the fruit to the
ground or on sheets of canvas, the quality always suffers because : first, in
falling the fruit becomes bruised or dirty, or both ; and second, immature fruit
is always mixed with mature. Shaking should be done only for figs and prunes.
All other fruits should be picked by hand from the tree or vine in order to
secure the best quality.

Storage of Fresh Fruit. — Mature fruit should be dried as soon as possible
after it has reached maturity. If it must be held for more than 1 or 2 days it
should be placed in cold storage. Some fruits cannot withstand the tempera-
ture in warm weather for more than a day without showing bruises or molding.
In cool weather firm-ripe fruit may well stand 2 or 3 days of storage at room
temperature. Crafts and Brooks'2 have observed and pointed out that apricots
showing a slightly green cast will ripen evenly, though slowly, in cold storage
or more rapidly at room temperature. Spoilage losses, however, may be very
high in years of serious brown-rot infection. One successful grower in the
Santa Clara Valley prolongs his drying season several days by holding firm-
ripe apricots, of canning quality, in cold storage.

Prunes are ordinarily considered capable of withstanding considerable
abuse, but in humid seasons they will mold in the boxes if allowed to stand
several days after picking. Moldy fruit should not be dried, even though an
alkaline dip removes the obvious evidence of mold.

Freestone peaches do not keep well in cold storage ; they should be dehy-
drated as soon as possible after they are harvested. Firm clingstone peaches
may be held, in carefully operated cold-storage warehouses, in reasonably
good condition for several days, and apples and pears for considerably longer.

12 Crafts, A. S., and E. Brooks. Unpublished data.

12 California Experiment Station Bulletin 698

For details concerning correct handling practices, storage temperatures, and
humidities for fresh fruits see Rose, Wright, and Whiteman (1941).

Washing Fruit Prior to Drying. — All fruits should be washed, but it is
particularly important that ones picked from the ground should be thor-
oughly washed to remove adhering bits of soil, straw, and other foreign
material. Prunes and grapes are washed at the time of dipping; apples and
pears are rather effectively washed during the process of spray-residue re-
moval; in addition, pears are (or at least should be) washed after cutting
and traying. Whatever the fruit, washing should be thorough and should be
conducted in such a manner as to avoid recontamination of the fruit with dirt
and microorganisms.

The cleaning of prunes is frequently unsatisfactory, particularly where
alkaline dips are no longer used for checking the skin. Frequently, where
washers employing recirculation are used, a given volume of cold or warm
water is sprayed over the fruit many times a day. After several hours' use
this water becomes quite muddy and, in some instances, so dirty that the dried
fruit actually shows dirty splotches. Recirculation is not recommended, but
if unavoidable it must be carried out in a manner that will not impair the
washing process. The latter can be done by applying an effective cold fresh-
water spray after the initial washing. This will remove dirty water which
remains after the first spray.

Prunes washed properl}T before dehydration need no further washing after
drying, if other handling equipment is kept clean. Under current practices,
however, washing is nearly always necessary before dehydrated prunes are
packed. Even though most foreign material may be removed easily, dehy-
drators find it very difficult to remove from fresh prunes adhering particles
of leaves and stems. These can be removed by means of a leaf and trash re-
mover, which is equipped with a blower and an efficient slotted screen shaker.
In addition, foreign objects and decomposed prunes should be removed at a
sorting belt,

Pears should be washed after cutting to remove adhering particles of dirt
and evidences of insect infestation. As the trays are removed from the cutting
tables, they should be sprayed with cold water. Figs are usually not washed
before drying, because they are washed effectively at time of packing. Never-
theless, it would be a desirable practice to wash figs before dehydration. After
lye-dipping, grapes are usually washed in a spray shaker or in an immersion-
type washer. Clingstone peaches must be thoroughly washed after lye-peeling.
It is not customary to wash freestone peaches and apricots. These soft fruits,
when allowed to stand with free moisture on the surface, may undergo rapid
deterioration through growth of the brown -rot fungus. Cutters also object
to handling wet fruit, because it reduces their output considerably.

8 pray -Residue Removal.— -Home spray materials leave residues on the fruit
which, above certain quantities, are considered injurious to human health.
Although the extent of the hazard in spray residues is not always measurable,
the greatest danger to the consumer is in chronic rather than acute poisoning.

The health hazard of arsenic- and lead-spray residues has been stronglv
voiced by Geiger, Becker, and Crowley (1936) and Hanzlik (1937). Cardiff
(1937), on the contrary, has argued strongly against the possible dangers of

Fruit Dehydration : I. Principles and Equipment 1 3

ingesting spray residues. Fairhall and Neal (1938) fed two human subjects
100 milligrams of lead arsenate over a period of 10 days, during which time
they were on a controlled diet. The degree of absorption, path of excretion,
and toxicity of this dosage were evaluated. Fairhall and Neal observed that
"While the lead arsenate was completely broken down in the body, no un-
toward effects on the well-being of these two individuals attributable to this
quantity were noted. The greater part of the lead and arsenic derived from
the ingested lead arsenate was directly recovered, and it was found that the
lead was excreted with the feces and that the arsenic was excreted in the urine."

Laws exist — and are enforced — which limit the amounts of arsenic, lead, or
fluorine that may remain on or in foods offered for sale for human use. Definite
limits, or "tolerances," for these spray residues have been established by the
Food and Drug Administration of the Federal Security Agency. In 1940 the
Agency announced that the tolerance for lead and arsenic had been liberal-
ized because of the experimental results obtained by the United States Public
Health Service. The new tolerances, as of August 10, 1940, are : 0.025 grain
arsenic as arsenic trioxide per pound (3.58 mg. per kilogram) and 0.05 grain
of lead per pound (7.15 mg. per kilogram). Fluorine was not included in the
study made ; consequently, the tolerance remains at 0.02 grain of fluorine per
pound (2.86 mg. per kilogram) as of November 14, 1938. These limits apply
to both fresh and dried fruits. The tolerances for spray residues are changed
Prom time to time. Growers uncertain of current tolerances should obtain this
information from the Federal Security Agency, Washington, D. C, or from
the Federal Food and Drug Administration, Federal Building, San Francisco,
California.

There is greater necessity for removing spray residues from impeded fruit
that is to be dried than from fruit that is to be consumed fresh. This is true,
because during drying the percentage of residue increases proportionately to
the drying ratio, and the Food and Drug Administration tolerance does not
differentiate between fresh and dried fruit. In order to cope with this problem,
the fruit should be washed before drying, and the use of toxic insecticides
should be avoided for as long as possible before harvesting. These procedures
are discussed more fully in publications by Allen (1937), Hough (1936),
Haller, Smith, and Ryall (1935), Robinson and Hatch (1933 and 1935),
Overly, St. John, et al. (1933), and Hartman (1929). Hoffman (1937) has
discussed the removal of spray residues from cherries.

Apples and pears, which are the most important fruits from the stand-
point of methods for spray-residue removal, are discussed on page 29 of this
bulletin.

Sorting and Trimming. — As a rule, to harvest cull fruit for drying is in-
advisable and uneconomical ; nevertheless, it is frequently done. Neither imma-
ture, moldy, or heavily infested fruits, nor those imbedded with dirt should
be harvested. Since pickers and cutters are usually paid by the box or by some
other unit of volume or weight, however, it is unsatisfactory to depend on
their sorting the fresh fruit. Sorting may be done as a separate operation be-
fore cutting or dipping. Tf defective parts of individual fruits are small, they
may be trimmed out and disposed of with the culls. Trimmings and cull fruit
should be collected in a separate box, in order to avoid soiling the good fruit,

14 California Experiment Station Bulletin 698

or, in the instance of apricots, contaminating the pits which have commercial
value. Because of its sugar content, waste fruit may have value as a source
of alcohol manufacture.

Cutting, Pitting, and Peeling. — Fruits are cut, pitted, or peeled to facilitate
drying and to produce dried fruit of better appearance and eating quality.

Apricots and freestone varieties of peaches and nectarines are halved and
pitted, but are not peeled in commercial practice. These fruits are cut by hand
knife around the suture, the halves separated, and the pit removed. On a few
occasions some freestone peaches have been peeled by hand after sulf uring ;
this practice loosens the skin of mature fruit to such an extent that it can be
removed without the use of a knife. This process, however, has never been
found feasible because it is too costly. Cutting these fruits into smaller pieces —
such as slices — facilitates drying, but offers difficulties in handling after
drying because the fruit sticks to the trays. Occasionally apricots have been
dried whole, but usually only after dipping them in lye and exposing them
to prolonged sulf uring.

Clingstone varieties of peaches and nectarines are cut around the suture,
but require the use of a pitting spoon to remove the pit, which adheres to the
flesh. In large establishments mechanical pitters, such as those described by
Cruess (1938), are used. A superior product is obtained when cling peaches
are lye-peeled after pitting. However, they may be dehydrated without being
peeled. The quality of the dehydrated unpeeled peach is comparatively poor
and there is relatively little demand for it. Peeling is accomplished by im-
mersing the halved peaches in a hot 1.0 to 2.5 per cent lye solution for 30 to 60
seconds, then by washing in sprays of cold water.

Apples are peeled and cored by machine. Since they are extremely suscep-
tible to enzymatic browning, they are then immersed in a dilute sodium
chloride (salt) or bisulfite solution to prevent initial discoloration. After
apples are hand-trimmed to remove blemishes or pieces of skin, they are me-
chanically cut into rings, quarters, or smaller sections ; preferably, they should
be dipped again in a dilute bisulfite solution. The peels, cores, and trimmings
amount to about 20 to 30 per cent or more of the fresh apple weight.

Pears are stemmed and halved and the calyx is removed by hand or mechani-
cal operations. It is not customary to peel, remove the core, or cut pears into
thin slices, although these procedures do offer certain advantages. Sliced pears
dry more rapidly than do half pears, but are rather difficult to remove from
the trays. Pears that are peeled and cored before drying are more attractive,
but these operations involve additional labor costs and loss in weight.

Persimmons are cut into halves, quarters, sixteenths, or slices and may or
may not be peeled. Cranberries are pulped or sliced before drying. Grapes,
figs, prunes, berries, and other small fruits are dried whole and without peel-
ing. Grapes are lye-dipped and usually sulfured.

Equipment such as the cutting shed, cutting tables, and pit boxes has been
described in detail by Mrak and Long (1941) .

Treatment of Pits and Waste Materials. — Apricot pits should be collected
in clean containers ; they should never be mixed with trimmings or with decom-
posed fruit. The pits are dried in the sun by spreading them about 4 inches
deep on trays or on concrete drying surfaces, where they are periodically

Fruit Dehydration : I. Principles and Equipment

15

stirred by raking (see fig. 1). Apricot pits should not be sulfured, because
this reduces their value, and the pits from sulfured whole fruit should never
be mixed with unsulfured pits. The pits are sold to by-product plants where
they are cracked and the kernels separated by brine flotation. The kernels are
utilized in the production of sweet oil and of bitter-almond oil for the candy
and bakery trades. The shells, after being pulverized, are used in several in-
dustrial applications, principally as a filler for plastic and rubber products.
Peach pits are principally used in the production of charcoal to be mixed
with chicken feeds, but are also used for other purposes. Activated carbon

Fig. 1. — Apricot pits spread on concrete surface for drying
in the sun prior to sacking.

made from peach pits for gas masks compares unfavorably with that made
from other sources, for example, cocoanut shells. At one time, peach pits were
subjected to a destructive distillation to recover methyl alcohol, acetic acid,
pyroligneous acid, and other chemicals. But, as more efficient competitive
processes have changed economic and market conditions, most of the destruc-
tive distillation plants have discontinued operation except for the production
of charcoal.

Apple peelings and cores are utilized largely for making industrial alcohol,
apple brandy, or cider vinegar. Much of the press-cake pomace is dried and
sold as stock feed or for the manufacture of pectin.

Pear trimmings and rotten fruit have no commercial value at present. They
do have some feeding value for hogs, however, if fed with grain and other dry
feed. This waste fruit is an excellent breeding source for insects which may
later damage the dried fruit; it should not be dumped near the cutting shed.
In order to eliminate this hazard, the waste material should be spread thinly
for rapid drying ; or, if it is placed in holes or in piles, sufficient lye, quicklime,
or chlorinated lime should be added to prevent fermentation, and resultant
odors. The drying yard should always be kept free of such refuse in order to
reduce insect infestation and to promote cleanliness.

16

California Experiment Station Bulletin 698

Prior to the war, canners dried cherry stems to be sold in Europe for use in
pharmaceutical preparations and as flavoring for liqueurs. Table 5 gives the
types and percentages of waste of a number of fruits.

Dipping. — Certain fruits are treated in an alkaline bath in order to remove
from the skin the waxy coating termed the "bloom," or to induce the forma-
tion of many minute skin cracks, called "checks," to facilitate drying. This
may be accomplished by submersion or by passing the fruit under a spray of
the hot alkaline solution. Many producers have found, however, that checking

TABLE 5

Type and Percentages of Waste in Preparing Various
Fruits for Dehydration

Fruit

Type of waste

Amount
of waste

Apples

Apricots.

Bananas ....

Cherries

Figs

Peelings and cores ....

Pits

Skins

Stems and pit if pitted. .

Only culls

per cent
20-30

6-10
45-50

8-20

20-45

Grapes

Nectarines

Only culls

Pits

Pits

Skins. .

Pits

Steins and calyx ends

9 14

11 15

Poaches (freestone)

Pears

10-15
4 8
,:,

25 35

Pineapples

Pomegranates

Prunes

Crown, shell, and cores:

( 'row n removed

Crown present

Skins and sector tissues
Only culls

35 45
45-55
50-60

causes the prunes to bleed or juice badly during dehydration, and, as a result,
have resorted to cold- or hot-water washes instead of the alkali treatment. Hot
water not only removes the bloom, but also reduces or eliminates bleeding.
Cruess (1943) has reported, however, thai the drying lime increases when lye
is omitted.

In the production of golden-bleached raisins, Thompson Seedless grapes are
treated with a % to % per cent hot lye solution for 5 to 15 seconds before sul-
furing. By this procedure, the skins are checked and the bloom is removed.
After the dipping and washing process, the checked areas tend to discolor
rapidly. Hussein, Mrak, and Cruess (1942) found that the oxidase activity of
these grapes was stimulated by a 5-second dip in hot lye but was greatly re-
duced when the dip was extended to 30 seconds. The behavior of the enzyme
was similar when hot water was used as a dipping bath. If in the latter type of
bath the temperature of the water was lowered from 205° F to 180° more than
twice as long a time was required to reach the same degree of inactivation.
Unfortunately, the longer dip cannot be used on Thompson Seedless grapes
because it results in overchecking and mushiness of the fruit. Furthermore,
any discoloration resulting from oxidase activity in Thompson Seedless grapes
bleaches out during sulfuring and leaves no noticeable effect on the final dried
product.

Fruit Dehydration : I. Principles and Equipment 17

Muscat grapes are sometimes dipped in a lye or carbonate solution before
dehydration. The skin is not checked, but the waxy bloom is removed. The
finished product is known as the Valencia raisin.

It is advisable to lye-dip sweet cherries to be dried whole, but not sour varie-
ties, which should always be pitted.

Blanching. — Apricots, peaches, pears, and nectarines are usually sun-dried.
Characteristically, the dried product is translucent, somewhat glossy, and is
of good color and texture. Fruit sulfured and then dehydrated without other
pretreatment is ordinarily opaque and dull in appearance, less uniform in
color, and of tougher texture. Because of these differences, dehydrated cut
fruits, with the exception of apples, have been placed in lower quality grades.
It has long been known that the commercially acceptable appearance of sun-
dried fruits can be attained by exposing freshly sulfured halves to the sun
for 3 to 4 hours before dehydration. This process, however, has all the disad-
vantages of sun-drying.

It would seem that a rational basis for dried-cut-fruit standards would give
greatest weight to such factors as nutritive value and sanitation. These factors,
however, have been almost completely ignored by the trade, which substitutes
appearance for food value. Appearance has been the primary trade require-
ment for dried fruit for such a long time that a new product, whether or not
it rates higher in all other respects, must resemble an older product to be
acceptable. A dehydrated fruit, therefore, must meet the trade requirements
for appearance.

Mrak, Phaff, et al. (1943) have pointed out that the approved appearance
of sun-dried cut fruit can be obtained in the dehydrater by using ultraviolet
rays. They were able to demonstrate this experimentally by exposing cut sul-
fured fruit to an H-5 (G.E.) lamp for 2 hours before dehydration. They also
showed that steam-blanching, correctly applied to cut fruits, likewise yields a
dehydrated product which resembles sun-dried fruits in appearance.

Nichols and Christie (1930) called attention to the fact that steam-blanching
shortens the drying time. They did not recommend it, however, for with the
short blanching times used in their experiments they failed to realize its pos-
sibilities. When longer blanching times were tried (Mrak, Phaff, et al., 1943)
this method was found to produce dehydrated cut fruit similar in color and
appearance to the sun-dried product.

To date, this required appearance of dehydrated fruits can be obtained only
by the two preliminary treatments described above. Of the two, blanching
appears to be the most workable and practical.

Fruit removed from cold storage should attain room temperature before
it is cut. Cold fruit causes excessive condensation of the steam and increases
blanching costs. Apricots, in particular, may slab and stick to the trays.

Blanching is best accomplished by exposing fruit spread on dehydrater
trays to steam at atmospheric pressure. This can be done in continuous or dis-
continuous cabinet blanchers. The time of blanching will vary with the size,
variety, and maturity of fruit, the efficiency of the blancher, the characteristics
of the trays, and the extent of continued heat penetration into the fruit after
stacking of the trays. Phaff, Perry, and Mrak (1945) pointed out that the
temperatures attained and maintained in different parts of fruit pieces should

18

California Experiment Station Bulletin 698

be known if the blanching process is to be understood thoroughly. When a cut
fruit half — a freestone peach for example — is exposed to steam at atmospheric
pressure, the surface temperature at once rises very rapidly, as shown in
figure 2, then gradually approaches steam temperature. In the center of the
fruit piece the temperature does not change at first ; but it begins to rise slowly
and then more rapidly as it follows the surface temperatures. Small fruits heat
through more quickly, but about 11 minutes are needed to heat the centers of

6 8

MINUT E S

Fig. 2. — Temperatures in a laboratory blancher, and in freestone peach halves during
blanching. Curves A, B, and C, blancher temperatures for runs 1, 2, and 3, respectively.
Curves Al and A2, Bl and B2, and CI and C2, peach flesh temperatures for runs 1, 2, and S,
respectively. Positions at which peach flesh temperatures were measured are indicated in the
sketch at the lower right.

larger peaches to above 200° F. When trays are removed from the blancher
the surface temperature of the fruit begins to drop almost at once, if heat can
escape readily from the stack, while the center temperature drops more slowly.
Since the center was slower in heating, with its temperature better maintained
on cooling, the blanching effect on the fruit is quite even.

Each operator must learn to determine the proper degree of heat penetra-
tion in order to avoid over- or underblanching. The cut fruit should be heated
more than two thirds of the way through by the time it emerges from the
blancher. The extent of heat penetration can be determined visually by cutting
apricots, freestone peaches, pears, or nectarines with a knife, or by testing for
the enzyme peroxidase33 on the cross section (see figs. 3 and 4). The cutting

1:1 The test for peroxidase is made as follows: the cut cross section is flooded with a 0.1
per cent alcoholic solution of guaiacol or benzidine. A few drops of a 0.3 per cent hydrogen
peroxide solution is then applied to the surface. In a few minutes any area where the enzyme
peroxidase has not been heat-inactivated will turn brownish if guaiacol is used or dark blue
if benzidine is used.

Fruit Dehydration : I. Principles and Equipment

19

Fig. 3. — Appearance of sections of freestone peach halves blanched in steam for
various periods of time. Blanching times were, from top to bottom, 0, 3, 6, and 9
minutes. Sections at the left were flooded with guaiacol and hydrogen peroxide,
whereas those at the right were not. Dark-colored areas at the left and light-colored
areas at the right indicate underblanching. (From Food Industries, vol. 15, no. 4,
p. 59.)

Fig. 4. — Appearance of sections of Bartlett pear halves after blanching in
steam for various periods of time. Blanching times were, from left to right,
0, 4, 8, 12, and 16 minutes. Underblanched areas appear dull and chalky and are
firmer than the blanched areas. (From Food Industries, vol. 15, no. 4, p. 59.)

20 California Experiment Station Bulletin 698

test is not satisfactory, however, for clingstone peaches. Properly blanched
fruit, after standing in the stack for some time, should be heated through, but
not to the point where excessive bleeding or tissue breakdown occurs. Under-
blanched fruit will retain firm areas, easily observed as described above.
Bartlett pears in particular, when underblanched, tend to discolor because of
oxidase activity. To avoid poor sulfur dioxide absorption and excessive bleed-
ing in the sulfuring house, blanched fruit should be cooled sufficiently before
sulf uring. Artificial cooling by means of fan blast is more efficient than cooling
by natural draft, particularly with close-sided trays, or if cabinet-blanching,
or mechanical car-loading pits are used.

Most of the excessive bleeding caused by overblanching takes place during
subsequent sulfuring. Lovell and Muir peaches in particular have a strong-
tendency to bleed heavily if the fruit is overblanched. Sulfuring also causes
the fruit to soften and become mushy, a condition accentuated by previous
blanching. Phaff, Marsh, et al. (1945) found that bleeding can be eliminated by
drying the fruit for 30 to 40 minutes before sulfuring, in a parallel-How tunnel
at a hot-end temperature of about 185° F. Fruit treated in this way should
then be sulfured in the usual manner (Cruess, Friar, et al., 1944) and dehy-
drated to completion in a countercurrent tunnel. It was found that soft-ripe
apricots and most freestone peaches, with the exception of the Curry variety,
tend to bleed unless predried as described above. Pears do not bleed, even
when overblanched, because the skins are tough enough to prevent collapse
of the fruit half.

An objection to blanching is the tendency of soft halves of apricots to flatten
out and form slabs. This tendency is not so apparent in peaches and is absent
in pears. Steam-blanching also tends to fix the green color (chlorophyll) so
that, in order to produce an attractive dried product, the fresh fruit should
be ripened evenly.

There are, however, several advantages in blanching freshly cut fruit halves.
Blanched fruit is translucent and glossy, similar in appearance to the sun-
dried product. Tilton apricots and peeled clingstone peaches assume a deep
orange color very similar to that of the Santa Clara Valley sun-dried Blenheim
or Royal apricot. Dehydrated unblanched fruit is dull and opaque. According
to Crafts (1944 a and b) the air pockets in the tissues of unblanched fruit
disperse light and give the dried product this dull, opaque appearance. (See
fig. 5). Blanching rids the tissues of air and improves the appearance of the
fruit. Three effects of blanching account for the displacement of intercellular
air. First, the air is expanded by the heat, and most of it escapes from the cut
surfaces through intercellular spaces. Second, the cells are killed by the heat
and rendered permeable ; in this way, the sap is allowed to escape. Third, the
cell walls are softened, so that they bend and give under the compressional
forces of surface tension.

Mrak, Phaff, et al. (1943) have observed that the retention of provitamin
A (carotene) and ascorbic acid (vitamin C) is greater in blanched dehydrated
fruit than in sun-dried. Other advantages of blanching are: (1) improved
cleanliness of the product; (2) reduced drying time and higher sulfur dioxide
retention; (3) superior retention of the fresh apricot flavor; and (4) reduc-
tion in time required for rehyd rating and cooking. Dehydrated fruit is not

Fruit Dehydration : I. Principles and Equipment

21

, ■: ■ ■ ... , V;.y. ,

d;

Fig. 5. — Royal apricot tissues, thin hand sections: A, Fresh tissue, showing thin-
walled parenchyma cells and prominent air-filled intercellular space. B, Dried fruit,
not blanched or sulfured. Large black dots are air bubbles. Transparent areas are
cavities filled with air. (Section mounted in glycerin.) C, Blanched, sulfured, and
dehydrated fruit. Note the almost complete absence of air. Very small black dots are
carotene masses. (Section mounted in glycerin.) D, Same as C, but rehydrated. Small
black dots are carotene masses. Note that blanching and dehydration have not
destroyed cellular structure. (Photograph by A. S. Crafts.)

exposed to infestation and contamination while drying. Blanched apricots
dehydrate in about two thirds of the time required for the unblanched fruit.
With peaches and pears, the differences are greater. Large-scale cooking tests
have shown that dehydrated blanched apricots require about 2 hours of soak-

22 California Experiment Station Bulletin 698

ing and V2 hour of cooking to become tender, whereas sun-dried fruits require
much longer.

The storage qualities of the dehydrated blanched fruit are at least equal to
those of the commercial sun-dried product, unless heat damage has occurred
during dehydration.

Sulfuring. — Certain fruits are sulfured before dehydration to preserve
their natural color and flavor, to prolong storage, to retard the loss of pro-
vitamin A and ascorbic acid, and to prevent microbial deterioration. Sulfuring
is usually done after the fruits are cut or the skins are checked by lye-dipping
to facilitate absorption of the sulfur dioxide.

The fruits sulfured before drying are : apricots, apples, peaches, pears, nec-
tarines, Thompson Seedless grapes to be made into golden-bleached raisins,
and a limited quantity of Kadota figs. Sour and light-colored varieties of sweet
cherries should also be sulfured.

The best method of sulfuring is exposure of the cut fruit to fumes of burn-
ing sulfur in a closed chamber. In addition, apples are dipped in dilute bisul-
fite solution. Phaff (1945) has pointed out a number of factors which influence
the absorption and retention of sulfur dioxide by the fruit. Time, temperature,
and concentration of gas in the sulfuring house are factors of primary impor-
tance. Even when very high concentrations of sulfur dioxide are used, a
minimum period for adequate sulfuring is required to obtain sufficient pene-
tration of the gas into the flesh of the fruit.

Blanched apricots usually require from 2 to 3 hours, peaches 3 to 4, and
pears 10 to 15 hours. Fisher, Mrak, and Long (1942) showed that cut fruits as
well as whole fruits, such as grapes and figs, absorb less sulfur dioxide during
sulfuring, but retain more after drying, when they are sulfured at a relatively
high temperature, such as 100° to 120° F.

The concentration of sulfur dioxide in the average sulfuring house will
range from 1% to 2 per cent by volume. Concentrations as high as 3 per cent
by volume may be attained by burning sulfur in a tightly constructed house.
Concentrations of about 2 per cent are advisable if satisfactory sulfuring is
to be accomplished in a reasonable length of time. Long, Mrak, and Fisher
(1940) have discussed fully the time and concentration factors related to the
sulfuring of fruits.

The absorption of sulfur dioxide also varies greatly with the kind, variety,
size, and maturity of the fruit. Apricots usually absorb more of the chemical
than do peaches or pears. Immature fruit usually absorbs more sulfur dioxide
than does mature fruit, but loses it much more rapidly during the process of
drying.

Grapes and cherries dipped in an alkaline solution absorb sulfur dioxide
more rapidly than does untreated fruit. During the process of drying, sulfur
dioxide losses are more rapid when the skins are checked by lye-dipping. The
use of certain salts, such as sodium citrate, will facilitate sulfur dioxide reten-
tion; these salts, however, have been used only experimentally. (See Mrak,
Fisher, and Bornstein, 1942.)

Apples may be sulfured after they are peeled and cored, or after they are
trimmed and sliced or quartered. Sulfuring fruit before cutting arrests dis-
coloration and gives more uniform penetration than sulfuring sliced fruit

Fruit Dehydration : I. Principles and Equipment 23

which is piled deeply on kiln floors. The disadvantage is that, after subsequent
cutting, the freshly exposed surfaces are susceptible to oxidation and dis-
coloration. Sulf uring after cutting is more uniform if the fruit is quartered,
cubed, or sliced, and spread thinly on trays. To prevent discoloration before
sulfuring, some driers, immediately after peeling, dip the fruit in a 2 per cent
salt solution, although a 0.5 per cent solution of sodium bisulfite is preferable.

PREPARATION EQUIPMENT

Plant Layout. — The plant layout should be planned so that the fruit can
move through the preparation line in a continuous flow, with a minimum
amount of labor, power, and equipment. The preparation line may be operated
during a 24-hour day or for any fraction of a day convenient to the operator.
Operation periods of 8, 10, 12, or 16 hours are most commonly used for the
preparation line. For economic reasons, if at all possible, the dehydrater must
be operated continuously during the full season. As the preparation line cus-
tomarily operates for a fraction of a day, storage tracks to hold extra cars of
prepared fruit must be provided.

A plant design similar to that illustrated in figure 6 provides for a straight-
line flow from the preparation line through the sulfuring house and tunnel
to the finishing area ; here the fruit is removed from trays to boxes. Storage
tracks are located between the sulfuring houses and tunnels and near the
finishing area. The tray washer is so located that every tray is washed before
it is loaded. A plant of this type is suitable for grapes and prunes, but it would
have to be modified considerably to be used for apricots, peaches, or pears. To
accommodate these cut fruits it would need continuous cutting lines and a
steam blancher (see fig. 7). Since the double-decking of trays in the blancher
gives satisfactory results and doubles the output, a number of parallel cutting
lines preferably should be built. These lines would then make a junction with
the main feeding line into the blancher, and at this point double-decking could
be done.

For apples, the plant layout must be different. In general, apples move from
the receiving room through a peeling and coring machine, through a dipping
trough, and on to a trimming table. From the trimming table the fruit is
passed through a slicer into a sulfite bath equipped with a walker which con-
veys the fruit either onto trays or into a wTheelbarrow for transfer to the kiln
floor. Caldwell (1923) has described an apple-drying layout. He provides for
a continuous sulfuring tunnel, which is still used in the Pacific Northwest.
In this the whole apples, after being trimmed, cored, and dipped in a dilute
bisulfite solution, are sulfured for about 45 minutes.

In planning a plant layout, sanitation should be given serious consideration.
Cleaning equipment should be amply provided. Wash bowls, soap, and paper
towels, conveniently placed in the cutting shed encourage the cutters to keep
their hands clean. Drinking fountains are a great convenience. Sanitary toilets
should be conveniently located. Employees should be instructed to wash their
hands frequently and especially after use of the toilets. Sewage lines from
toilets and lavatories should run to a septic tank, with the final disposal line
outside the plant area or to a city sewer. Rotten and waste fruit should be
removed daily and treated as previously described to prevent insect breeding.

fcd

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i^p#tin

Fruit Dehydration : T. Principles and Equipment

25

Hosing and sweeping the cutting-shed floor should be part of the daily routine
of cleaning. Dusty roads should be oiled or should be constructed on the side
of the plant away from the prevailing winds, and no barnyards or corrals
should be near the plant.

Fig. 7. — Cut fruit preparation room showing continuous cutting table, blaneher,
and tray stacker. (Courtesy of John Leonard, Cupertino, Calif.)

Roads, cutting-shed floors, and paths along distribution tracks should be
treated to settle dust, which otherwise may contaminate the fruit. Deliquescent
salts, such as calcium chloride, spread on an earth surface absorb moisture
from the atmosphere and keep the soil surface moist. The calcium chloride
may be distributed at the rate of V2 pound per square yard, and may be either

26 California Experiment Station Bulletin 698

spread dry and raked into the surface, or dissolved in water and applied with a
sprinkling can. In very arid localities, the atmosphere may not give up suf-
ficient moisture to the chloride ; here, treated areas may require one or two
light sprinklings during the season. Being soluble in water, the chloride will
be leached away by winter rains and will have to be replaced annually. On a
cutting-shed floor it can be expected to last longer. Asphaltic oils of a heavy
grade, with sufficient asphalt content to cement the soil surface may also be
used. Lighter oils which do not consolidate the soil are objectionable, for coated
particles that adhere to the fruit are difficult to remove by washing.

Fruit Boxes. — Wooden lug boxes about 15 x 24 x 9 inches, to hold 40 pounds
of fruit, are commonly used in transporting fruit to the cutting shed. The box
should be made of lumber having a thickness of ^ inch for the sides and bot-
tom, and 1 inch for the ends; triangular corner posts should be 2 x 2 inches;
and top cleats across the ends, %x2 inches. All material should be of pine,
nailed on about 3-inch centers, around sides and bottom, with 6-penny, cement -
coated box nails. The top cleats are essential to the protection of the fruit;
they also give a space for ventilation wrhen the boxes are stacked. In handling
pears, boxes holding 44 or 45 pounds of fruit are used. The dimensions of
these boxes are 14% x 20 x 10 inches, including a %-ineh top cleat ; or
14% x 20 x 102%2 inches, including a 2%2-inch top cleat. Larger-sized boxes —
holding 60 pounds of fruit — are not recommended for orchard use; they are
difficult to handle and tend to bruise the fruit,

Boxes with the sides and bottom of exterior-grade Douglas fir plywood have
been introduced ; although higher in initial cost, they are durable and light
in weight. Sheet-metal boxes have been used experimentally ; their proponents
claim for them the advantages of cleanliness, improved ventilation, less bruis-
ing because of improved shape, lower weight, and longer life. Of additional
advantage is the fact that the boxes, when empty, require less storage and
hauling space, since their design permits the stacking of one within the other.

Cutting Tables. — Plans for three types of cutting tables are obtainable from
the Agricultural Extension Service, University of California, Berkeley, Cali-
fornia. Continuous cutting tables are preferable ; they eliminate much labori-
ous handling of trays and facilitate the movement of freshly cut fruit directly
from the cutting table into the blancher. Such a table is shown in figure 7.
Where continuous cutting tables are used, the cutters may be paid either by
the hour or by piecework. When the latter system is used, cutters should be
shifted along the table periodically, or the last four cutters should be paid by
the hour. If the fruit fails to fill the trays by the time it reaches the end of the
table, a discontinuous system of operation may be used.

Blanchers. — Since apricots and freestone peaches are blanched for rela-
tively short periods and may be easily overblanched, the use of a continuous
blancher is advisable. Cabinet-blanching with present equipment does not
appear feasible for them; there is too great a possibility of uneven heating
throughout the tray stack, in the short blanching period that may safely be
used. Cabinet blanchers may be used for pears and clingstone peaches, which
require a long blanching period and show only a slight tendency to bleed.

Continuous vegetable blanchers, similar to those used for blanching trays
of cabbage, are suitable for cut fruits. Because fruit must be blanched on trays,

Fruit Dehydration : I. Principles and Equipment 27

care must be taken to attain smooth movement of trays through the blancher
in order to avoid turning the fruit. Openings for steam jets should not be
pointed toward the fruit, because of the danger of mechanical damage; they
should slant downward, about 2 inches apart. To avoid steam currents which
would overturn fruit, the openings should face each other in each successive
steam pipe inside the blancher. Either a narrow blancher accepting trays
in lengthwise position or better still, a wide blancher carrying trays in
crosswise position may be used. The latter is more suitable for large-scale
operations.

When some blanchers were found to form bottlenecks, as compared with
the operation of other plant equipment, the expedient of using trays double-
decked was tried ; examination of the product indicated little difference in the
results between top and bottom trays. The trays should be separated or so
constructed that steam may flow freely to the fruit on the lower tray.

If the following factors are known, one can easily calculate the required
length of a continuous blancher.

1. Maximum blanching time in minutes for a given fruit (= t).

2. Average tray load in pounds for a given fruit (=L).

3. Dimension in feet of a tray measured in the direction of travel through
the blancher (= W) .

4. Desired capacity in pounds of prepared fruit per hour (= C) .
Assuming that double-decking of trays is used, the necessary blancher

length should be computed by substituting the proper values for the letters
indicated in the following formula :

Blancher length = — — -r—-
Lx 120

The following table shows maximum blanching time and average tray load
for a few fruits :

Maximum blanching Average tray

Fruit time, minutes (t) load, lbs. (L)

Apricots 3.5 35

Clingstone peaches 12 45

Freestone peaches 8 50

Pears 20 70

Example:

A plant manager wants to cut 6 tons of apricots per hour. The waste
may be assumed to be 10 per cent. Amount of cut fruit per hour 10,800
pounds (=C). # = 3.5; L = 35; W = 3 for a blancher in which 3x6 foot
trays go crosswise. With double stacking, the effective required blancher

i m CxWxt 10,800 x 3 x 3.5 0_ „, __■ . _ A, _. ,

length = — = — ynQ- = — ~oE — ^ =27 ft. With single trays, the blancher

length should be twice as long.

After the trays leave the blancher they must be stacked on cars. Since the
lifting of loaded and hot trays is hard on the workmen, loading pits or a
mechanical tray stacker similar to the one illustrated in figure 8, are recom-
mended.

28

California Experiment Station Bulletin 698

Boilers.— 'OH- or gas-fired boilers are used to supply steam for blanching,
and sometimes for heating washers, dippers, and peelers. Steam is also used
for cleaning purposes. Because of the relatively short operating season, low
initial cost is more important than high fuel efficiency. In many instances,
reconditioned oil-field-type boilers have been installed. A permit to operate
any boiler carrying over 15 pounds per square inch pressure, with the boiler
subject to inspection, is required by the California Industrial Accident Com-
mission. The purchaser of a used boiler should require that the boiler be in
condition to pass inspection.

Fig. 8. — Mechanical tray stacker in operation. The loaded trays pass from the con-
tinuous cutting line at the extreme left, through the blancher onto the tray stacker,
Avhere they are lifted, carried over to the car being loaded, and then lowered onto
the stack. (Courtesy of John Leonard, Cupertino, Calif.)

The custom of rating boilers in horsepower persists in the industry, although
steam is seldom used for the generation of power in food-processing plants.
The "builder's rating" is based on a unit of 10 square feet of heating surface
per rated boiler horsepower. The rate at which steam is produced in a boiler
is often spoken of in terms of "developed boiler horsepower," or DB hp. A
developed boiler horsepower is equivalent to a heat absorption rate of 33,480
B.t.u. per hour from the furnace. This will convert per hour about 30 pounds
of water into steam under dehydration-plant conditions.

Some modern types of boilers are so designed and built that their perform-
ance will exceed the builder's rating. The oil-field- or locomotive-type boiler,
however, cannot be operated much above its rating without a considerable drop
in efficiency. The horizontal return tubular boiler and the stationary Scotch
boiler, which are used in some plants, will develop about 50 per cent above

Fruit Dehydration : I. Principles and Equipment 29

their rating. Some types of water-tube boilers, installed in large industrial
plants, develop from 250 to 600 per cent of the builder's rating.

The size of boiler required for a blancher can be estimated roughly from the
amount of fruit to be blanched, by use of the following formula :

j^j, ! _ Pounds of fruit per hour x temperature change in °F
Blancher efficiency per cent x 334.8

To use this formula, the blancher efficiency must be estimated. Continuous
straight blanchers with adequate manifolds and end baffles will have efficien-
cies between 30 and 40 per cent. Thus, to blanch fruit at the rate of 1 ton per
hour, heating the fruit from 70° to 190° F, at an efficiency of 30 per cent,

.i i i i u -i i .u 2,000 x (190- 70) .. __ ,.

the developed boiler horsepower must be — —r^ — ., . . , , or 24. Blanching

1 1 30 x 334.8

3 tons per hour, with a horizontal return tubular boiler which will develop
150 per cent of its rating, will then require a rating of — — - — , or 48 rated

-L.D

horsepower. This is equivalent to 480 square feet of heating surface. If the
boiler is to be used for other services, additional capacity must be provided.

The thermal efficiency of a cabinet blancher may range from 46 to 60 per
cent, an indication that it requires less steam than a continuous blancher. The
steam demand, however, is intermittent; consequently, the boiler must be at
least as large as for a continuous blancher.

Fruit Washers. — A shop-built continuous washer has proved satisfactory
for washing apricots. In the use of this washer, the fruit is carried on an end-
less belt between water sprays, over a sorting table, and weighed onto trays
ready for delivery to the cutting shed.

Immersion washers are usually employed for grapes and prunes. In a
washer more commonly used in Europe, the fruit moves through a trough
with a perforated false bottom. Aeration causes the fruits to rub against one
another, with subsequent removal of dirt particles. For prunes, it is advisable
to include in the preparation line an air blower, trash remover, and strong
water sprays to remove adhering leaves, stems, clods, dirt, and other foreign
materials.

After pears are cut — but before the trays are stacked — it is advisable to
pass them under a water spray to remove adhering dirt and evidences of insect
infestation. A hose with nozzle may be used, but more positive results will be
obtained by passing the trays slowly under a battery of fixed spray heads,
arranged to wash both top and bottom. At times the washing has been carried
out after the trays were stacked on the cars. This is an unwise procedure, since
the foreign material removed from one tray is merely deposited on the first
tray below. The area under the spraying equipment and dripping tray stack
on the car should be smooth concrete with suitable drain. A continuous-cut
pear washer is shown in figure 9.

Washers and Materials for Spray -Residue Removal. — Spray-residue re-
moval has been discussed by Allen (1937), Essig and Hoskins (1944), and
Haller, Smith, and Ryall (1935).

30

California Experiment Station Bulletin 698

For the removal of spray residue, the fruit should be passed through a vat
or a spray of acid at 70° F. When an excessive amount of wax has formed, or
when oil sprays have fastened much lead arsenate on the fruit, the use of higher
temperatures or an alkaline wash may be necessary. Most alkaline washes
leave much of the lead and impair the keeping quality of the fruit, although
they do remove arsenic efficiently. In California, pears are nearly always
sprayed with lead arsenate and require thorough washing before drying.
Apples are also sprayed with lead arsenate; they do not need the spray
residue so thoroughly removed as do pears, however, since they are usually
peeled and cored before drying. Washing the fruit in a 0.5 to 1.0 per cent

Fig. 9. — Cut pear washer. The loaded trays are placed on a roller conveyor and passed
into a spray chamber where rows of spray heads, two from above and one from below, wash
the fruit and the bottom of the tray. The conveyor and catch basin are extended to permit
the trays to drain. (Plans through the courtesy of California Packing Corporation, San
Francisco, Calif.; from California Agr. Exp. Sta. Cir. 350.)

solution of hydrochloric acid is a successful method. With simple immersion,
a period as long as 3 minutes may be necessary ; but with power-spray washing
machines, 30 seconds may be sufficient. Removal of the calyx, or blossom end,
and the stem of the pear after washing helps greatly in reducing the spray
residue, which is found chiefly in these areas.

Two general types of washing machines are in common use. Simple dipping
vats have not proved to be so effective as machines of larger capacity, such as
conveyors and dipping vats, spray chambers and damp cloth or rubber wipers.
They may be built from available designs, or they may be purchased pre-
fabricated, as shown in figure 10. Somewhat simpler designs of washing
equipment are being manufactured in local shops in fruit-producing areas
of the state, the local designs usually being adequate. Responsible care in
operation is required to maintain the solutions used at the concentrations and
temperatures necessary for optimum results. In several instances smaller
growers have found it advantageous to make use of large community fruit
washers.

Fruit Dehydration : I. Principles and Equipment

31

Peeling, Pitting, and Cutting Machines. — Where cling peaches are lye-
peeled, a standard peeler and pitter such as is used in canneries may be em-
ployed. Apples are always peeled and cored by machines, which may be either
hand- or power-operated, and are sliced by power-driven machines. A pear-
cutting machine has been evolved and tried in a few drying yards. Six stalls
are provided where girls push the fruit against rotating spindles to remove

lap

Fig. 10. — Fresh-fruit washer and drier for spray-residue removal :
A, section sketch; B, photograph. (Courtesy of the Cutler Manufac-
turing Company, Portland, Ore.; from California Agr. Exp. Sta.
Cir. 350.)

the calyx, then place it on a rope-belt conveyor, or specially constructed
"cupped" conveyor leading over a rotary knife. Unless the fruit is properly
placed on the belt it may be cut diagonally. Four women are required to tray
the fruit as it leaves the knife ; the entire crew handles about 60 boxes of
fruit per hour.

Miscellaneous Small Equipment. — Sharp cutting knives and satisfactory
equipment for sharpening them should be provided. A container for rotten
and cull fruit should be placed conveniently for each cutter; its contents
should be emptied frequently for removal to a dumping place remote from
the drying yard. Pit boxes which clip to the side of the fruit box, or hang from
the side of the cutting table, should be provided for stone fruits. Cutters

32 California Experiment Station Bulletin 698

should be informed that whole fruit dumped on the tray bruises unnecessarily
and tends to smear rotten particles over fruit and tray. Pits must be kept
separate from rotten fruit. They should not be thrown into the fruit box ;
they make the boxes sticky, which, in turn, mars the appearance of the fruit.
A wooden or concrete platform adjacent to the cutting shed should be pro-
vided for the drying of pits.

A satisfactory record for payment of the cutters consists in the use of a
system of numbered cards and a foreman's punch to indicate the number of
boxes cut each day.

Tray and Box Washers. — It is imperative that trays and fruit boxes be kept
clean. Some fruit juice adheres to the wooden surfaces with each handling;
this results in an accumulation of dirt and mold that may contaminate and
injure the appearance of the dried product. Soaking the boxes or trays in a
vat of clear water, or in a heated solution of trisodium phosphate, or in some
other cleansing material, then scrubbing them thoroughly with a stiff bristle
or wire brush, and rinsing in clear water will cleanse satisfactorily. Or a
strong lye solution may be applied by a power orchard sprayer. It should
likewise be vigorously brushed with a wire brush, and then rinsed off. The
strength of the cleansing solution will be determined by the condition of the
trays, and whether or not brushes are used. These alkaline cleansing agents
must be used with care, because they can cause serious skin burns. A con-
tinuous tray washing machine installed in the line is desirable. One such
washer has a rotary fiber brush under which the trays move continuously.
Another type has reciprocating-motion fiber brushes, with interrupted move-
ment of the trays to permit brush operation from the side. The second type has
the advantage of working more closely into the corners than the rotary brush,
and of rubbing with the grain of the wood in the tray bottoms. I £ heated water
is used, 2 operators can clean 3 to 5 trays a minute at a total cost of about
% cents a tray.

To reduce mold growth on trays during off-season storage, they should be
thoroughly washed and dried, then stored under a shed where ventilation
is good.

Skin-checking Equipment . — Grapes and prunes are dipped before deh\ < I pa
tion. The simplest dipper consists of a semicylindrical dump basket, with a
perforated sheet-metal or wire-screen bottom, hinged to one side of the lye
tank. The fruit is poured from the boxes into the heated ]ye solution while
the basket is submerged. After the desired period of immersion, the basket is
raised by a hand-operated lever; the fruit is then discharged upon a shaker
or a chute, which is provided with water sprays and leads to the trays.

For large-scale operations, a power-driven rotary or a conveyor-type
dipper is more suitable. The rotary dipper used for prunes consists of a per-
forated metal cylinder mounted on a horizontal axis and containing a helical
baffle to maintain positive discharge. The lower third of the cylinder is sub-
merged in the lye bath. The prunes are introduced continuously at one side
and discharged at the other. A variable control on the speed of rotation governs
the time of immersion. The conveyor-type dipper has either baskets sus-
pended from chains or a metal belt with cress vanes to carry the fruit. This
type of dipper enters at one end of an elongated lye tank and emerges at the

Fruit Dehydration : I. Principles and Equipment oA

other. The conveyor discharges the fruit on a shaker or a chute leading to
tiie trays, where water sprays remove excess lye.

The lye tanks are usually built of black iron instead of galvanized iron,
because zinc galvanizing dissolves in lye. The vats are generally heated by oil
burners, but gas, wood, coal, or steam may be used. In order to maintain the
temperature of the solution at or near the boiling point, a relatively large
heating surface and ample heat are necessary. The tanks usually hold 100 to
200 gallons or more; this relatively large volume of solution tends to maintain
a more uniform temperature.

The use of a trash screen, before the fruit enters the lye bath, is a material
aid in maintaining a clean dipping solution. It is not usually provided in hand-
operated basket dippers, but is commonly installed on power dippers. All
types of dippers should have a screen between the lye tank and the tray, where
additional debris and the film of hot lye may be removed with sprays of cold
water. This screen is usually mounted as a shaker.

Pricker or needle boards for prunes are sometimes used between the lye
bath and the tray; they are not very satisfactory, however, because they are
difficult to clean and keep in good order. Furthermore, they appear to be of
little value except for use with the thin-skinned varieties of prunes, such as
the Imperial, the dipping of which requires little or no lye.

Prune dippers are often provided with grading screens, which segregate
the large, plump prunes from the small or shrunken ones and permit the trays
for the two grades to be handled separately. This process accomplishes more
uniform drying.

Commercial caustic soda, or sodium hydroxide in flake form, is commonly
used in the preparation of dipping solutions. It quickly absorbs moisture
and carbon dioxide from the air ; therefore, it should be kept in tightly closed
metal containers. The amount in the vat, or concentration of the solution,
varies with the character of the fruit and with other conditions. Usually it is
adjusted to give a uniform checking of the fruit skin to the degree found best
by experience. Overchecking, with consequent loss by excessive bleeding, must
be avoided. The concentrations of lye commonly used range from ]/> to 1 per
cent at a temperature of about 200° F. Some large operators maintain the lye
concentration by allowing a concentrated solution of lye to drip very slowly
into the dipping tank.

Small producers will find it advisable to purchase lye in 1-pound containers ;
larger producers will find purchases in either flake or liquid forms in drums
more economical.

Sulfuring Houses and Burners. — In large driers, as a precaution against
fire, the practice is considered advisable to construct the sulfuring houses as
two or more separate structures, rather than as one continuous building. They
are of diverse design and construction. In a survey, made by Long, Mrak, and
Fisher (1940), very few houses were found adequate. The requirements for
these houses are economy in construction and durability under severe usage.
Their design should promote rapid, uniform sulfuring, and should make them
permanently tight against air infiltration and gas leakage. The compartment
may be of a size to hold 1 or more stacks of trays, but should not contain excess
space beyond the 6-inch clearances for convection circulation of air about the

34 California Experiment Station Bulletin 698

stack and the open ends of the trays. The most positive convection distribution
of sulfur dioxide gas is secured by making the compartment longer than the
tray stack, so that the burner may be placed on the floor between the car and
the door without fire hazard.

For sulfur burners, clean metal pans are recommended; earth pits into
which sulfur is sometimes dumped for burning not only are grossty wasteful
of sulfur, but are also a major factor in the production of poor-quality fruit.
A tin pan 10 inches in diameter and about 3 inches deep makes a very satis-
factory burner for a single-car house. Concrete hearths, of an area equivalent
to that of the pan burners, and shallow enough to permit easy cleaning, are
satisfactory. Insulated, regenerative, or forced-draft burners may be advisa-
ble for burning the poorer grades of sulfur, or sulf urs which do not ignite well
because of contamination.

Care must be taken to prevent the burning rate from becoming so rapid that
some sulfur is carried through the flame and sublimed on the fruit. It is
preferable to use pan burners and sulfur that burns readily. An operator can
test the extent to which a particular lot of sulfur will burn, by igniting a
weighed quantity of the sulfur in a weighed 10-inch pan in an empty, but
closed, sulfuring house. After the sulfur has ceased to burn, the pan should
be removed and weighed with any remaining slag. The percentage of sulfur
burned can then be calculated.

Where the compartment is short and the burner pan must be set in a pit
under the end of the tray stack, a metal baffle sheet may be fastened under the
end of the car for fire protection. Loose baffles laid over the pit are likely to
reduce the opening materially and to restrict the burning rate, with resultant
lower sulfur dioxide gas concentrations.

In a house of sufficiently tight construction to prevent drafts, vents will
be required to provide air for the fire — an inlet of not more than 1x2 inches
beside each track at the base of the door, and outlet holes of 1-inch diameter
located at the top center of both the rear wall and the door. When a 10-inch
diameter pan is used as a burner, these outlet vents provide about 1 square
inch of area to 50 square inches of burner surface. This ratio must be main-
tained and the vent area modified, if the burning area is changed by varying
the pan size or by using more pans — which may be necessary in a large house.

A good grade of refined sulfur is recommended because it is more economical
and less troublesome than cheaper grades. The sulfur should burn completely,
leaving not more than 1 or 2 ounces of residue from the standard 4- or 5-pound
charge per car containing about 1,000 pounds of cut fruit. If an appreciable
amount of slag remains, the cause may be surmised from its color; a clean
yellow color indicates an insufficient air supply, whereas a black residue of
carbonaceous appearance indicates a poor sulfur and one probably contami-
nated by oil or other organic material. Proper handling and storage of the
sulfur supply is essential ; it must be kept dry and must not be permitted to
come in contact with oily surfaces or vapors. It should not be stored in garages,
or near oil tanks. (See Bisson, Allinger, and Young, 1942) .

Maintenance of the sulfuring house is a most important phase of plant man-
agement. To attempt to remodel old houses is poor economy, unless the frame-
work is in good condition. If the structure is on mud sills it should be raised,

Fruit Dehydration : I. Principles and Equipment 35

and a continuous concrete slab for floor and foundation poured. This base
imparts a rigidity to the structure which helps keep it tight, and also holds
the track firmly in position. Top-heavy, vertically sliding doors and swinging
counterbalances for top-hinged doors cause warping and should be replaced
by closely fitted and weatherstripped side-hung doors. All cracks and struc-
tural joints of the interior walls and ceiling, particularly those at the sill and
plate, should be calked with asphalt mastic, with two coats of asphalt paint
applied in a continuous film to seal the structure. All visible leaks other than
vents must be closed. If the interior cannot be sealed readily, it should be lined
with plywood and sealed at all joints with mastic or asphalt-impregnated
felt with sealed edges. For a more extensive discussion of the principles and
practices of sulfuring, the reader is referred to another publication of this
station by Long, Mrak, and Fisher (1940).

As previously indicated, apples may be sulfured in a continuous sulfuring
box or by immersion in a bisulfite solution for a short period of time, either
alone or as a preliminary step to sulfuring with burning sulfur. The immersion
equipment usually consists of a square wood tank with a walker, which moves
the apples through the solution in about 2 minutes. The speed of the walker,
however, should be adjustable. Iron should be avoided in sulfiting equipment.

Sulfur dioxide (S02), or sodium bisulfite (NaHS03) solutions may be used.
Sodium bisulfite is most widely employed, however. It is economical to use,
easy to handle, and has given very satisfactory results. The use of sulfur
dioxide solutions has been limited by the difficulties of controlling concentra-
tion and the unpleasant environment for workers created by escaping gas.

PRINCIPLES RELATING TO DEHYDRATION

The principles underlying dehydration may be considered best with respect
to the design and efficient operation of the dehydrater. The principles of
dehydration have been discussed by Walker, Lewis, et al. (1937), Van Arsdel
(1942), Cruess and Mackinney (1943), Eidt (1938), and Marshall (1942-43).
Pierce (1942) and the Heating, Ventilating Air Conditioning Guide (Ameri-
can Society of Heating & Ventilating Engineers, 1945) contain valuable
production supervision, engineering, and operation data.

The term dehydration as used in this bulletin may be defined as the removal
of water from a product under controlled conditions of air flow, temperature,
and humidity. Although drying in vacuo is being used for certain products,
air is the common medium used in drying fresh fruits.

Process of Evaporation. — Water escapes from exposed wet surfaces by
evaporation. The evaporation reduces the amount of water at the surface
and increases the concentration of vapor in the adjacent space. Evaporation
is a process which absorbs heat energy; consequently, a certain amount of heat
energy known as the latent heat of evaporation must be supplied. This heat
energy is immediately secured from the material composing the surface, and
its loss lowers the temperature of the surface. Any reduction in fruit tem-
perature enables heat energy to flow from the adjacent air by convection, from
other directly visible surfaces by radiation, and from still other surfaces in
direct contact through conduction. In a conventional dehydrater heat energy
is supplied mostly by connection from rapidly moving heated air.

36

California Experiment Station Bulletin 698

The rate at which evaporation proceeds depends upon the difference between
the concentration of vapor at the wet surface and in the adjacent space, and
also upon the resistance offered to the passage of the vapor through a relatively
stagnant layer of air (air-vapor mixture) next to the surface.

The vapor concentration at the wet surface depends upon the nature and
the moisture content of the material, and upon its temperature. For most rapid
evaporation, the surface must be kept at as high a temperature as can be main-

TABLE 6

Kelation between Moisture Content Expressed as T (Parts of

Water per Part of Bone-Dry Matter) and as

Per Cent on the Wet Basis

Per cent
moisture

T

Per cent
moisture

T

Per cent
moisture

T

Per cent
moisture

T

90.0

9.00

84.8

5 58

71.0

2.45

25.0

0.333

89.8

8.80

84.6

5.49

70.0

2 33

24.0

.316

89.6

8.62

84.4

5.41

68.0

2.12

23.0

.300

89.4

8.43

84.2

5.33

66 0

1.94

22.0

.282

89.2

8.26

84.0

5 25

64.0

1.78

21.0

.266

89.0

8.09

83.5

5.06

62.0

1.63

20.0

.250

88.8

7.93

83.0

4.88

60.0

1.50

19.0

.234

88.6

7.77

82.5

4.71

58.0

1.38

18.0

.220

88.4

7.62

82.0

4.56

56.0

1.27

17.0

.205

88.2

7.47

81.5

4.41

54.0

1.17

16.0

.190

88.0

7.33

81.0

4.26

52.0

1.08

15.0

.177

87.8

7.20

80.5

4 13

50.0

1.00

14.0

.163

87.6

7.06

80.0

4.00

48.0

0.92

13.0

.150

87.4

6.94

79.5

3.88

46.0

0.85

12.0

.136

87.2

6.81

79.0

3.76

44.0

0.79

11.0

.124

87.0

6.69

78.5

3.65

42.0

0.73

10.0

111

86.8

6.58

78.0

3.55

40.0

0.67

9.0

.099

86.6

6.46

77.5

3.44

38.0

0.61

8.0

.087

86.4
86.2

6.35

6.25

77.0
76.5

3.35
3.26

36.0
34.0

0.56
0.52

7.0
6.0

.075
.064

86.0

6.14

76.0

3.17

32.0

0.47

5.0

.053

85.8

6.04

75.5

3.08

30.0

0.43

4.0

.042

85.6

5.94

75.0

3.00

29.0

0.41

3.0

.031

85.4

5.85

74.0

2.85

28.0

0.39

2.0

.020

85.2

5.76

73.0

2.70

27.0

0.37

1.0

.010

85.0

5.67

72.0

2.57

26.0

0.35

0.0

0.000

tained without damage to the material. The vapor concentration at the surface
decreases as the moisture content is reduced ; therefore, the evaporation rate
diminishes as the material becomes drier.

The vapor resulting from evaporation must be dispersed as rapidly as it
forms, in order to prevent the concentration in the adjacent space from rising
and interfering with evaporation. Vapor is removed by permitting it to
escape by natural convection, by sweeping it away with an air stream of lower
humidity, or by operating in a vacuum chamber. Most dehydrators employ
a stream of air moved at a rapid rate through the tunnels or cabinets by a fan.

The resistance to the escape of vapor, offered by the stagnant air layer at the
fruit surface, depends upon the thickness of the layer, which is influenced
by the turbulence and the velocity of the air stream. For rapid drying, the air
velocity should be as high as is consistant with economy in fan power.

Air then performs two functions in the conventional dehydrater : it carries

Fruit Dehydration : I. Principles and Equipment 37

away the vapor which is formed ; and it supplies to the material being dried
the heat energy which will be absorbed in evaporation. The air temperature
should be high enough to keep the material at the maximum temperature which
it can sustain without damage. A higher air temperature is permissible when
the material is wet, not only because it is then less sensitive to heat, but also
because it is cooled by rapid evaporation. The air- volume rate (cubic feet per
minute) must be great enough to carr}^ away the vapor without causing too
high a moisture content in the exhaust air. The air velocity must be reasonably
high in order to reduce the resistance to escape of vapor from the evaporating
surface.

These generalizations can be applied effectively in dehydrater design and
operation when physical properties of air-vapor mixtures, characteristics of
the material to be dried, and principles of performance of fans, furnaces, and
different types of dehydraters are known.

Moisture in Fruit. — The moisture content of a substance is usually ex-
pressed by the analyst in percentage by weight on the wet basis, or, in other
words, in grams of water per hundred grams of sample. This method of ex-
pression might give an incorrect impression if it were to be used in describing
the rate of drying, since both the moisture content and the basis on which it
is figured are variable. If, however, the moisture is expressed as moisture ratio,
part of water per part of bone-dry matter (water-free solids), a correct rep-
resentation of the drying rate can be given, since the amount of dry matter
remains constant, whereas the moisture evaporates.

The relation between the moisture content M, percentage on the wet basis,
and the moisture ratio T, the pounds of water per pound of dry matter is

M

T - ztfu\ — tTt- Values of moisture ratio corresponding to given values of mois-
ture content are given in table 6. For example, if the moisture content of
fresh prunes is reported by the analyst to be 70 per cent, the moisture ratio is

70
100-70

or 2.33 pounds of water per pound of dry matter. In this case, 3.33,

(that is, 2.33 + 1), pounds of fresh prunes contain 2.33 pounds of water and
1 pound of dry matter. If these prunes were dehydrated to a moisture content

of 16.7 per cent, the final moisture ratio would be .,Ar. \n _, or 0.20 pounds of

10U— lb. 7

water per pound of dry matter. The amount of water evaporated in the ex-
amples given would be 2.33-0.20, or 2.13 pounds of water per pound of dry
matter. In a typical drying curve, figure 13. page 44, it can be seen that the
drying rate, which is represented correctly by the steepness of the moisture -
ratio curve, is quite different from that shown by the slope of the moisture-
content curve, both plotted against time.

The drying ratio — the pounds of fresh material required to yield a pound of
dried fruit — can be obtained either from the initial and final total solid con-
tents 81 and 82, or the moisture ratios T} and T2, thus

S„ 100 -M2 T, + l

drying ratio

8, 100 -M, T2 + \

38 California Experiment Station Bulletin 698

For the initial and final moisture contents given above, the drying ratio is

100-16.7 w ig g Q . could be found ag2.o' + wMch ig algo 2 7g
100-70 0.20 + 1

The pounds of water which must be evaporated from 1 pound of fresh
material to yield a product of the desired moisture content may be calculated
as follows :

. . M1-M2 T,-To
Pounds of water evaporated per pound of fresh material = _ " = ™ . •

The pounds of water which have been evaporated from the amount of fresh
material needed to obtain 1 pound of dried product of the desired moisture
content can be computed in the following way :
Pounds of water evaporated to

M -M T - T*>

produce 1 pound of dried product = inn_ -J = "^ T '

The moisture content and moisture ratio of the edible portions of certain
fresh fruits that are commonly dried or dehydrated are as follows :

RaZl^!Ure Range of moisture

Fruit content (M)

per cent of water

•atios (T)

Apples 82 to 86 4.56 to 6.14

Apricots 83 to 86 4.88 to 6.14

Grapes 80 to 83 4.00 to 4.88

Peaches 83 to 89 4.88 to 8.09

Pears 82 to 85 4.56 to 5.67

Prunes 70 to 80 2.33 to 4.00

The moisture ratio gives a better indication of the amount of moisture to be
removed than does the moisture content in percentage on the wet basis.

Properties of Air-Vapor Mixtures. — The operator of a dehydrater must
realize that there is a maximum limit to the absolute humidity, vapor density,
or concentration of vapor, expressed as pounds of vapor per cubic foot, that
can exist at any given temperature, as shown in figure 11, A. At the maximum
concentration of vapor, the space in which the vapor occurs is said to be
saturated — that is, if any more vapor were introduced, condensation would
proceed until only the original amount remained.

The dehydrater must be controlled so that saturation will not be approached
at any point lest the drying rate be too slow, and spoilage by microorganisms
occur in unsulfured products. In the early stages of dehydration of sulfured
fruits, excessive losses in sulfur dioxide occur if humidities are high and
drying is prolonged, for, although the air may be saturated with moisture,
it is far from being saturated with sulfur dioxide.

The term relative humidity is used to designate the degree of approach to
saturation with water vapor at any temperature. Relative humidity is defined
as the ratio of the concentration of vapor, for some given condition, to the con-
centration for saturation at the same temperature. Curves representing vapor
concentrations for several relative humidities are also shown in figure 11, A.

Fruit Dehydration : I. Principles and Equipment

39

In certain dehydrater calculations, confusion might arise in dealing with
the pounds of water vapor per cubic foot, because the vapor is mixed with the
air, which changes in density as it passes through the dehydrater. It is then
desirable to use the humidity (pounds of water vapor per pound of air),
which avoids this difficulty, because the weight of the air (weight of air free
of water vapor) remains constant as it passes through the system, unless leaks
occur. The relation between humidity and temperature is shown in figure 11, B,
for several relative humidities.

40

60 80 100 120 140 160 180
TEMPERATURE, °F.
A

40 60 80 100 120 140 160 180
TEMPERATURE, °F.

B

SATURATED
VAPOR

0.10

40 60 80 100 120 140 160 180

TEMPERATURE, °F. (DRY BULB)

C

— i 1 1 ry-B

DRY-BULB TEMP.

/

WET-BULB TEMP °

DEW
POlNT*.n/

REL.^
HUM"*/
o\«/ "

0.10 *
<

0.06

40 60 80 100 120 140 160 180

TEMPERATURE, °R (DRY BULB)

D

O to

Fig. 11. — Temperature charts: A, Kelation between vapor density, or absolute humidity,
and temperature, at several relative humidities; B, relation between temperature and
humidity (pounds of vapor per pound of dry air) at several relative humidities; C, relation
shown in B, with wet-bulb temperature lines added; D, illustration of relations shown by
psychrometric chart.

Measuring Air Conditions. — In order to adjust dehydrater controls for best
operation, the operator must be able to determine the conditions of the air
entering and leaving each unit. This can be done readily by means of a psy-
chrometer, or wet-and-dry-bulb hygrometer. The instrument consists of a pair
of thermometers, one of which has its bulb covered with a porous wick sup-
plied with distilled water. An air velocity of about 600 feet per minute across
the wick is desirable. For measurement in quiet or slowly moving air, hand
instruments are available which permit whirling of the thermometers to
secure the desired speed.

Various combinations of dry- and wet-bulb thermometers are available
for permanent stationary mounting. The dehydrater should be equipped with
such instruments ; they should be properly placed at the air-exit end of the
tunnel, conveniently located behind small glass windows in the tunnel walls.

40 California Experiment Station Bulletin 698

To secure a correct reading, it is extremely important that air at high velocity
should pass over the wet-bulb thermometer. Only distilled or rain water should
be used to fill the reservoir that moistens the wick. Because tap and well waters
contain salts, which accumulate on the wick and interfere with the evaporation
of water, the wet-bulb thermometer will give too high a reading, and this will
result in serious error. An error often occurs in tunnels drying sulfured cut
fruit, because some of the sulfur dioxide is transformed into sulfuric acid ;
this deposits on the wick, digests the fabric, and causes erroneous readings.
When sulfured fruits are being dehydrated, the wicks should be renewed
every 2 weeks.

When the air stream passes over the bulbs, the bare thermometer indicates
merely the temperature of the air (called the dry -bulb temperature, to dis-
tinguish it from the temperature of the wet-wick-covered thermometer). The
air passing over the wet-bulb thermometer is usually not saturated with vapor ;
consequently, evaporation can occur and cool the wick. As the wick gets colder,
the evaporation rate declines, while heat flowing in from the adjacent air
tends to maintain the wick temperature. A balance is soon reached, at which
point the heat flow to the wick is just rapid enough to supply the heat absorbed
in evaporation ; the temperature of the wet bulb remains constant as long as
the wick is wet. In saturated air, no evaporation can occur to cool the bulb,
whereas in moisture-free air maximum cooling results. From the dry- and
wet-bulb readings, the relative humidity and other properties of the air can
be found by reference to appropriate tables or charts.

The Psychrometric Chart. — Charts which show only the relative humidity
corresponding to given values of dry- and wet-bulb temperatures are quite
simple; but a much clearer understanding of the processes which occur in
dehydration can be gained from psychrometric charts which show additional
properties of air-vapor mixtures, drawn so that the changes which occur in
the dehydrater can readily be depicted. A chart which shows the maximum
amounts of moisture that can be removed without approaching too close to
saturation is especially useful.

In the psychrometric chart which seems best adapted to use by dehydrater
operators, the air temperatures (dry -bulb temperatures) are represented on
the horizontal scale, and the humidity (pounds of vapor per pound of dry
air) on the vertical scale. On these coordinates, curves are drawn showing the
relation between the temperature and the humidity for a series of relative
humidities. Figure 11, B thus forms the skeleton of this type of chart," but
it lacks the wet-bulb temperature lines, which are added in figure 11, C. It
will be noted that at saturation, the wet-bulb temperature equals the dry-bulb
temperature, for example, point 0, in figure 11,7). For a low relative humidity,
the wet bulb is much colder than the dry bulb, point X (for 140° F dry bulb
and 100° wet bulb 26 per cent relative humidity).

14 Those interested in a complete chart may purchase an enlarged copy of a chart by H. J.
Garber, reproduced from the Chemical Engineering Catalog, by writing to the Book Depart-
ment, Eeinhold Publishing Corporation, 330 West 42 Street. New York, N. Y. Instead of
relative humidity curves, Garber's chart shows per cent humidity curves. Per cent humidity
is defined as the humidity for a given condition, divided by the humidity for saturation at
the given temperature. It differs only a little from relative humidity, and a conversion chart
is provided.

Fruit Dehydration : I. Principles and Equipment

41

The use of the chart is illustrated in figure 11, D for a typical condition of
a dry-bulb temperature of 140° F and a wet-bulb temperature of 100°. This
air condition is found at point X, the intersection of line AB, which represents
all conditions with a dry-bulb temperature of 140°, and line CD, which repre-
sents all conditions with a wet-bulb temperature of 100°. The point X is just
above the curve EF, a 26 per cent relative-humidity curve.

The humidity for this condition is also illustrated in figure 11, D. All points
on the horizontal line GH passing through X are at the same humidity. The
numerical value is read from the right-hand scale at II, as 0.033 pounds of
vapor per pound of air.

O.IO

180

113.5 125

60 80 IOO 120

TEMPERATURE, °F,

Fig. 12. — Use of psyehrometric chart in dehydration problems.

140 160
DRY BULB

The dew point of an air-vapor mixture is the temperature at which conden-
sation would begin, if the mixture were cooled without change in composition.
This is illustrated in figure 11, D at the point G, which is on the saturation
curve at a temperature of 92° P. During much of the fruit-dehydration season
in California the humidity ranges between 0.008 and 0.010, corresponding
to dew points from 51° to 57°.

The psyehrometric chart is particularly useful because changes in air con-
ditions which occur in the dehydrater are readily represented on it. The three
principal changes are : ( 1 ) heating the air before it is delivered to the tunnel
or cabinet; (2) passing the air across the fruit, where it picks up moisture
and gives up heat; and (3) mixing exit air from the tunnel with fresh air
when recirculation is used.

Keating of air without addition or removal of moisture is illustrated by
line AB on figure 12. Air at 70° F dry-bulb and 60° wet-bulb temperatures,
point A (humidity of 0.0088 pound of vapor per pound of dry air, relative
humidity 55 per cent, dew point 54°) is heated to 165°, point B. At B the
humidity is the same as at A. The wet-bulb temperature is found from the
chart, to be 86.5°, and the relative humidity 4 per cent.

The changes which occur in air as it passes through the trays of fruit can
be found readily from the psyehrometric chart. The vapor from evaporation
mixes with the air and raises the humidity. The air must supply the latent heat
of evaporation, and also the heat for warming the fruit, trays, and trucks

42 California Experiment Station Bulletin 698

and the heat losses through the walls, ceiling, and floor. The evaporation of
a pound of moisture at a typical fruit temperature of 130° F during evapor-
ation requires 1,020 B.t.u. Warming the trays, trucks, and dry matter of
the fruit to the temperature at which they leave the dehydrater, and warm-
ing the moisture which does evaporate to its evaporating temperature, re-
quire in addition only about 80 B.t.u. A pound of nearly dry air provides
about 0.24 B.t.u. for each degree drop in temperature. Thus, a pound of air

can supply the heat energy for evaporating — — j- — — , or 0.000218 pound of

1020 + 80

moisture, for each degree drop in air temperature.

For an illustration on the chart, typical prune dehydrater temperatures
will be chosen, namely, an air temperature of 165° F entering the tunnel, and
125° leaving the tunnel. With this temperature drop of 165°-125°, or 40
degrees, the humidity will rise by 40 times 0.000218 or 0.00872 pounds of
vapor per pound of air. With the initial condition of point B (165° dry bulb,
0.0088 humidity, and 86.5° wet bulb) the final condition will be a dry-bulb
temperature of 125° and a humidity of 0.0088 + 0.00872, or 0.01752. It is
notable that when a point to represent this condition (point C) is placed on
the chart, its wet-bulb temperature is found to be 86.5°.

The above example illustrates a very nearly exact general rule : When air
supplies the heat energy for evaporation, its exit wet-bulb temperature is the
same as its entering wet-bulb temperature. This is a very convenient, albeit
fortuitous, circumstance which greatly simplifies dehydrater design and con-
trol. Lines which represent wet-bulb temperatures on the psychrometric chart
also represent the drop in temperature and simidtaneous rise in humidity of
air passing through the dehydrater. It is thus possible, by following a wet-bulb
line, to estimate the amount of vapor which can be absorbed by air passing
through the dehydrater by the time its relative humidity has risen to a point
where dehydration will be materially retarded. With fruits in a counter-
current tunnel, for example, this point has been found to be a relative
humidity of about 60 per cent.

Calculation of the efficient utilization of heat energy of the air in the tunnel
can be illustrated by using the data from the preceding examples. Line AB,
figure 12, represents heating the air from 70° to 165° F, while line BC repre-
sents the useful drop in temperature of the air which is supplying the energy
for evaporation. The heat energy involved in each of these is proportional

165—125
to the temperature changes of the air. The tunnel efficiency is then or

JLoo — i\j

42 per cent. In this illustration the efficiency was low because, although the
air had to be heated from 70°, it was exhausted at 125°.

When a psychrometric chart is not available, a simple rule of thumb can be
used to estimate the evaporation rate in a dehydrater. As shown above, a pound
of air can pick up 0.000218 pound of vapor for each degree drop in tempera-
ture. In rough calculations, it is often more convenient to use the volume of
the air, which can be more readily measured, than the weight. In the dehy-
drater, a pound of air occupies about 16 cubic feet. Each cubic foot can then

Fruit Dehydration : I. Principles and Equipment 43

pick up — — — — , or 0.0000136 pound of moisture for each degree drop in

temperature. The evaporation rate is approximately equal to 0.0000136, mul-
tiplied by the air flow and the air temperature drop, or
evaporation rate, in pounds of vapor per minute = 0.0000136 x air flow x tem-

cubic ft. per minute x (air temperature drop)
perature drop, or = * — 80000 '

The value of 60,000, sometimes given as the constant in the above equation,
is based upon measurement of the air volume at a temperature of 60° F, which
is seldom applicable to fruit dehydration.

Recirculation. — The psychrometric chart is of great help in considering the
recirculation of air in a dehydrater. With slow-drying fruits, such as prunes
or grapes, air of the usual outside humidity will not reach a practical limit
of its evaporative capacity in a single passage through the trays, unless the
tunnel is unusually long or the velocity unduly low. In the example given on
page 42 for operation without recirculation, fresh air was heated to 165° F
and cooled by evaporation in the tunnel to a typical temperature of 125°,
where it had a final relative humidity of only 20 per cent. The final relative
humidity can be higher, up to 60 per cent, without materially lengthening the
drying time of fruits. By opening the by-pass, or recirculation door, and, if
necessary, partly closing the exhaust door, some of the humid air leaving the
fcrays will pass into the heating chamber and the humidity will rise throughout
the dehydrater. When part of the air is being recirculated, less fresh air has
to be taken in and heated. The recirculated air itself will require less heating
than the fresh air. Thus, a very substantial fuel saving may be realized.

Although to measure the volume of the fresh and recirculated air, or the
temperature of the mixture, is usually inconvenient or impossible, the condi-
tions can be readily determined from the psychrometric chart. For example,
the same initial and final air temperatures. 165° and 125° F, will be taken as
in the preceding example for operation without recirculation.

At the final air temperature of 125° F, with the relative humidity permitted
to rise to 60 per cent, the final condition is represented by point G, figure 12,
where the wet-bulb temperature is found to be 109.5° F and the humidity
0.0543 lb. of vapor per pound of air. Since, in passing over the fruit, the
air conditions must have followed the wet-bulb line, the initial condition must
have been at point E (109.5° wet bulb and 165° dry bulb), where the relative
humidity is found to be 13 per cent, and the humidity 0.0446. The air given
by point E is the mixture of fresh and recirculated air, which has been heated
by the furnace. Before it was heated, its humidity must have been the same
as at E, or 0.0446 ; its temperature before it was heated may be found from
the chart. This can be done because the mixture has been formed from fresh
air at A, and exit air at G.

The rule for mixtures of humid air is : The condition of mixture D, made up
partly of air of condition A, and partly of air of condition G, must lie on the
straight line from A to G. The Quantities of each component are inversely
proportional to the distances of the final condition D from the condition of

44

California Experiment Station Bulletin 698

each component. Instead of distances DA and GD, the horizontal or vertical
projections (KA and JD representing temperature or DK and GJ represent-
ing humidities) may be used.

Since the condition of the mixture lies on the line AG with a humidity of
0.0446, it is represented by the point D. The temperature of the mixture is
found from the chart to be 113.5° F. Thus, in these examples, fresh air must
be heated from 70° to 165°, or 95 degrees when recirculation is not used, while
the mixture resulting from recirculating need be heated only from 113.5° to
165°, or 51.5 degrees. Neglecting the slight differences in heat capacity and

100

8Q_ \ WEIGHT OF SAMPLE (LEFT SCALE — )
' , MOISTURE RATIO (RIGHT SCALE*-)

Fie. 13.-

LJ

<r

D
H

O

2 0

-2.o|

MOISTURE CONTENT^ ~~

(LEFT SCALE-)

Q

.5C0

15

20

I ME,

HOURS

-Chan

test

ge in weight of a sample of prunes during n laboratory dehydration
in a cabinet drier at constant temperature and humidity.

volume, the saving in heat can be estimated from the temperature changes.
Recirculation saves heating from 70° to 113.5°, or 43.5 decrees. The fuel sav-
ing is 43.5 out of 95, or 46 per cent. The tunnel efficiency with recirculation is

165-125

165-113.5

77.8 per cent,

Drying Rates and Drying Times. — The time required for drying is deter-
mined by the amount of moisture to be removed and by the removal rate. In
order that conditions favorable to rapid drying may be economically provided,
the several factors which influence drying rates should be understood.

The drying rates of many commodities, including fruits, have been found
to be very nearly proportional to the moisture ratio, that is, to the pounds of
moisture per pound of dry matter. This relation between drying rate and
moisture ratio is illustrated in figure 13, which shows the weight of a sample
being dried in a cabinet where the air temperature and humidity are kept
constant during the process. It is apparent that the drying rate is high when
the moisture ratio is high, and decreases as the amount of water decreases.
Even if the sample were kept in the cabinet for a prolonged length of time,
the moisture ratio would not drop below a certain value, the equilibrium

Fruit Dehydration : I. Principles and Equipment 45

moisture ratio. The equilibrium moisture ratio depends principally upon the
relative humidity and is influenced to a smaller extent by the temperature.
The moisture that is free to evaporate can be found by subtracting the equili-
brium moisture ratio from the total moisture ratio. Fortunately, fruits are
not dried to low moisture contents; as a consequence, drying can ordinarily
be completed without providing air of extremely low relative humidity. The
drying rates of fruits are almost exactly proportional to the free moisture
ratios, except when moisture ratios below commercial practice are approached.
The drying rate also depends upon the fruit temperature, but, since this
cannot be readily measured in commercial operation, it is more convenient
to refer to the temperature of the air passing around the fruit. For prunes
and raisins, drying rates have been found to be proportional to the fourth
power of the Fahrenheit temperature of the adjacent air. The relative effect
of temperature is as follows :

Temperature, Relative drying

° P rafo

150 0.68

155 0.78

160 0.89

165 1.00

170 1.12

The influence of humidity upon the drying rate is different with materials
of different nature. For commodities of such a nature that freely evaporating
surfaces are maintained while most of the water is being evaporated (diced
or julienne cut, sliced, and shredded vegetables) the rate is roughly propor-
tional to the wet-bulb depression (dry-bulb-wet-bulb difference). The drying
rate of prunes, on the contrary, has been found to be practically independent
of the wet-bulb temperature, if the relative humidity is not above 40 per cent
(if the dry-bulb-wet-bulb difference is not less than about 30 degrees F). At
60 per cent relative humidity, the drying rate of prunes is about two thirds
of the rate at 40 per cent or below. Blanched cut fruits in their behavior are
probably intermediate between vegetables and prunes.

Air velocity affects the drying rate, because the higher the velocity the
thinner the rather stagnant air layer at the fruit surface, and the less the
resistance to escape of vapor and acquisition of heat. Changes in air velocity
have less influence upon the drying rate of slowly drying fruits, such as prunes
and pears, than upon the drying rate of rapidly drying material, such as
steam-blanched cut fruits. The relative effect of velocity is as follows :

Telocity feot
per mi nn to

Relative drying
raff, prunes

Relative drying:
rate, vegetables

300

0.87

0.70

400

0.92

0.80

500

0.97

0.90

600

1.00

1.00

800

1.06

1.15

1,000

1.11

1.30

Small pieces dry more rapidly than do large ones, because the volume is
smaller in proportion to the surface, and because the moisture within the piece
does not have to travel so far to reach the surface.

46

California Experiment Station Bulletin 698

In dehydration, since the air drops in temperature as it passes over the fruit,
drying conditions vary from one end to the other in a commercial dehydrater.
Conditions encountered in a typical counterflow prune tunnel are shown in

!400

100

a.

z>

o

1200

80 r

tr

u

1000

60 0-

sooi

Q

40^

o

2

600

?0 ^

h-"

X

400^

u

<

0*

£

o

z

200

>

oc

6 12 18

TIME. HOURS

12 24 36

DISTANCE FROM START OF TUNNEL, FEET

Fig. 14. — Average air and fruit temperatures, weight of fruit ou a car, and drying
rate, as a car of fruit proceeds through a counterflow dehydrater tunnel.

Fig. 15. — Kelation between air velocity and drying time, with final exhaust air tempera-
ture also given, for counterflow dehydraters of different lengths. Initial air temperature
165° F, wet-bulb temperature not over 105°. Initial prune moisture content 70 per cent,
final prune moisture content, 16.7 per cent. Prune size, dry count of 50 per pound.

figure 14. Most of the moisture is removed in the first few car positions. The
air temperature changes most rapidly along the tunnel where the drying rate
is greatest.

If the air velocity had been greater or the tunnel shorter, the air tempera-
ture at the fresh-fruit end would not have dropped so low, and the drying time

Fruit Dehydration : I. Principles and Equipment 47

would have been shorter. An example of the relation between velocity, tunnel
length, drying time, and final air temperature is given in figure 15, which is
for a counterfiow tunnel drying French prunes with a final size count of 50
per pound.

Fruit Temperatures during Dehydration. — High air temperatures give
rapid drying rates and short drying times. But if the fruit temperature rises
above a certain critical value for more than a very limited time, when the mois-
ture content is low, the quality of the product is impaired and the storage life
is reduced. Critical temperatures depend upon the kind of fruit and upon the
moisture content. Although fruit temperature cannot be readily measured
in commercial dehydraters, it can be estimated from the air temperature and
the drying conditions.

When fruit is first moved into the relatively cool, humid end of a counter-
flow tunnel, its temperature rises rapidly until it is slightly above the wet-bulb
temperature. Because of the high moisture content of the fruit at this point,
the evaporation rate is fairly high if the relative humidity is not excessive.
The fruit, therefore, remains considerably cooler than the air, until its mois-
ture content has been materially reduced. As the car is moved through the
tunnel, the air temperature is higher at each successive car position. When
the fruit decomes drier, the evaporation rate becomes slower and, finally, the
fruit temperature approaches that of the air. Typical temperatures in a
counterfiow tunnel are shown in figure 14. Special care must be taken to avoid
exceeding the critical temperature near the end of dehydration, and to remove
the fruit before its moisture content drops so low that its susceptibility to heat
damage materially increases. Maximum air temperatures for typical counter-
flow tunnel operation are given as follows :

Maximum recommended
Product air temperature, °F

Apricots 155*

Freestone peaches 155*

Clingstone peaches 160*

Nectarines 155*

Pears 140*

Golden-bleached raisins 150

Black Mission figs 140

Prunes 165

* These figures to be used only if the fruit is dried to a moisture content not below 25 per cent.

In two-stage dehydration, the air temperature at the hot end of the primary
parallel flow tunnel can be considerably higher than the values given above.
The fruit is much cooler than the air, because of the high evaporation rate.
Moreover, because of its high moisture content, it is not so susceptible to heat
damage.

Air Flow. — Air is drawn in by the fan and then forced through the tunnel
where it passes between the trays, over and around the fruit, and out of the
tunnel. In some designs the furnace is installed before, in others after, the fan.
As the air flows through this circuit, pressure losses result from changes in
air velocity and direction, and from friction by contact with the sides of the
tunnel, travs, and fruit.

48

California Experiment Station Bulletin 698

In dealing- with air flow, dehydrater performance, and friction losses, to
attempt measurement of the actual velocities at all points adjacent to the
fruit on the trays within the tunnel is seldom feasible. It is more practicable
to measure the total air flow through the tunnel, bearing in mind that, with a
given total air flow, the performance will vary with the tray spacing, tray
loading*, and side and top clearances. The air velocity over the fruit will be
consistently higher than that in the free tunnel cross section.

Air velocities can be measured by the pinwheel anemometer, the balanced-
vane impact instrument called the velometer, or the impact tip called the Pitot
tube, which is read with a manometer or diaphragm chamber (fig. 16). In
using- any of these instruments to find regions where the flow is uniform
enough to give steady and representative readings is sometimes difficult.

Fig. 1(5. — Instruments for measuring air velocities.

For measuring the total volume rate of flow (cubic feet per minute passing
through the tunnel) the pinwheel anemometer lias the advantage of giving
an average reading. The recommended procedure is to move the instrument
at a uniform, slow speed, in a traverse which will cover the entire cross section
of the tunnel, at a point at least 2 feet from the downstream face of the last
truck. Care must be taken to avoid speeding up the traverse, or dwelling in
regions of high or low velocity, particularly at the walls or ceiling. The total
air flow is found by multiplying the average velocity so obtained by the cross-
sectional area of the tunnel. To investigate the uniformity of flow, the area
can be divided into sections which can be traversed separately.

For taking instantaneous readings at particular points, the velometer is
useful. Variations of velocity across the tunnel and pulsations of Aoav make
the averaging of results difficult, but the instrument is helpful in exploring
quickly the uniformity of flow.

Since a Pitot tube shows a differential of only a/j (5 inch of water at a velocity
of 1,000 feet per minute, it is not well adapted for dehydrater-tunnel measure-
ments.

Air Pressures and Friction Losses. — The air pressures which are developed
in dehydraters are only a fraction of a pound per square inch. Conventional

Fruit Dehydration : I. Principles and Equipment

49

pressure gauges are not suitable for measuring these low pressure differences.
Installation of a manometer to measure the pressure within a duct (the pres-
sure difference between the inside and outside of the duct) is shown at A in
figure 17. The pressure connection must be made carefully at the duct wall,
witli the face of the tube opening parellel to the direction of air flow, and free
from any projections or burrs. With a vertical water-filled manometer, the
pressure is read from the difference in the levels in the two legs of the tube,
as inches of water. Since a column of water 1 inch square and 1 inch high

AIR FLOW

IMPACT TIP OPENING

Id
tc

D

—

U 1

5 *

h

0.

■

</>
<o
u
cc
a.

o

<

<\

V)

y

o ■

h

W

WALL TAP FOR

IMPACT TIP FOR

STATIC PRESSURE

"TOTAL PRESSURE

M

EASUR

EME

NT

MEASl

JREI^

4ENT

PITOT TUBE FOR „
VELOCITY "PRESSURE
MEASUREMENT

ABC
Fig. 17. — Installation of tubes for measuring air pressure.

weighs 0.0361 pound, an inch of water is equal to 0.0361 pound per square
inch. In the measurement of air flow, it is customary to express pressures
directly in inches of water rather than in pounds per square inch.

Energy is required to put air in motion, that is, to give it velocity. It is con-
venient to express the velocity of air in feet per minute, and the velocity or
kinetic energy in terms of the equivalent pressure in inches of water. The
energy related to velocity is

v \4,005/ 0.075

in which

Pv - Pressure equivalent of velocity energy, inches of water

V = Average velocity, feet per minute

D = Density of air, pounds per cubic foot.
For example, when air of a density of 0.06 pound per cubic foot is drawn
through an opening, where it acquires a velocity of 2,000 feet per minute, the

velocity energy becomes f _ — — ) v ttt^t^, or 0.20 inch of water.

\4,005/ 0.0 /o

50 California Experiment Station Bulletin 698

Velocity energy is dissipated by turbulence resulting from sudden changes
in direction of air flow, and by changes in the size of the duct system through
which the air flows. Sharp turns and restrictions in the dehydrater should be
avoided as much as possible. The performance of some poorly designed dehy-
draters could be considerably improved by increasing the area of air supply
and exhaust openings.

Friction losses are unavoidably incurred in moving air. The losses are pro-
portional to the square of the velocity, and to the ratio of the area of the
surfaces contacted to the cross-sectional area of the passageway. Friction also
depends upon the roughness of the surfaces. In the tunnel, where additional
losses are caused by the continual change in velocity and direction as the air
passes over and around the fruit, the pressure drop will vary greatly with
the product and the tray loading. When trays are overloaded, so that the free
space between them is reduced, the area through which air can flow is greatly
restricted, and the air flow reduced.

Dehydrater-tunnel friction factors can be conveniently used when expressed
as average pressure drop in inches of water per foot of tunnel length for an
average air velocity of 600 feet per minute in the whole tunnel cross section.
Friction factors for eounterflow tunnels, based on observations by Moses,
Guillou, Lorenzen, and Perry15 are as follows :

Type of load Friction factor

Empty trays 0.01

Prunes, normal tunnel, 3 lb./sq. ft 0.02

Peaches, normal tunnel, 2% lb./sq. ft 0.018

Grapes, normal tunnel, 3% lb./sq. ft 0.022

Grapes, all green, 3% lb./sq. ft 0.040

Grapes, all green, 5 lb./sq. ft 0.065

Grapes, normal tunnel, 5 lb./sq. ft. . . . 0.025

The pressure drop through cars of fruit which is nearly dry, is not so great
as through cars of fresh fruit. When it shrinks, fruit leaves more free space
for air flow. The values in this table, designated for "normal tunnel," repre-
sent the average drop throughout a tunnel with the usual tray spacing, where
fruit is fresh at one end and nearly dry at the other. For other operating condi-
tions, appropriate friction factors would have to be found by tests. In using
these factors,

Pressure drop, in inches of water = fxLx

(-Y

\600/ '

In this expression,

/ = friction factor, inches of water per foot of tunnel length, at a velocity of

600 feet per minute
L = tunnel length occupied by cars of fruit, in feet
V = average velocity in free cross section, in feet per minute
For example, the friction loss is to be found in a 12-car (36-foot) prune
tunnel, which is 75 x 77 inches (40 square feet) in cross section, with an air
flow of 30,000 cubic feet per minute. The average velocity from this data is
15 Unpublished data.

Fruit Dehydration : I. Principles and Equipment 51

: ' , or 750 feet per minute. The pressure drop is then 0.02 x 36 x ( — — )

or 1.14 inches of water.

Power Required to Move Air. — The power required by a fan is determined
by the capacity, or air-volume rate in cubic feet per minute, the difference
in air pressure between the inlet and discharge, and the fan efficiency. Al-
though the pressures encountered seem very low, the energy for moving air to
overcome them is by no means negligible. The power required by a fan is given
as follows :

Fan input horsepower = tt^-tt; — —

63.56 xE

in which

Q = air-volume rate, in cubic feet per minute

P = total pressure difference, including static and velocity pressure in inches
of water

E = fan efficiency, in per cent
Thus a fan of 60 per cent efficiency, delivering 48,000 cubic feet per minute,
against a static pressure of 1.25 inches and giving the air a velocity energy of

/ 1 25 + 0 5 \

0.5 inch, will absorb 48,000 x ( ' — j— V or 22 horsepower.

/ 1.25 + 0.5 \
\63.56 x 60/

DRYING EQUIPMENT

The many different types of equipment used for drying foods have been
described and illustrated by Eosseau (1939). Only a few of these, however,
are suitable for fruit. Whole fruits and cut halves are usually dried at at-
mospheric pressure on trays placed in a forced air stream which provides the
necessary heat energy and carries off the moisture. Sliced apples are also dried
at atmospheric pressure, but in an air stream which moves upward through
a bed of material by natural or forced convection. Other types of driers are
used for special purposes.

Evaporaters. — Apples are customarily dried in natural-draft evaporaters,
or occasionally in more modern dehydraters. Drying in old-style evaporaters
is slow and inefficient, and does not yield a uniform product ; in recent years,
however, material improvement has been effected by placing a propellor fan
operated by a small motor in the roof vent of the natural-draft evaporater.
These fans draw the air upward from the heater through layers of the fruit
being dried on slatted floors, and force it out of the vents (fig. 18) . The drying
time may be decreased by one fourth, and the uniformity of the product
greatly improved; however, it is still necessary to turn over the floor charge
with shovels periodically during drying.

Some rather unsatisfactory attempts have been made to dry prunes in
natural-draft kilns and rotary bake-oven driers. The natural-draft Oregon
tunnel evaporater has found wide use in Oregon, because of the wet drying
season. In recent years the performance of these has been materially improved
by installation of fans, although they are not so efficient or economical as
regular countercurrent-tunnel dehydraters.

52

California Experiment Station Bulletin 698

Dehydraters. — The construction and operation of fruit dehydraters has
been discussed in detail by Christie (1926), Nichols, Powers, et al. (1925),
and Guillou and Moses (1943). Christie and Ridley (1923), Eidt (1938),
Guillou (1942), and Van Arsdel (1942) have treated dehydrater design. A
dehydrater which is built with roughly equal dimensions of length, width,
and height is usually spoken of as a cabinet, or compartment, dehydrater. A
dehydrater which is very long in proportion to width and height is called

a tunnel.

Although forced draft and recirculation are used in most tunnel dehy-
draters, the methods of heating and conducting air flow through the plants
vary considerably with the dehydration units designed by different engineers.

Fig. 18. — A slat-bottom kiln floor, partially loaded with freshly sliced apple rings.
The air is drawn upward by a fan through the narrow openings between the slats.

The air to be passed over the drying fruit is heated by direct heat, direct
radiation, or indirect radiation. By direct heat is meant the direct mixing of
the products of combustion and the air without the intervention of furnace
walls or flues. This can only be done by use of a fuel whose combustion products
do not damage the fruit ; natural gas is such a fuel. Oil is used in some plants,
but a careless operator may easily damage a charge of fruit. It is impossible
to use coal or wood in a direct-heat system. The advantages of direct heat are
high fuel efficiency, 90 to 100 per cent furnace and 36 to 50 per cent over-all
efficiency, low cost of installation and upkeep, and instantaneous temperature
regulation through the absence of stored heat. The disadvantages of direct
heat are the possible contamination of fruit by products of incomplete com-
bustion and the necessity of a high-grade fuel.

Direct radiation refers to the systems in which heat from burning fuel is
transferred through the furnace walls, flues, or radiators to the dehydrater
air. This system offers the advantage of cleanliness and permits the use of any

Fruit Dehydration : I. Principles and Equipment

53

kind of fuel. Since the furnace efficiency usually ranges from 60 to 90 per
cent, the over-all efficiency of direct-radiation systems will be between 25
and 45 per cent. The disadvantages of direct radiation are greater cost of in-
stallation and upkeep, lack of instantaneous temperature regulation because
of temporary heat storage, and lower fuel efficiency from stack losses.

Indirect radiation by means of steam or hot-water radiators is seldom used
in California, where inexpensive, clean-burning fuels are available. The ad-
vantages of such a system are automatic regulation, freedom in choice of fuel
used, and distribution of radiators where desired. The great disadvantages
of indirect radiation are its expensive installation and upkeep and its low
fuel efficiency. The furnace efficiency usually ranges from 60 to 70 per cent,
and the over-all efficiency from 24 to 35 per cent.

TABLE 7
Heating Values and Prices*

Fue

Approximate
heating
values

Comparison

based on

Diesel oil

as unit

Prices

Relative cost

based on

Diesel oil

as unit

B.t.u.
133,000/gallon
103.000/gallon

1,103/cu. ft.
3,412/kw.h.

1 gallon
1.29 gallon
121 cu. ft.
39 kw.h.

cents

6.25/gallon
6.00/gallon
50.00/1000 cu. ft.
1.5/kw.h.

1,00

Butane

Natural gas

Electricity

1.24
0.97
9.35

* After B. D. Moses, unpublished data.

Cost data on fuels are given in table 7. The efficiency of the several dehy-
drater types as given above must be considered in applying the relative cost
data. Prices may vary from time to time in different regions.

The countercurrent, cross-flow, and center-inlet systems of air flow are most
commonly used today, and the two-stage, parallel-counter system is being tried
for drying grapes. Cabinet driers are occasionally used for batch operation.

Countercurrent Dehyd raters. — Countercurrent tunnel dehydraters are the
most widely used for fruits. Movement of cars of fruit through the tunnel is
in opposite direction to the flow of air. The tunnel is generally about 6% feet
wide, 7 feet high, and 40 to 60 feet long. Ordinarily, the tunnel length is suffi-
cient to accommodate from 12 to 14 cars each holding from 25 to 28 trays,
6x3 feet, or from 50 to 56 trays, 3x3 feet. When the latter are used, 2 tray
stacks are made on each car. The use of double cars, each holding two stacks
of 6 x 3 feet trays, is not recommended ; the product dries too unevenly and
the weight of the cars makes them too difficult to handle. The loaded cars enter
at the cool- and moist-air exhaust end of the tunnel and move forward toward
the hot end at 1- to 2-hour intervals, their movement being dependent on the
product and the rate at which it dries.

Air is heated in a chamber adjacent to the drying tunnel. The heating
chamber is usually located at one side of or above the drying tunnel. Air is
drawn from the heating chamber and forced through the drying tunnel. A
considerable quantity of the air may be recirculated.

Countercurrent tunnel dehydraters are satisfactory for slow-drying prod-

54

California Experiment Station Bulletin 698

ucts, such as prunes, grapes, pears, and peaches. If they are used for apples,
however, the tunnel load must be decreased, or the air flow and heat supplied
in the drying tunnel must be increased. A typical countercurrent tunnel dehy-
drater is illustrated in figure 19.

Cross-flow Dehydraters. — In the cross-flow, or combination compartment
and tunnel, arrangement is such that air flow is transverse to the axis of the
tunnel.

Since the air can be reheated in the side chambers, or ducts, after a short
passage through the trays, operation is possible at a low air velocity without

Fig. 19. — Double-tunnel, counterflow, direct oil-fired dehydrater with centrifugal fan :
E, exhaust air; F, fresh air; G, gases heated in furnace; E, hot air delivered by fan to
tunnels ; M, mixture of fresh air, gases heated in furnace, and recirculated air entering the
fan; P, primary fresh air for initiating combustion in the furnace; B, recirculated air;
S, secondary fresh air for completing combustion in the furnace.

an excessive temperature drop. This possibility, which would permit good
performance with low fan power, has not always been realized in practice.
Some of the older and larger units have been quite difficult to control.

Recently Guillou and Moses (1943) presented plans, construction details,
and operating instructions for a modified type of cross-flow dehydrater. It is
inexpensive to construct and efficient to operate. The general plan is so flexible
that the possible size range is wider than for many other designs. Units already
in existence vary in size from one built to accommodate the fruit from a 6-acre
prune orchard to another to care for the fruit from a 60-acre orchard.

The Guillou and Moses dehydrater is shown in figure 20. Heated air is drawn
from a furnace room by a fan and is forced into a chamber, from which it flows
across trays of fruit into an air duct. From the air duct the air is reversed
and flows across the trays of fruit of a higher moisture content, a controlled
portion being discharged through a door, while the remainder returns to the
furnace room. The draft of the fan draws fresh air partly through the furnace
and partly through openings adjacent to the furnace.

As will be noted in figure 20, a large volume of air passes along a short path
at a high velocity through the wetter fruit and at a low velocity through the

Fruit Dehydration : I. Principles and Equipment 55

fruit that is partially dried. According to Guillou and Moses, this type of air
movement provides the following advantages :

1. The moderate air pressure required to move the air along a short path
through the fruit may be provided by an inexpensive fan and a small motor.
Units up to 8 tons per day green capacity can use a 5-horsepower motor. A
single-phase motor of this size, or smaller, may usually be connected to the
same electric meter as farm or household heating equipment, the user paying
at the heating rate for the energy used by the motor.

Fig. 20. — Cross-flow dehydrater, with axial-flow fan: 1, first position of fruit, when
fresh ; 2, second position of fruit, partially dried ; S, third position of fruit, nearly dried ;
E, exhaust air, escaping out of adjustable door; F, fresh air, entering beside furnace; G,
gases heated in furnace ; H, hot air delivered by fan to fruit in second and third positions ;
/, intermediate air passing from fruit in second and third positions to that in the first posi-
tion ; M, mixture of fresh air, gases heated in the furnace, and recirculated air entering the
fan; P, primary fresh air for initiating combustion in the furnace; R, recirculated air
passing from the fruit in the first position to the furnace chamber ; S, secondary fresh air
for completing combustion in the furnace.

2. The temperature drop through the charge is small. Heat is supplied
rapidly to the fresh fruit by air moving at high velocity, and more slowly to
the partly dried fruit by air moving at low velocity. This arrangement gives
satisfactory drying time, with a moderate finishing temperature, and lessens
the danger of caramelization and scorching. By slow, final drying, it reduces
the differences in dryness caused by fruit of different sizes or of different
initial moisture content.

3. One third of the fruit in the dehydrater may be changed at a time, the
plant operating unattended between changes.

56

California Experiment Station Bulletin 698

4. The fruit advancing through the dehydrater is subjected to a reversal of
air flow, which tends to equalize drying.

Center-Inlet Dehydraters. — In the center-inlet tunnel the air flow is so ar-
ranged that the highest air temperature is at the center of the drying tunnel.
The tunnel may be straight, with the air flowing toward the ends from the
center, or it may be U-shaped, with the air flowing from the bend in the U
toward each end. In either case the fruit enters at a lowrer temperature and
moves toward the hotter air at the center, reaching it after about half of the
drying period is completed. Then the fruit moves past the center and finishes
drying in air of progressively decreasing temperature. Since a good deal of
the moisture is removed in the first section of the tunnel, the temperature drop
in the second section is not very great. This arrangement was probably devel-
oped in an effort to avoid what was believed to be retardation of drying caused
by case hardening. Although the system has been used rather extensively, it

Material In

DDD

PARALLEL-FLOW SECTION

:ddddd

COUNTERFLOW SECTION

Fig. 21. — Two-stage dehydrater, consisting of two separate sections.
(Courtesy W. B. Van Arsdel, Food Industries, October, 1942.)

is more difficult to control than the simpler count emu-rent type. Great care
must be taken to avoid overheating in the center section ; and drying may be
found to be extremely slow toward the end.

Center-Exhaust or Tivo-Stage Dehydraters. — The center-exhaust system is
the reverse of the center inlet. Heated air enters at the two ends and circulates
toward the outlet, which is usually located about one third of the tunnel length
from the place where fresh cars enter. Moist material moves in the same direc-
tion as the air flow at the wet end and counter to the air flow at the dry end
(see fig. 21). This dehydrater is particularly suitable for dehydrating fast-
drying products, such as apples and blanched cut fruit. The rate of drying is
faster than in a countercurrent tunnel and, according to Eidt (1938), the
quality of apples dried in a two-stage dehydrater is superior to that of apples
dehydrated in a countercurrent tunnel or an evaporater. The construction and
operation of these two-stage dehydraters has been discussed by Eidt (1938).
In some instances two tunnels are used as two-stage driers. The two-stage
system has been tried satisfactorily with grapes in a few plants.

Cabinet Dehydraters. — While the tunnel is best suited to continuous opera-
tion, the cabinet can be adapted to batch operation. It is thus used for inter-
mittent operations in small plants. For the best quality of product, capacity,
and economy, the air temperature and the amount of recirculation must be
adjusted as drying proceeds. Beavens (1944) has given details of construction
and operation of cabinet dehydraters. The method is less efficient and requires
more labor than a tunnel dehydrater of equal capacity.

Conveyor Dehydraters. — In this drier, an endless belt is used to carry the
fruit through the tunnel. The advantage is continuous operation and low labor

Fruit Dehydration : I. Principles and Equipment 57

cost, but the disadvantages are high cost of installation and limitation to rela-
tively dry products. Conveyer driers are used to reduce the moisture content
of sun-dried raisins sufficiently before cap-stemming-. In at least one instance
a conveyor drier was used effectively to complete the drying of golden-bleached
and Valencia raisins after their removal from a tunnel dehydrater ; this addi-
tional operation reduced the time required in the tunnel dehydrater.

Vacuum Dehydraters. — These driers, described by Havighorst (1944) are
used commercially only for reducing the moisture content of dried fruits to a

Fig. 22. — Vacuum tray dehydrater. (Courtesy of
Vacudri Corporation, Emeryville, Calif.)

lower level (1 to 3 per cent) in the preparation of "nuggets," "powders," and
"ribbons." Various designs and arrangements are in use. Installation costs
are high and experience in operation is necessary in order to secure the most
satisfactory results. One design of vacuum drier used for preparing fruit nug-
gets is shown in figure 22.

Drum Driers. — Drum driers are used to convert cranberry pulp into flakes.
The driers may be single or double drums, operated under atmospheric pres-
sure or vacuum. The single drum, atmospheric type, is used for cranberries.
Fruit puree of a definite consistency is spread uniformly on the outer surface
of a revolving steam-heated drum ; the speed of rotation of the drum is such
that the product is dried by the time the adhering film has moved to position
for removal.

Furnaces. — For direct-heat dehydraters, the furnace must be carefully
designed and installed, especially if oil is to be burned. For gas, there are

58 California Experiment Station Bulletin 698

several types of burners which can fire directly into the heating chamber (see
fig. 23 ) . However, a refractory firebox partially surrounding the burner helps
to maintain good combustion with a low flame.

Oil burners should be placed to fire into firebrick-lined tubes with two
checkerbrick baffles, as shown in figure 24. Some builders, although skilled in
erecting furnaces for boilers, have installed units which are not suited to
dehydraters.

The furnace tube for an oil burner must be large enough to permit complete
combustion within, but not so large that its temperature is too low with a light

Fig. 23. — Full-view picture of the end of the furnace showing the automatic
gas burners in operation and the other automatic controls on the outside of the
heat chamber which operates them. (Courtesy of the Fresno Dehydrating

Company.)

fire. From y5 to % of a gallon of oil can be burned per hour, per cubic foot of
furnace volume. A 36-inch diameter tube can be used for 20 gallons per hour,
while a 60-inch tube is needed for 50 gallons per hour. The length of tube up
to the first baffle should follow the recommendations of the burner manufac-
turer. The first baffle insures a hot firebox with a low flame, whereas the second
baffle provides a hot refractory -lined space in which combustion is completed
with a high flame. To avoid undue restriction of air flow through the furnace,
the baffles must be built as open as possible without sacrificing their strength.
The flow of air must be great enough to keep the furnace temperature below
the softening point of the firebrick. In boiler furnaces, or indirect-fired dehy-
draters, where a large part of the heat released by combustion is absorbed by
relatively cold heat-exchange surfaces, maximum efficiency is secured with

Fruit Dehydration : I. Principles and Equipment

59

an air supply only large enough to give complete combustion — about 15 per
cent above the theoretical amount for gas or oil. In a dehydrater with direct
heat, however, the furnace gases warm the dehydrater air by mixing with it,
and dehydrater efficiency is not reduced by having a considerable fraction of
the fresh air enter through the furnace. Moreover, the furnace, transferring

Fig. 24. — Construction of a terminal eheckerbrick baffle in a furnace tube for a direct-
heat oil burner. (Courtesy of the Fresno Dehydrating Company.)

practically no heat by radiation to cold exchange surfaces, would quickly rise
above the softening point of the brick if only 15 per cent excess air were pro-
vided. The air supply must be sufficient to dilute the burned gases enough to
keep the temperature at the maximum firing rate below 2,500° F in furnaces
built of high-grade firebrick. This requires about 2,700 cubic feet of air per
gallon of oil, nearly twice the requirement for a boiler furnace. The secondary
air openings through the furnace i'ront must be ample, because the furnace
should operate with a low draft in order to economize on dehydrater fan power.

60

California Experiment Station Bulletin 698

Fans. — For any design of fan, there is theoretically a particular size and
speed which will result in operation at the maximum efficiency inherent in the
design for each given combination of static pressure and volume. If there
were no limitation of size, speed, or initial cost, the most efficient type of fan
would be selected. In practice, however, it is found that certain types of fans
are more suitable than others for tunnel dehydraters.

Until recently, centrifugal fans of the backward-curved multiblade type
(fig. 25) were most widely used. With improvement in design, and a trend

Fig. 25. — A single-width, single-inlet, centrifugal fan
with backward-curved blades. (Courtesy of the Western
Blower Company.)

toward shorter tunnels, axial-flow pressure fans (fig. 26) have been installed
with success in some modern dehydraters. The peak efficiencies of some of the
centrifugal fans are higher than those of most axial-flow fans. Because of
their greater initial cost and their bulk, however, somewhat smaller than
optimum sizes of centrifugal units have in some instances been installed for
short seasonal operation, with the result that the maximum efficiency has not
always been realized. When the static pressure to be overcome has been over-
estimated, the fans have been connected to operate at an unnecessarily high
speed, and the power required has been excessive.

Three types of axial-flow fans are now in use : flat, wide-blade propellers,
aerofoil-section narrow-blade propellers, and air screws in which the projected
area of the blades covers a large proportion of the fan circle. The wide-blade

Fruit Dehydration : I. Principles and Equipment

61

air screw is relatively efficient and quiet, but highest in initial cost. Some
aerofoil propellers are efficient, but noisy at high speeds. The flat, wide-blade
propeller is likely to have a lower efficiency than the other two, but is lowest
in initial cost. A large-diameter hub is an aid to securing high efficiency in most
designs of axial-flow fans intended for operation against appreciable static
pressures.

The fan must be able to deliver the required volume of air, in cubic feet per
minute, against the static pressure caused by the friction and turbulent losses

Fig. 26. — Air chamber and axial-flow fan. (Courtesy of the Fresno
Dehydrating Company.)

of the heating chamber, tunnel, and air openings. Counterflow-tunnel friction
losses, based on the friction factors shown on page 49 are illustrated by
the data presented in table 8. This table is based on an air velocity of 600 feet
per minute in a 12-car (36-foot working length) tunnel, with the usual tray
design. The designation "normal tunnel" indicates that the fruit has the usual
range of size on the trays corresponding to the progressive change from fresh
to dried fruit. Higher velocities may be desired in dehydraters intended to
handle those cut fruits that dry rapidly, and they will entail greater friction
losses than those indicated in table 8. In addition to the friction loss in the
tunnel, the pressure drop in the heating chamber and in the turns entering
and leaving the tunnel will be about % inch of water. With inlet air openings
which permit an air velocity of say 2,000 feet per minute (24 square feet for
the 48,000 cubic feet per minute supplied to a double tunnel), an additional
pressure of V4 inch of water is required to bring the air up to the speed with
which it enters the unit. Thus, the fan must be able to operate against a static

62

California Experiment Station Bulletin 698

pressure at least Y2 inch greater than the friction loss in the tunnel itself. The
fan horsepower required to deliver 24,000 cubic feet per minute against the
static pressures listed, is also given in table 8, based on a fan static efficiency
of 50 per cent. In a fixed installation, if a fan were selected to operate against
the highest pressure shown in the table, its power requirements would be
greater than if it were selected to operate against the lower static pressures.
The operating characteristics of a typical centrifugal fan are illustrated in
figure 27 ; those of an axial-flow fan in figure 28. In these figures, the relation
between the static pressure encountered and the volume to be delivered is
given for a moderate fan speed by curve 1, and for a high fan speed by curve 2.
The resistance offered by the dehydrater is given by curve 3 for a tunnel with
normally loaded cars of prunes, and by curve 4 for a tunnel with heavily

TABLE 8

Air Friction and Fan Operating Pressures and Horsepower in a Twelve-car Tunnel
at (i00 Feet per Minute Air Velocity*

Commodity, tray leading, and condition

Prunes — 3 lb./sq. ft., normal tunnel

Thompson Seedless grapes — 3XA lb./sq. ft., all green

Thompson Seedless grapes — 3J^ lb./sq. ft., normal tunnel
Thompson Seedless grapes — 3H lb./sq. ft., all dry

Tunnel

friction loss

(inches

of water)

Fan static
pressure
(inches

of water)

inches
0.72
1.44
0.80

0.40

inches
1.22
1.94
1 . 30
0 90

Fan

horsepower

in a
single tunnel

hp.
9.2
14.0

9.8
6.8

* Based on 24.000 cu. ft. per minute and a fan efficiency of 50 per cent.

loaded grape cars. With moderate speed and normal resistance, the static
pressure encountered and the volume delivered are shown by point A, and the
power required is indicated by point a. Point B gives the volume and static
pressure for moderate speed and high dehydrater resistance, and point b gives
the power required for this condition. With high fan speeds, the correspond-
ing pressures and volumes are given by points D and C, the power require-
ments are indicated by points d and c. It can be seen that if the fan were
selected to deliver air against an anticipated high pressure, point C, but in-
stead encountered only the lower resistance, the actual operation would be
given by point I). If only the volume represented by point C were needed,
the power could be reduced from that of point c to point a, by reducing the fan
speed to give the operation at point A.

Tunnel-Dehydrater Operation. — The drying characteristics of most fruits
which are dehydrated are known only in a rather general way. Each operator
must ascertain the conditions of his own plant which will give the best combi-
nation of high quality, capacity, and efficiency. For a particular plant, the
conditions may differ somewhat from general recommendations.

For a given tunnel length, initial air temperature, and air velocity, the time
of drying and final air temperature are fixed by the characteristics of the
material being dried. This is illustrated for prunes in figure 15. With a slow-
drying material, the average temperature drop per foot of tunnel length is
small, so that long tunnels can be used. If slow-drying materials are to be dried

Fruit Dehydration : I. Principles and Equipment

63

(f) STATIC PRESSURE OF
FAN AT HIGH SPEED

0 STATIC PRESSURE OF FAN
AT MODERATE SPEED ^

® RESISTANCE OF

TUNNEL WITH 12

- HEAVILY LOADED

CARS OF GRAPES

(D RESISTANCE OF TUNNEL
WITH 12 NORMALLY LOADED
CARS OF PRUNES OR GRAPES

I

(g) POWER REQUIRED BY
FAN AT HIGH SPEED "

POWER REQUIRED
FAN AT
MODERATE
SPEED

20

15

10°
Id

CO
DC
O
I

0 20,000 4 0.000

AIR RATE, CUBIC FEET PER MINUTE

Fig. 27. — Characteristics of a backward-curved blade centrifugal fan,

or 4

(g) STATIC PRESSURE
OF FAN AT
HIGH SPEED

(D POWER REQUIRED BY
FAN AT HIGH SPEED-

® STATIC
PRESSURE OF
FAN AT MODERAT
SPEED

® RESISTANCE OF TUNNEL
WITH 12 HEAVILY LOADED,
CARS OF GRAPES -

d> RESISTANCE OF TUNNEL
WITH 12 NORMALLY
LOADED CARS OF
PRUNES OR GRAPES

(5) POWER REQUIRED
BY FAN AT
MODERATE
SPEED_

20

-15

-10

AIR RATE

20,000
CUBIC EEET PER

40,000

MINUTE

Fig. 28. — Characteristics of an axial-flow fan.

in short tunnels, the temperature drop will be small and the efficiency will be
low unless recirculation is used.

With a fast-drying material, the temperature drop per foot of tunnel length
is large, and recirculation may not be practical even in a moderately short
tunnel. If a fast-drying material is to be dried in a long tunnel, the tunnel
should not be filled with cars, or the drying conditions at the cool, humid end
will be unsatisfactory.

G4

California Experiment Station Bulletin 698

A very helpful indication of the operating conditions within a tunnel can be
obtained from observation of the difference between temperatures of dry- and
wet-bulb thermometers at the cool, or fresh fruit, end. Each time a car of fresh
fruit is introduced, the final air temperature drops considerably, for the fruit
on this car is relatively cold. In addition, each of the cars which has been
moved ahead in the tunnel contains fruit that is not so dry as that on the ear
previously in the same position,' and the evaporation rate throughout the tun-
nel is greater than before the recharge. This also causes a greater temperature
drop in the tunnel and a cooler final air temperature. The temperature grad-
ually rises, as the fruit on each car becomes drier, until again it is time to
remove the driest car and put in another fresh one.

cr

D
H

DRY-8ULB TEMPERATURE
OF EXHAUST AIR

\

EXHAUST AIR TEMPERATURE TO BE
REACHED BEFORE PUTTING IN NEXT CAR
/ CARS PUT IN AT THESE TIMES

<

rr

*1 ^*^\j>^\^^^^

u

U.

A ^MINIMUM RECOMMENDED EXHAUST AIR TEMPERATURE
j ^WET-BULB TEMPERATURE THROUGHOUT TUNNEL

TIME

DRY-BULB TEMPERATURE
OF EXHAUST AIR

EXHAUST AIR TEMPERATURE TO BE

REACHED BEFORE PUTTING IN NEXT CAR

CARS PUT IN AT THESE TIMES

/

|5 "p MINIMUM RECOMMENDED EXHAUST AIR TEMPERATURE

| yWET-BULB TEMPERATURE THROUGHOUT TUNNEL

TIME

Pig. 29.— Exhaust air temperatures, :it the end <.f single car and
double-car dehydrater tunnels.

If the drying rate is slower and the drying time longer than anticipated,
the cold-end air temperature will be found to be a little lower each time a new
car is introduced. This is a sign that the tunnel is being overloaded. If this
happens, the time interval between cars should be lengthened, the amount of
recirculation reduced, or the air velocity increased. It' an adjustment is not
made, the cars will not be ready to be taken out when they reach the front end
of the tunnel, and the number of cars in the tunnel— if it is not already filled—
will be increased. The result at the cool end is that, with cooler and more humid
air, the drying rate of the fresh cars will be retarded and soon the tunnel will
be filled with cars, of which only the first few will be drying satisfactorily.
If this condition is anticipated from observation of cool-end temperatures,
poor operating conditions can be avoided. With appropriate attention, the
operator can learn for each commodity the dry-bulb-wet-bulb difference which
must be attained before the next car should be introduced.

An approximate rule is that, after introducing a fresh car, the dry-bulb
temperature should be 15 degrees F higher than the wet-bulb temperature.
The extent to which the dry-bulb temperature should be allowed to rise before

Fruit Dehydration : I. Principles and Equipment 65

a new car is put in depends upon the nature of the product, the air velocity,
and the fraction of the tunnel load which one car comprises.

Typical cool-end air temperature changes are illustrated in figure 29 for
dehydration of peaches on single and on double cars. With single cars, the air
temperature was found to drop about 7 degrees F when a fresh car was intro-
duced. In order to have a 15-degree difference afterward, a 22-degree differ-
ence between dry- and wet-bulb temperatures was necessary before a fresh
car was put in. With double cars, a 14-degree temperature change occurred
each time a new car was introduced; it was therefore necessary to have a 29-
degree difference between dry- and wet-bulb temperatures before putting in
another car. During much of the latter period, the air was not effectively used,
but the new car could not have been introduced sooner or poor drying condi-
tions would have resulted.

Another disadvantage of double cars is the appreciable difference between
the drying rate at the front and at the back sides of the trays. Furthermore,
because of the great weight, handling is so difficult that an extra man is
needed to move the double truck — yet he is frequently idle.

In starting the operation of a tunnel, some modification must be made from
the normal operating schedule. It is highly undesirable to put several cars of
fresh fruit into one tunnel on starting; the evaporation rate from a car of
fresh fruit in an empty tunnel is great, and the drop in temperature of the
air passing through the car is high. For example, if 1 car, 6x6 feet in size,
containing blanched clingstone peaches is introduced in an empty tunnel,
with an air velocity of 1,000 feet per minute and a temperature of 155° F, there
will be an initial temperature drop of almost 30 degrees. If more than 3 or 4
single cars are introduced at once, the air moving past the last car will be
nearly saturated. This is indicated by the fact that the dry-bulb temperature
will be nearly as low as the wet-bulb temperature. Thus, little evaporation can
occur from the last car which has entered until the first cars have lost a con-
siderable part of their moisture; at this time their evaporation rate will be
less and the air reaching the last car will be warmer and less humid. The lower
the air flow, the smaller is the number of fresh cars which can be introduced
in an empty tunnel without excessively building up the humidity.

One procedure in starting is to introduce cars at a regular schedule, with
the hot-end air temperature at first adjusted to about the average cold-end
temperature to be found with steady operation. Each time a new car is put in,
the hot-end temperature is raised until the normal operating temperature is
reached. The proper temperatures for the hot-end air can be selected from
an air-temperature curve like that shown in figure 14 (p. 46). The first car
under this procedure will dry in just a little less than normal time, for it was
subject to approximately normal tunnel temperatures, although the humidity
of the air passing by it was low during its entire drying period. The first car of
an unfamiliar product should be examined as the predicted drying time is ap-
proached, in order that the drying schedule can be modified if necessary.

The other procedure on starting is to keep the hot-end air at normal operat-
ing temperature and to introduce cars more frequently at first than would be
done under the normal schedule. The latter procedure is simpler and requires
less supervision than the former method.

66 California Experiment Station Bulletin 698

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25m-l,'47(A139)