UNIVERSITY OF CALIFORNIA

COLLEGE OF AGRICULTURE

AGRICULTURAL EXPERIMENT STATION

BERKELEY, CALIFORNIA

Investigations in the Sulfuring of
Fruits for Drying

J. D. LONG, E. M. MRAK,
and C. D. FISHER

BULLETIN 636

July, 1940

UNIVERSITY OF CALIFORNIA
BERKELEY, CALIFORNIA

CONTENTS

PAGE

Introduction 3

Purpose of the investigation 6

Review of literature 7

Scope of investigations 13

Experimental procedure 14

Effect of fruit characteristics on absorption and retention of sulfur dioxide. . . 18

Factors affecting the gas environment 23

Influence of tight construction 23

Convection distribution 23

Tray construction " 24

Sulfur dioxide gas concentration obtainable 24

Temperature 25

The favorable gas environment 26

Types of sulfur and factors affecting their use 27

Variability in sulfur burning 28

Liquid sulfur dioxide 29

Sulfur burners 30

Metal pan burners 30

Earthen pits and other burners 31

Burner baffles 31

Design of the sulfur house 33

House capacity 33

Type of house 33

Convection channels 34

Tight construction 36

Type of door 37

Natural ventilation 38

Forced ventilation 39

Construction material 40

Painting 41

Moisture condensation 41

Remodeling old houses 42

New construction 43

Retention of sulfur dioxide by the fruit during drying 43

Relation of loss of moisture and of sulfur dioxide 43

Effect of climatic differences 46

Placing of fruit on the drying yard in the evening 47

Securing high retention ratio 48

Fruit temperatures during drying 50

Effect of tray color 50

Dehydration 52

Summary 52

Acknowledgments 54

Literature cited 55

INVESTIGATIONS IN THE SULFURING OF
FRUITS FOR DRYING1 2

J. D. LONG,3 E. M. MRAK,* and C. D. FISHER5

INTRODUCTION

The production of dried fruit has been an important agricultural indus-
try in California since the latter part of the nineteenth century. Prunes
and raisins are the most important crops from the standpoint of produc-
tion but the sulfured dried fruits have been more important from the
standpoint of value per pound. The tonnage of dried sulfured fruits,
consisting of apricots, peaches, nectarines, pears, apples, bleached rais-
ins, and figs has totaled for the past few years about 90,000 tons annu-
ally, with an estimated total value of about fifteen million dollars. Dried
sulfured fruits are produced in nearly all valleys of the state. The prin-
cipal localities, varieties, and seasons are given in table 1. The season,
yield, drying ratio, and values vary with the variety and locality as well
as with the year. Detailed information concerning deciduous fruit sta-
tistics is given by Shear (1939).6

Sulfur dioxide has been used for the treatment of foods since ancient
times. Fruits are so treated in order to preserve their natural color,
flavor, and, in part, to protect certain nutritive values. Sulfuring also
prevents enzyme and microbiological deterioration; repels insects to
some extent during drying and storage ; facilitates drying by plasmolyz-
ing the cells ; and is sometimes used to prevent losses during rainy dry-
ing seasons (see Bioletti and Way, 1919, and Cruess, 1921a). Years of
scientific investigations and practical trials have failed to reveal another
pretreatment agent equal to sulfur dioxide in preserving the desired
qualities in cut, dried fruits.

When sulfur is burned in air it combines with the oxygen of the air
to form sulfur dioxide, a colorless gas. The white fumes commonly seen
in sulfur houses are due to the fogging of water vapor about particles of
sulfur trioxide simultaneously produced, but which represent a small

1 Received for publication January 11, 1940.

2 Part of the nontechnical assistance was supplied by the Works Progress Admin-
istration, in Project No. 431, Northern California Section, Area 8.

3 Assistant Professor of Agricultural Engineering and Assistant Agricultural En-
gineer in the Agricultural Experiment Station.

4 Instructor in Fruit Technology and Junior Mycologist in the Experiment Station.

5 Chief Chemist, Dried Fruit Association of California.

6 See "Literature Cited" for complete citations which are referred to in the text by
author and date of publication.

[3]

University of California — Experiment Station

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BUL. 63C] SULFURING OF FRUITS FOR DRYING 5

portion of the sulfur burned. According to B. Hatherell,7 approximately
10 per cent of the sulfur burned in air is converted into sulfur trioxide,
and the concentration of sulfur trioxide in the sulfur house is usually
equal to about 10 per cent of that of the sulfur dioxide concentration.
The formation of sulfur dioxide by burning sulfur in air may be stated
by the following equation :

S + 02 -» SO.,

(Sulfur) (Oxygen) (Sulfur dioxide)

When sulfur dioxide dissolves in water or fruit juice it forms sul-
furous acid which is a weak acid that should not be confused with sul-
furic acid. The equation for the reaction of sulfur dioxide with water
to produce sulf urous acid is :

S02 + H20 -* H2S03

(Sulfur dioxide) (Water) (Sulf urous acid)

Sulf urous acid is used as a preservative in numerous food products. This
acid, and its salts (sulfites) are reducing substances (having oxygen-
removing power) and it is this property which is probably responsible
for its action in preventing discoloration.

The control of sulfuring so that fruit is not undersulfured is of pri-
mary importance to the producer of quality dried fruits. The trade de-
mands a light and uniformly colored product that will not deteriorate or
darken during storage. For this reason the fruit must contain a suffi-
cient quantity of sulfur dioxide when first dried.

When fruit once darkens during storage, subsequent exposures to
sulfur dioxide will not change the appearance of the darkened fruit to
that of the originally prepared fruit. Consequently it is often necessary
to dispose of darkened fruit on a lower-quality basis and at a lower price.
Love (1937) has emphasized the importance of the first sulfur treatment
in regard to the retention of color in dried apricots.

To maintain the desired qualities in fruit it has been found necessary
to incorporate in it an excess of sulfur dioxide to allow for losses occur-
ring during handling and storage. The exact amount of sulfur dioxide
needed to preserve the color and other qualities of dried fruit varies with
the nature of the fruit and with storage conditions. For example, dried
cut fruits may be relatively low in sulfur dioxide content and still retain
the characteristic color of the fruit if properly and thoroughly dried. Cut
fruits of high moisture content will darken during storage even though

7 Unpublished data from sulfuring investigations project of the Research Labora-
tory of the Dried Fruit Association of California, 1928.

6 University of California — Experiment Station

they contain a high initial sulfur dioxide content. The amount of sulfur
dioxide which should be incorporated in the fruit at the drying yard to
maintain quality is approximately, in parts per million (1,000 p.p.m.
equals %0 of 1 per cent by weight), as follows : apricots, 2,000; peaches
and nectarines, 2,000 ; pears, 1,000 ; golden bleach raisins, 800, sulfur
bleach raisins, 1,500 ; and apples, 800. In certain districts as a result of
adverse climatic conditions, inadequate drying-yard practices and poor
sulfur absorption and retention by the fruit, it is difficult to obtain the
necessary quantity of sulfur dioxide in the fruit.

The technique of sulf uring fruits usually consists of exposing the fruit
on trays to sulfur dioxide in a closed chamber for periods of time, vary-
ing according to the variety and maturity of the fruit, the locality, and
the experience of the operator. The sulfured fruit is then dried in the
sun or in a mechanical drier. Nichols (1933) has described in detail the
sulfuring procedure commonly used in California. The treatment of
fruit with sulfur dioxide appears to be a relatively simple process. There
are, however, numerous variations in procedure, the relative merits of
which have never been determined. During the last half century or more
of the development of the dried-fruit industry, many practices have
been adopted and many designs of equipment developed which may not
be well founded, or which may be subject to change or modification un-
der current operating conditions. Within a given district it has been ob-
served that growers tend to adopt similar methods, some of which are
questionable. Practices may vary markedly between districts with no
apparent reason. Many problems and questions have arisen concerning
the factors affecting the burning of sulfur, and the absorption and re-
tention of sulfur dioxide by the fruit. They relate to sulfur-house design,
orientation, gas concentration, temperature of the compartment, and the
climate in general.

PURPOSE OF THE INVESTIGATION

The 1936 apricot drying season in the Hollister district found many of
the producers experiencing difficulty in their sulfuring practice. A
survey of the conditions existing in other areas revealed that the same
difficulties were being encountered in many districts, and that the
troubles reported in that year were merely a continuation of a series of
annual troubles. The investigation of sulfuring practices and problems
reported in this publication was conducted during the years 1937, 1938,
and 1939.

The purpose of this investigation has been to obtain information that
might enable growers to improve their sulfuring and drying procedures

BUL. 636] SULFURING OF FRUITS FOR DRYING 7

to secure better-quality products. Information was obtained concerning
factors involved in the burning of sulfur, sulfur dioxide gas concentra-
tion and distribution, and the temperature within the compartment.
Information was also obtained concerning the typical sulfuring tech-
nique and results from farm practices used in representative districts,
and an effort made to correct conditions responsible for such waste as
shown in figure 1. Furthermore, an attempt was made to correlate the

Tig. 1. — Sulfur slag waste resulting from poor burning in earth pits: Left,
two- or three-ton accumulation in a large drying yard; right, clinker weighing
69 pounds formed in one season from about thirty burnings.

interrelations of the important structural, chemical, and physical fac-
tors common to farm procedure with the absorption and retention of sul-
fur dioxide by fruit.

REVIEW OF LITERATURE

Certain phases of the sulfuring problem, including methods of sulfuring
fruits for drying, have been discussed by a number of authorities. It is
of interest to review these publications since they frequently show a di-
vergence of opinion and in certain instances disagree with the results
reported in this bulletin.

Nichols (1933) has given a recent summary of the sulfuring procedure
used in California.

Beekhuis (1935) made a comparative survey of drying-yard practices
and equipment used in the twelve major fruit-producing areas of north-
ern California. He paid particular attention to the construction of sulfur
houses, their number, condition and capacity of each unit, elapsed time
between cutting and sulfuring, exposure of the fruit to sulfur dioxide
and pounds of sulfur used per ton of fresh fruit. He considered the Sui-
sun district the best of the areas surveyed. The general practice in this
district is to employ a large number of sulfuring compartments, each
holding a single field car. The compartments are joined in a long shed

8 University of California — Experiment Station

with overhanging roof which protects the doors. The interior walls are
plastered. These compartments were not considered ideal, however, be-
cause : (1) their short length requires that the sulfur burner be placed
under one end of the car, which interferes with the gas circulation and
creates a fire hazard; (2) no provision is made for ventilation; and (3)
the doors of many of the compartments are built in a wall parallel with
the prevailing winds, a feature he considered conducive to uneven
draughts within the compartments. A second burner at the rear of the
compartment provided with a vent through the rear wall was recom-
mended to correct this fault. In the Patterson district, Beekhuis observed
the use of drying-yard carts for moving stacks of trays rather than the
conventional field car and tracks. This favored the installation of move-
able, "hood-type" sulfur houses of pressed fiberboard.

Beekhuis used, as a measure of variations in exposure to the sulfur
fumes, a "sulfuring factor," defined as the pounds of sulfur used per ton
of fresh fruit multiplied by the hours of exposure. Isolated cases were
cited of growers successfully sulfuring apricots with a sulfur factor of
10 to 20 (4 to 8 pounds of sulfur per ton and an exposure of 2% hours) .
The average factor was about 30-35 with extreme cases extending to 90.

The sulfuring procedures used in Argentina, as described by Croce
(1936 and 1937), and in South Africa, as described by Perkins (1925),
are very similar to those used in California. The Australian sulfuring
practices described by Quinn (1926a, b) and Jewell (1927a, b) and
recommended by Quinn and associates (1929), differ somewhat from
those in common use in California.

It is generally agreed that the variety, maturity, and general condition
of the fruit are important factors in determining the absorption and re-
tention of sulfur dioxide by the fruit. Jewell (1927a, b) stated that
peaches and pears do not absorb sulfur dioxide as readily as apricots and
for this reason are not as easily oversulfured. x\ccording to Culpepper
and Moon (1937), peeling or slicing of pears enhances the absorption of
sulfur dioxide. Quinn and associates (1929) indicated that fruit, to be
sulf ured, should be of "eating ripe" maturity. Nichols, Mrak, and Bethel
(1939) showed that color and sulfur dioxide retention by dried apricots
varied with the locality in which the fruit was produced.

There is no agreement concerning the desirability of sprinkling fruit
with water or brine solutions prior to sulfuring. Christie and Barnard
(1925) believed that because the reaction S02 -f- H20 — > H2S03 occurs
during sulfuring it was desirable to sprinkle the fruit with water before
sulfuring. Anderssen (1929) found that apricots moistened before sul-
furing contained more sulfur dioxide than unmoistened apricots, but

BUL. 636] SULFURING OF FRUITS FOR DRYING 9

the final quality was about the same. According to Jewell (1927a and
19275) the amount of moisture on the cut surface of the fruit influences
absorption. Chace, Church, and Sorber (1930), on the other hand, could
not detect differences in the absorption of S02 or in the appearance of
fruit sprayed with water before sulfuring. Beekhuis (19366) states that
the sprinkling of pears with water has been abandoned in many drying
yards as it does not seem to answer any definite purpose. Lyon (1930),
Nichols and Christie (1930a), and Nichols (1933) state that sprinkling
fruit prior to sulfuring has no beneficial effect. Storage of the fruit be-
tween cutting and sulfuring was considered undesirable by Beekhuis
(1936a) because it permitted drying of the fruit surfaces and he recom-
mended that the period between cutting and sulfuring be less than iy2
hours.

Blanching in steam or hot water prior to sulfuring does not alter the
sulfur dioxide absorption or retaining capacity of the fruit according to
Nichols and Christie (19306) and Chace, Church, and Sorber (1933).
Fruit blanched after sulfuring, however, retained 50 per cent more sul-
fur dioxide than unblanched fruit.

Various types of sulfur burners have been suggested and used. Nichols
and Christie (1930a), Beekhuis (1936a and 1938), and Kennie (1936)
described concrete pipe and magnesia burners which have proved satis-
factory. Nichols (1933) and Nichols and Christie (1930a), stressed the
importance of maintaining a dry burner and the latter authors recom-
mended protecting the burner from moisture by the use of oil or tar.
Quinn (1926a and 1935) suggested the use of a glazed vessel or an old
oil drum for a burner. He further recommended that the sulfur be lighted
with live coals obtained from a wood fire. On the other hand, Cameron and
associates (1929) , stated that a minimum of inflammable material should
be used to light fires. Nichols, Powers, Gross, and Noel (1925) indicated
that it is preferable to burn sulfur in from three to six shallow pans
stacked one above the other in zigzag formation in order to obtain a
high concentration of sulfur dioxide in a short time. Nichols and Christie
(1930a) and Nichols (1933) later pointed out the danger of subliming
the sulfur when using the stacked pan burner. Residue resulting from
the incomplete combustion of sulfur need not be discarded according to
Nichols (1933) for it will burn if mixed with other sulfur and given
the proper amount of draft. Mixing 1 pound of sodium nitrate (Chile
saltpeter) with 20 pounds of sulfur will promote burning under very
difficult conditions. Caution should be observed in the preparation and
handling of the mixture because of its inflammability.

Sulfur-house construction, design, materials, and location have been

10 University of California — Experiment Station

discussed in detail by Christie and Barnard (1925), Cameron and asso-
ciates (1929), Nichols and Christie (1930a), Rennie (1936), Croce
(1936) and Beekhuis (1936a and 1938). It is agreed that the sulfur
house should be tight and constructed of permanent materials if pos-
sible. The house should be so located that the prevailing winds will not
interfere with burning of sulfur or the distribution of sulfur dioxide gas
within the house. California investigators recommend that the sulfur
house be constructed of sufficient length to allow for a free space above
the burner at the front end of the house. Vents should be used on the
front or near both front and rear walls when needed to facilitate burn-
ing. Cameron and associates (1929), recommended the use of two con-
trollable vent holes, 1 inch in diameter and about 1 foot apart, located
in the roof of the chamber close to the wall farthest from the sulfur
burner when only one burner is used. "When two burners are used, one
at each end of the sulfur house, they recommend that the vents be located
in the center of the roof. Jewell (1927a and 19275) described two types
of houses in common use in Australia: (1) a practically tight, either
portable ("malthoid" covering over a wooden frame), or permanent
fixture (of brick or fibro-cement sheets) ; (2) a more or less gas-perme-
able chamber consisting of a wooden frame covered with hessian cloth
or bagging, generally washed with lime.

The amount of sulfur used varies considerably with the kind, variety
and condition of the fruit being sulfured. There are, however, several
other factors that have an effect on the quantity of sulfur used in a
single charge. According to Anderssen (1929) the theoretical maximum
of sulfur that can be burned per 100 cubic feet of air is about 1.5 pounds
and would yield a concentration of about 20 per cent of sulfur dioxide
by volume. Under optimum farm conditions it is doubtful, however, if a
concentration exceeding 7 per cent of sulfur dioxide can be obtained by
burning sulfur. Bioletti and Cruess (1912) found that in actual practice
the possible yield of sulfur dioxide from a given amount of burning
sulfur is much less than the theoretical value. This difference was at-
tributed to losses of oxygen resulting from the formation of sulfuric
acid, escape of air from the enclosure as it expands with rise in tempera-
ture, and the failure of some of the oxygen to combine when the propor-
tion in the air becomes too small to support combustion of sulfur.
Pacottet (1911) stated that the low yield of sulfur dioxide obtained
when sulfur is burned in wine casks is due to loss of sulfur which is
volatilized or melted without burning and to that which forms sulfuric
acid. It varies with the amount of sulfur burned in a given space and
with the physical condition of the sulfur. Lyon (1930) stated that the

Bul. 636]

Sulfuring op Fruits for Drying

11

amount of sulfur consumed bears little direct relation to the length of
burning period. Other factors that influence it are draught, air tempera-
ture, humidity, and area of the ignited surface. Jewell (1927) listed fac-
tors influencing absorption and hence the amount of sulfur used as
follows : (1) variety of fruit; (2) type of sulfur house; (3) quantity of

TABLE 2

Quantity of Sulfur Burned and Length of Sulfuring Period Commonly

Used in Various Dried-Fruit-Producing Eegions

Fruit

Region

Pounds sulfur
per ton of
fresh fruit

Length of

sulfuring

period, hours

Bibliography
reference

California
Argentina
Australia

k South Africa

California
Argentina

3-4
3-4

7-8

2*

3-4
3-4
Day, 7; night, 6
8

12

8
10-12

3
3

2-4

2-3
2-3
2-3

4-5

3-4
3-4
Day, 6; night, 12
4

24-36

24-36
24, maximum

12

4
4

2-3

Nichols (1933)
Croce (1936)
Cameron and asso-
ciates (1929)
Perkins (1925)

Nichols (1933)
Croce (1936)

Pears (Bartlett)

] Australia
[ South Africa

[ California

] Argentina
( Australia

Eastern
United States

California
California

Jewell (1927a)
Perkins (1925)

Christie and
Barnard (1925)

Croce (1936)
Jewell (1927a)

Figs (Adriatic)

Culpepper and
Moon (1937)

Raisins (sulfur bleach)

Raisins (golden bleach)

Barnard (1925)
Christie and
Barnard (1925)

Christie (19306)

* Two pounds per 100 cubic feet of space.

sulfur used per unit weight of fruit or per unit volume of air space ; (4)
amount of moisture on the cut surface of the fruit ; and (5) temperature
in the sulfur house.

The quantity of sulfur and the sulfuring period for various fruits as
recommended by different authorities are given in table 2.

According to Jewell (1927a) the bottom tray of fruit in the sulfur
house tends to absorb more sulfur dioxide than fruit on the top trays.
This seems to indicate unequal distribution and the concentration of
sulfur dioxide fumes at the bottom of the house. Nichols and Christie
(1930a) stated that the concentration of sulfur dioxide in the chamber

12 University of California — Experiment Station

is controlled by the amount of sulfur burned, the rapidity and complete-
ness of burning, the volume of space and quantity of fruit in the chamber
and the loss of sulfur dioxide from the chamber through ventilators or
leaks ; also that the decrease in concentration is further influenced by
wind velocity and direction. Nichols (1933) stated that for rapid, uni-
form sulfuring there must be dense white fumes in the house throughout
the sulfuring period.

Chace, Church, and Sorber (1930) found that the concentration of
sulfur dioxide fumes in the sulfur chamber influenced the rate of ab-
sorption of sulfur dioxide by the fruit. Apricots were satisfactorily sul-
fured when treated for 3 hours in a chamber having an initial sulfur
dioxide concentration of 2 per cent or for 2 hours in a chamber having
an initial concentration of 3 per cent. Temperature had less effect on the
retention of sulfur dioxide by the fruit than either time of exposure or
concentration of the fumes. Temperature above 120° Fahrenheit caused
apricots to become red in color. According to Nichols and Christie
(1930a) temperature affects absorption in several ways. As the tempera-
ture rises, the solubility of sulfur dioxide in water decreases. On the
other hand, an increase in temperature increases the rate of combination
of sulfur dioxide with other substances in the fruit and increases the rate
of penetration of it into the fruit tissues. High temperatures presum-
ably soften the fruit and facilitate absorption and penetration. Jewell
(1927&) found that a longer sulfuring period was required at night than
in the daytime because of the lower night temperature. In this connec-
tion, Nichols and Christie (1930a) indicated that the choice of construc-
tion material would depend upon whether a sulfur house is to be used
principally during the day or night. Observations showed that while
wood and metal houses reach higher temperatures in the day than do
concrete or brick houses, at night the temperatures in the concrete and
brick houses are higher. Quinn (1926a) and Love (1937) also stressed
the importance of temperature in sulfuring fruit. According to Love,
it is more difficult to sulfur fruit properly in cool weather.

The experiments of R. S. Hiltner8 have shown that apricots and
peaches can be sulfured in 3 hours with relatively small amounts of
sulfur by use of his "dense smoke method." The method consists of ex-
posing peaches or apricots for 3 hours in a warm and tight compartment
containing dense fumes of sulfur dioxide obtained from burning sulfur
in the sulfur house. A dense smoke must be maintained throughout the
sulfuring period. The dense smoke method, however, has not come into

8 Unpublished data from sulfuring investigations project of the Eesearch Labora-
tory of the Dried Fruit Association of California, 1928.

BUL. 636] SULFURING OF FRUITS FOR DRYING 13

general use because of periodical and hitherto unexplainable difficulties
encountered in obtaining complete combustion of the sulfur.

Beekhuis (1938) and Cruess (1938) suggested testing freshly sul-
fured fruit by cutting and noting the depth of penetration. Chace,
Church, and Sorber (1933) found that this was not a reliable method
for determining the extent of sulfur dioxide absorption.

Chace, Church, and Sorber (1933) and Nichols, Mrak, and Bethel
(1939) have shown that unfavorable drying weather or drying in the
shade favors the loss of sulfur dioxide by the fruit during drying.
Anderssen (1929) also found that shade-drying decreased its retention
during drying. Beekhuis (1935) cited data to show that apricots dried
rapidly retained more of this chemical than those dried slowly. C. F.
Love9 was able to increase the sulfur dioxide content of fruit dried under
unfavorable conditions by drying on white painted trays and by stack-
ing the trays at night to avoid excessive losses during the cold, foggy
night. Positive but less pronounced increases were noted in fruit spread
in the morning to secure the advantage of the morning sun rather than
at night.

Roleson and Nichols (1933) stated, in regard to sulfur dioxide reten-
tion during drying, that the content of fruit exposed in the drying yard
for one day would be nearly the same as that for the ordinary dried pro-
duct, if no correction for moisture content is made. Hanus and Vorisek
(1937) found that the sulfur dioxide content of apricots, dried in ordi-
nary atmosphere and compared on a moist weight basis, remained con-
stant for 35 days. It began to decline only when the apricots ceased to
lose water. This apparent constancy was attributed to the comparable
loss of water, for where sulfur dioxide was determined on a moisture free
basis it decreased during the entire drying period.

Cruess, Christie, and Flossfeder (1920), Cruess and Christie (1921),
Cruess (19215), and Caldwell (1923) have shown that fruits to be dried
artificially require less sulfuring than fruits to be sun-dried.

Nichols and Christie (1930a) described the characteristics of properly
sulfured and dried fruits. Eoleson and Nichols (1933) stated that for
cut fruits other than apples and pears, 2,000-2,500 parts per million of
sulfur dioxide in the fruit is considered the optimum.

SCOPE OF INVESTIGATIONS

Experiments to determine the effects of various factors on gas concen-
tration and temperature conditions in the compartment were conducted

9 Unpublished data from sulfuring investigation project of the California Prune
and Apricot Growers Association, 1935.

14 University of California — Experiment Station

at Davis in a three-compartment panel-board sulfur house, as designed
by Long, Catlin, and Nichols.10 The structure is shown in figure 15.

Tests involving the relation of certain chemical and physical factors
to sulf uring practices were made in the following fruit-growing areas :
Aromas, Brentwood, Esparto, Exeter, Fresno, Gridley, Hemet, Hollister,
Kerman, King City, Kelseyville, Ojai, Modesto, Suisun, Ventura, and
Vacaville. In the field studies data were obtained concerning : (1) num-
ber and size of trays to each sulfur house, size of house, and general con-
struction features; (2) weight of sulfur used and completeness of
combustion; (3) fruit variety, maturity, and size; (4) gas analyses
from at least four points within the compartment at frequent intervals ;
(5) temperature observations at six points within the compartment at
frequent intervals; (6) atmospheric and drying-yard temperature read-
ings during sulf uring and drying ; (7) wind velocity and direction ; (8)
sulfur dioxide analyses of freshly sulfured, of partially dried, and of
dried samples; (9) general observations concerning drying-yard prac-
tices and climatic condition during drying; (10) results of correcting
obvious sulfur-house defects.

Three test houses of different construction materials erected for the
work are shown in figure 14.

Fruits used in the various tests were : Royal, Blenheim, Moorpark,
Hemskirke, and Tilton apricots ; Muir and Lovell freestone and Paloro
cling peaches; New Boy nectarines; Bartlett pears; and Thompson
seedless grapes.

EXPERIMENTAL PROCEDURE

Burning tests were made with sulfur samples collected from growers
located in different drying areas. These samples varied considerably
in the time and manner in which they had been stored and in certain
physical and chemical characteristics. The burning tests were conducted
as follows : Four pounds of sulfur were placed in a round metal pan, 10
inches in diameter and 2% inches deep. The loaded pan was then placed
on the bottom of the burner pit and ignited by touching a lighted match
to a depressed area in the sulfur at the center of the pan. The match was
subsequently discarded to avoid contaminating the burning sulfur with
carbon. The sulfur-house door was then closed and left undisturbed
until burning ceased, after which the remaining slag was weighed and
observed for surface characteristics.

The effect of sulfuric acid, black-surfaced sulfur slag, and rock sulfur

10 Long, J. D., H. E. Catlin, and P. F. Nichols. Fruit sulfuring house. California
Agr. Ext. Service Farm Building Plan C-173. 1934. (Available from the California
Agricultural Extension Service, University of California, Berkeley. Price 25 cents.)

Bul. 636]

SlJLFURING OF FRUITS FOR DRYING

15

on the burning quality of good sulfur was determined by mixing various
quantities of these materials with the sulfur before lighting the sulfur.
For the guidance of chemists certain technical procedures are given
in some detail in the following paragraphs.

Fig. 2. — Equipment used in the experiments. A, Test equipment used with sul-
f uring at the College of Agriculture, Davis ; B, test in progress on the house from
which the pile of slag shown in figure 1 was taken ; C, close-up view of analytical
equipment for determining sulfur dioxide gas concentration ; D, small sulf uring
compartment used to secure constant concentration of sulfur dioxide gas for
making tests; E, small electric dehydrater used in studies of sulfur dioxide re-
tention during drying.

Sulfur dioxide concentration attained at various points within the
sulfur house during operation were determined with the equipment
illustrated in figure 2. Copper-tubing (%6 inch outside diameter) gas
inlets were fastened on tray surfaces at the center and about 1 foot from
each end where they were surrounded by fruit. These inlets were located
at three elevations, as illustrated in figure 9, for the 1937 experiments.
In 1938 and 1939 only four gas inlets were used, located at points 1, 3,

16 University of California — Experiment Station

7, and 9 of the tray stack (fig. 9) because the results of 1937 indicated
that these points were sufficient to determine gas distribution. The cop-
per tubes extended out of the sulfur house, through the track openings,
and connected with the gas-analysis equipment (fig. 2, C) by means of
rubber tubing. Gas samples were withdrawn from the house through
an absorption vessel, by means of suction resulting from the drainage of
water from an air-tight reservoir connected to the absorption vessel. The
connections were "swept out" prior to each determination. The volume
of water withdrawn from the reservoir constituted a measure of the
volume of gas removed from the house. A hydrometer jar, 12 inches in
height and 2 inches in diameter, served as the absorption vessel, and gas
from the compartment was drawn into the bottom of this jar which con-
tained water, a few drops of starch solution and 1, 2, or 5 cubic centi-
meters of tenth normal iodine solution. The depth of solution in the
vessel was maintained at about 9 inches. The volume of the iodine solu-
tion used was adjusted according to the gas concentration within the
house so at least 100 cubic centimeters of gas were withdrawn for each
test. Gas was slowly drawn through the solution in the hydrometer jar
until the blue color resulting from the starch iodine reaction disappeared.
At this point all of the iodine had been reduced by sulfur dioxide. The
gas concentration was then read on curves prepared for each amount of
standard iodine used, plotting cubic centimeters of gas withdrawn from
the sulfur house against per cent by volume11 of sulfur dioxide in the
gas sample.

Data for the curves were calculated using Bureau of Standards
values for the volume of one pound of sulfur dioxide gas at 80° Fahren-
heit and 1 atmosphere pressure which is equal to 5.968 cubic feet. Vari-
ations of 20 degrees from this temperature cause changes in this volume
of about 4 per cent. The maximum probable error in gas concentration
determinations was approximately 5 per cent, caused chiefly by these
temperature variations within the gas absorption vessel. This degree
of accuracy was considered sufficient for the purposes of these investi-
gations.

Temperature measurements were made with resistance thermometers
placed on the trays adjacent to the gas lead inlets. Thermometers were
also suspended in the front and rear air spaces inside the house. Atmos-
pheric temperature was determined by suspending thermometers in the
shade near the sulfur house. Mercury-bulb thermometers were used to
secure the various drying-yard temperatures. Wind velocities were

u Per cent by volume of sulfur dioxide at 80° Fahrenheit multiplied by 0.167 =
pounds sulfur dioxide per 100 cubic feet.

BUL. 636] SULFURING OF FRUITS FOR DRYING 17

measured by use of a vane-type anemometer and wind direction was
determined by use of a cloth streamer. Readings were made in most
instances at 15-minute intervals during the first 3 hours of each test.

Other experiments included absorption studies made by periodically
withdrawing fruit samples during sulfuring, through a small opening
in the door of the sulfur house or from a specially constructed gas-tight
portable sulfur chamber. Samples were withdrawn in such manner that
the gas concentration in the chamber was not appreciably affected.
Liquid sulfur dioxide was used in the small chamber and distributed by
means of a small electric fan located within the chamber. Liquid sulfur
dioxide was also used in a limited number of full-scale sulfuring runs to
determine any possible advantages over the customary method of burn-
ing "flowers of sulfur," and also the comparative costs.

Dehydration studies were made with a specially constructed mechani-
cal drier consisting of a lower air chamber containing electric heaters
placed immediately in front of a motor-driven fan and eight 18 X 25
inch trays in an upper chamber. Wet- and dry-bulb thermometers were
located in the air-stream at the dry end of the top compartment. A
thermostat, the bulb of which was placed adjacent to the thermometers,
controlled one of the three 660-watt electric heaters. The other two elec-
tric heaters were manually operated. Adjustable ports for control were
provided at the fan end of the drier.

Samples of fruit for analyses were collected in glass-topped jars and
sealed until sulfur dioxide determinations were made. In the full-scale
tests of actual sulfuring practice, at least four samples from the extreme
corners of the tray stack, and others from mid-stack points when deemed
necessary, were taken and averaged for the final results. The jars were
filled with from 10 to 50 pieces of fruit, according to their size and stage
of drying. The time between sampling and analysis varied from 1 to 4
days. Trials indicated that sulfur dioxide loss during this interval was
insignificant in well-sealed glass jars. Sufficient fruits were always used
in the analysis to give a good random sampling.

In the small-scale tests with the tight sulfur box used for the constant
gas concentration and similar studies, and in the tests with the de-
hydrator, the single samples taken for the different stages of the test
procedure were representative.

The method of analysis used for sulfur dioxide determination was that
described by Nichols and Reed (1932) which involves the distillation of
a weighed ground sample of fruit in hydrochloric acid solution into
standard iodine solution and titrating the unused iodine with standard
sodium thiosulfate solution. Results were reported in parts per million

18

University of California — Experiment Station

by weight (1,000 p.p.m. equals 1/10 of 1 per cent by weight). Moisture
tests were made on fresh fruit samples by the method of Nichols, Fisher,
and Parks (1931) by distillation of a weighed sample of fruit with
xylene and the collection of the moisture in a calibrated sedimentation
tube under a reflux condenser. Dried samples were tested for moisture
with the electric moisture tester developed by Fisher.12

EFFECT OF FRUIT CHARACTERISTICS ON ABSORPTION
AND RETENTION OF SULFUR DIOXIDE

The physical and chemical factors influencing the absorption and reten-
tion of sulfur dioxide by the fruit are as yet incompletely understood.
Among the variable factors which are believed to affect the process are

TABLE 3

Effect of Size of Tilton Apricots on Their Absorption and

Eetention of Sulfur Dioxide

Diameter

Absorption,
p.p.m.

Retention,
p.p.m.

Retention
ratio*

3,410
3,510
3,730

510
640
490

0.15

.18

0.13

* The retention ratio is derived by dividing the sulfur dioxide content of the dried
fruit by the content of the freshly sulfured fruit; this necessitates no correction for mois-
ture. It is obvious that the higher the ratios the greater the retention. The retention ratios
given above are very low, but typical of the drying conditions of the Aromas district.

variety, composition (probably as related both to variety and to locality
and other conditions of growth) , maturity, and size.

Investigation of the physical and chemical processes of absorption and
retention are fundamental to a complete solution of the f arm-sulfuring
problem. Further work is in progress, particularly with regard to the
effect of temperature and added fixative agents. It is known that sulfur
dioxide is readily absorbed by the liquid present on the freshly cut sur-
face of the fruit, forming sulfurous acid which gradually penetrates the
tissues. Some penetration proceeds simultaneously through the skin-
covered surface of the fruit, but at a slower rate.

The size of the fruit is believed to be a factor in the absorption of sul-
fur dioxide and later loss. Table 3 gives the results of a test in which apri-
cots selected for uniform maturity in three size ranges were exposed
simultaneously to sulfur dioxide in a concentration of 2 per cent, for 3

" Fisher, C. D. Apparatus for determining moisture content of dried fruits, etc., by
electrical conductivity. U. S. Patent No. 1,961,965. 1934.

Bul. 636]

SULFURING OF FRUITS FOR DRYING

19

hours. The data obtained are not very conclusive. In spite of care in se-
lecting the fruits, the maturity may not have been as uniform as indi-
cated by appearance; furthermore in analyzing for sulfur dioxide the
limit of accuracy is considered to be about 50 parts per million.

A comparison of maturity differences secured in a field test is shown
in table 4. The results indicate that maturity is a critical factor in suc-
cessful sulfuring practice. From this and other tests by the authors and
those by other investigators, notably Quinn (1926a and 19265) and
Jewell (1927a and 19275) it is obvious that ripe fruit frequently absorbs
less sulfur dioxide than green, but always has a much higher retention
ratio. The term "retention ratio" expresses the relative amount of SUI-
TABLE 4
Absorption and Eetention of Sulfur Dioxide as Affected by
Maturity in Apricots

Variety

Maturity

Absorption,
p. p.m.

Retention,
p. p.m.

Retention
ratio

f Green
\ Ripe

( Green
\ Ripe

3,490
3,490

3,810
3,380

230
590

210
610

0.07

Tilton

.17
.05

0.18

fur dioxide present in the dried fruit as compared with that absorbed
by the fresh fruit during sulfuring. This method of comparison obviates
the necessity of correcting for moisture differences.

Variation in size and difference in composition are believed to be pri-
marily responsible for the differences in absorption (not retention) ex-
pressed in table 5 wherein the gas environment is expressed in terms
involving both sulfur dioxide concentration and exposure periods of
typical farm-sulfuring practice. Since the exposure period tends to be
constant for each kind of fruit this gas-environment factor essentially
is based on the average sulfur dioxide gas concentration during sul-
furing.

The data in the last column give an indication of the absorption char-
acteristics of the various fruits and varieties. It is interesting to note
that pears exhibit the lowest factor, that the large Moorpark and Hems-
kirke apricot varieties have the highest, and that the Paloro cling peach
has a slightly higher value than the two freestone varieties. A similar in-
dication of the effect of varietal differences is given in table 9. Studies
under controlled conditions must be made before the exact role of pos-
sible variations in composition as related to variety can be determined.

20

University of California — Experiment Station

A comparison by districts of the absorption factors of Blenheim or
Royal apricots, is given in table 6. Using as criteria the results of the
tests made in experimental houses at Davis, in 1937, and at Aromas, in
1939, it is obvious that the gas-environment rating secured in ordinary
farm sulfuring houses in the coastal areas is appreciably below that
which can be readily maintained in good sulfuring practice.

TABLE 5

Eelation of Fruit Variety to Sulfur Dioxide Absorption

Fruit and variety

Number

of

tests

Average

exposure

in minutes

Average

absorption

factor*

Apricots:
Tilton

7

29
2
3

2

3

2

2
2

210

220
185
180

260

245

275
240

2,180

160

10.43

12.25

19.00

19.50

Nectarines:

8.40

Peaches:
Muir

8.73

10.20

11.20

Pears:
Bartlett. . .

0.89

Raisins:

10.50

* "Absorption factor" is a numerical expression used here to compare the absorption of sulfur dioxide
by various fruits in different sulfur houses, operating under farm conditions, throughout the state. The
factor number represents absorption by the fruit in parts per million divided by "gas environment"
value which in turn is equal to the percentage gas concentration in the sulfur house multiplied by the
minutes of exposure of the fruit while sulfuring. The gas environment was conveniently determined in
each case by planimeter measurements of the area enclosed by the graphs ; a representative graph is shown
in figure 6.

The results of previous investigators, particularly of Nichols, Mrak,
and Bethel (1939) have indicated considerable variation in the absorp-
tion characteristics between fruit grown in the interior valleys and that
in the coastal areas. It appears that the variations may be due to differ-
ences in composition, as previously explained, and perhaps also in part
to the higher relative absorption in the concentrations found in sulfur
houses of the coastal areas. In tests with all kinds of fruit it has been
noted that fruit exposed to the lower gas concentrations absorbs a higher
proportional amount than that exposed to higher concentrations. For
example, apricots exposed to a 1 per cent maximum concentration might
absorb 1,500 p.p.m. of sulfur dioxide, but would take in appreciably

TABLE 6

Belation of Locality to Sulfur Dioxide Absorption by Blenheim
or Royal Apricots

Locality and year

Number

of

tests

Average gas

environment

value*

Average

absorption

factor

Hemet (1938)

3
2

1
8
4
5
7
2

272
339
175
247
318
184
291
155
205

10.50

Davis (1937)

11.05

Upper Ojai Valley (1938)

11.20

Suisun Valley (1938)

11.70

Aromas (1939)

11.25

Aromas (1938)

12.40

Hollister (1937)

12.94

Ventura (1938)

13.30

King City (1938)

15.80

For meaning of "gas environment value" see footnote to table 5.

5000

t 4000

D

a:

* 3000

z
o

I-

CL

o

g 2000

<

1000

3 0

SULFUR

O
O

O^

*^p

^"-0

3

c

^ <

)

o

8J

' o

V

/

/

y

V

/

i

1 •

/
/

/

A

/
o

7C

LEGEND
o Apnir^"1*^ — —

of

^ D r a r i

-IF ^ ■•

1LO ™—

100 200 300 400
DIOXIDE CONCENTRATION

500 600
x TIME CMIN)

Fig. 3. — Showing the sulfur dioxide absorption by apricots as related to the
environmental factor defined as sulfur dioxide concentration in per cent by
volume in the sulfuring chamber multiplied by time in minutes. It is evident
that under farm conditions an increase in the environmental factor would not
greatly increase absorption above the 4,500-p.p.m. level. The few tests that were
made with peaches illustrate their relatively slower absorption.

22

University of California — Experiment Station

less than 3,000 p.p.m. if subjected to a 2 per cent sulfur dioxide environ-
ment for the same period. From table 6 it is apparent that in the Hol-
lister area tests a high concentration was maintained, but that similar

100
uj or

a. i

FRONT OF TRAY STACK

^ 90

UJ 0

o. uj 80
2 Q

,•

„

*

f£.

«" 70

2 M

_i

>

«° 2.0

ow

«0 1.5

H

z 10

o

£ 0.5

/'

\

\

\

1
1

\

T_

CENTER

OF

TRAY STACK

-.'

j^"-.

t

^

"\

J

1

Wk

y

/

\

/

f

REAR

OF TRAY

STACK

2^.

^

■"T*

30 60 90 120 ISO 180

30 60 90 120 150 180
TIME IN MINUTES

WEATHER CONDITIONS FOR THE TEST

90 120 150
N MINUTES

uj or
>oJ 100

cp|qz|qi|oqqi

dz

WIND

r

DIRECTION
VELOCITY

30 60
TIME

90 120 150
IN MINUTES

Fig. 4. — Typical curves and analytical results showing the type of data taken dur-
ing each field test. These data are from test 53, Davis, sulfuring Tilton apricots in
the house illustrated in figure 2, A. Note the uniformity of the sulfur dioxide concen-
tration curves taken from nine points within the tray stack, and the fact that a con-
centration above 2.5 per cent was maintained for 90 minutes — two characteristics of
a desirable gas environment. Temperature curve 1 was taken just outside the tray
stack, and curve 2 at the rear track level. Missing temperature curves in this test were
due to broken contacts. The temperature curves at the top front and at the rear track
level indicate the direction of the convection currents within the compartment. Cor-
responding data on the absorption and retention of sulfur dioxide by the fruit used
in these tests are given in table 7.

data in the Ventura and Aromas (1938) areas were low. The differences
in the absorption ratios for these districts do not correspond with those
of the gas-environment values.

Sufficient data were secured in 24 experiments at Davis to plot the sul-
fur dioxide absorption of apricots (fig. 3) using the planimeter measure-
ment of the sulfur dioxide concentration and the time as described in

Bul. 636]

SULFURING OF FRUITS FOR DRYING

23

table 5 for the independent variable, and plotting the absorption on a
logarithmic scale. A limited number of tests with peaches are also given
on the curve, and show the slower absorption of this fruit. It is obvious
that in typical farm sulfuring practice apricots tend to reach a maxi-
mum absorption of 4,500 p.p.m. under a gas environment such as is il-
lustrated in figure 4. In short, further sulfuring, under these conditions
would not noteworthily increase absorption. The slopes of the absorption
curves in figure 3 give the rate of change of the proportion of sulfur di-
oxide in the fruit in parts per million with respect to the environmental
value (percentage of sulfur dioxide in gas chamber X time in minutes) .

FACTORS AFFECTING THE GAS ENVIRONMENT

As indicated in table 6, and in figure 3, the conditions under which fruit
is exposed to the gas influence the absorption of sulfur dioxide in the
fruit. Obviously, for uniformity of absorption at all points in the tray

TABLE 7

Data of Absorption and Betention of Sulfur Dioxide for the Bespective

Graphs Shown in Figure 4

Location (top to

bottom) of fruit

in tray stack

Front of tray stack

Center of tray stack

Rear of tray stack

Absorption
of SO2 by
fresh fruit

Retention
of S02 by
dried fruit

Absorption
of S02 by
fresh fruit

Retention
of SO2 by
dried fruit

Absorption
of SO2 by
fresh fruit

Retention
of S02by
dried fruit

Top

Middle

p.p.m.
4,060
3,900
3,960

p.p.m.
1,950
1,970
2,230

p.p.m.
4,000
3,960
3,920

p.p.m.
1,960
2,240
2,340

p.p.m.
3,840
4,000
3,840

p.p.m,
2,230
1,990

2,710

stack it is necessary to have a uniform distribution of gas within the
compartment. The gas concentration, the exposure period, and the tem-
perature of the fruit are all factors in the rate and amount of absorption.
Field tests in which these factors were considered were conducted in the
major dried-fruit-producing areas of the state. Data from the field tests
were plotted for analysis, as shown in figure 4. For the 4 pounds of sulfur
burned, the gas-concentration curve shows an ample concentration main-
tained for a suitable time, as evidenced by the average sulfur dioxide ab-
sorption of 3,942 p.p.m. (table 7). The distribution throughout the tray
stack, taken at the nine points illustrated in figure 9, is also fairly
uniform.

Influence of Tight Construction. — In many of the field tests disturb-
ances were noted attributable to drafts caused by moderate breezes blow-
ing through relatively tight-appearing construction joints, poorly fitted

24 University of California — Experiment Station

doors, or poorly designed ventilation openings. In the test of a four-car
tandem house made with a moderate breeze blowing, sulfur dioxide con-
centrations varied uniformly from 0.75 per cent maximum, at the loosely-
fitted door end, to 1.5 per cent maximum at the other. A variation from 1
per cent at the bottom of the tray stack to 2 per cent at the top is shown
in figure 12 ; this difference was due to a poorly designed system which
caused excessive ventilation.

Convection Distribution. — In single-car houses a tendency for a lower
temperature and a lower concentration of sulfur dioxide at the lower
front portion of the tray stack just behind the sulfur burner has been
noted. This is caused by sluggish convection currents through the lower
trays, and sulfur dioxide removal through absorption by the fruit. The
rapid rise of gases immediately above the burner appears to retard diffu-
sion from the front. Under such conditions the atmosphere at this point
may take half an hour to reach concentrations of sulfur dioxide equiva-
lent to those at other points in the tray stack, and sometimes remains
slightly lower throughout the sulfuring period. This variation occurred
in all types of sulfur houses tested, but is not sufficiently marked to be
considered a serious fault.

The use of a structural design which approximates the convection-
channel clearances shown in figure 9 would appear to be a satisfactory
solution. This difficulty is the reverse of that noted by Jewell (1927a)
in the tendency of the fruit on the bottom trays to have the highest ab-
sorption. No corroborating evidence was found for this situation, but it
may have been caused by a temperature differential between top and
bottom of the tray stack.

Tray Construction. — Where free convection currents maintain a uni-
form gas concentration about the tray stack, and open spaces are left
across the ends of the trays, the gas diffuses through the tray length rap-
idly. In only a few tests was the accumulation of gas at the central tray
points noticeably slower than at the ends of the trays.

The three sizes of standard tray design used in California, the 2X3
foot open-end raisin tray, and the 3X6 foot and 3X8 foot closed-end
trays used for other fruits, appear to be well designed for their purpose.
For sulfuring the largest-sized fruit, it is desirable to increase the depth
of the side rail, or the thickness of the bottom cleats, to maintain ap-
proximately % inch clearance between the fruit and the bottom of the
tray above.

Sulfur Dioxide Gas Concentration Obtainable. — Theoretically, sulfur
burning within a closed compartment and utilizing all of the oxygen in
the air develops a 21 per cent sulfur dioxide concentration. Several f ac-

BUL. 636] SULFURING OF FRUITS FOR DRYING 25

tors contribute to materially reduce this percentage in practice. The
maximum gas concentration reached in an empty compartment was 7
per cent. In actual sulf uring practice the rapid absorption of the gas by
the fruit still further reduces the concentration, a maximum of 3.5 per
cent being reached in this series of tests. With conditions similar to those
of figure 7, the empty compartment maximum was about 6 per cent and
the buffer effect of the fruit reduced the maximum sulfur dioxide concen-
tration to about 3.0 per cent. Large sulfur-burning area, adequate air
supply, and freedom from sulfur impurities increase the rate of burning
and develop higher gas concentrations. Leaky houses permit a ready es-
cape of the gas with low concentrations as a result.

Temperature. — Sulfur-house temperatures depend on the following
factors : rate and duration of burning of the sulfur ; convection and cir-
culation characteristics of the house; outside air temperature; house
construction material, or design features that absorb and transmit solar
radiation to the compartment ; the temperature of the fruit entering the
compartment ; and the cooling effect of the evaporation from the moist
fruit surfaces.

Burning sulfur within the sulfur house generates heat. As shown in
figure 4, the heat generated by free-burning sulfur in a 10-inch-diameter
pan is sufficient to raise the air temperature within the tray stack about
15° Fahrenheit. Opposing this temperature rise is the evaporative cool-
ing effect of moisture leaving the freshly cut fruit surfaces, as well as the
heat capacity of the fruit mass. Burners with large surface areas increase
the temperature rapidly, and to relatively high maximum points. Burner
baffles decrease the rate of burning, so the temperatures rise slowly and
somewhat more uniformly throughout the house. The tests on mechan-
ical forced-draft burning showed little temperature difference over nor-
mal natural-draft burning. Air temperatures surrounding the fruit
consistently ranged between 80° and 100°, with a maximum record of
134° for the interior valleys, and a minimum of 70° for the coastal areas.
A temperature differential of 12° to 18° exists between top and bottom
trays in houses located in coastal areas; those in the interior valleys usu-
ally have a much lower variation.

Outside the tray stack in the free air of the compartment the tempera-
ture commonly ranges up to 15° Fahrenheit higher ; temperatures taken
6 feet above the sulfur burner may be 30° above the average air tempera-
tures within the tray stack. The hot gases tend to travel back under the
ceiling, down the rear wall and forward under the tray stack to the
burner again, as indicated in figure 9. A temperature differential of 20°
between ceiling and floor is not uncommon, and as much as 50° has been

2() University of California — Experiment Station

recorded. These differences tend to equalize when burning has ceased.
Such temperature variations induce convection currents which facilitate
uniform gas distribution within the compartment. Locating burners at
each end of the compartment or midway of the floor area under the tray
stacks tends to cause conflicting convection currents and uneven distribu-
tion of sulfur dioxide.

Atmospheric temperatures during the sulfuring period and the solar
heat absorption by the structure have some effect on interior tempera-
tures. As previously mentioned, the thermal insulation and heat absorp-
tion characteristics of certain construction materials may play a part in
reducing the effects of the solar radiation. Nichols and Christie (1930a)
observed differences in the temperatures occurring in concrete or brick
houses as compared with those in wood structures. A horizontal ceiling
helps to insulate the structure. Although house construction material or
design may influence the interior temperatures, these differences are not
usually large enough to greatly affect the temperature of the fruit. Tests
on houses of different construction have failed to indicate any marked
variations in sulfur dioxide fixation due to their interior temperature
differences. Small-scale, controlled experiments with liquid sulfur di-
oxide, however, indicated that fruit temperature does exert a marked
effect on absorption and retention. High temperatures result in lower
absorption, but in higher retention. Royal apricots sulf ured at a constant
sulfur dioxide concentration of 2 per cent for 3 hours at an average tem-
perature of 62° Fahrenheit had an absorption of 3,870 p. p.m. while at a
temperature of 117° the absorption was 2,970 p. p.m. Placed on the same
drying yard they produced dried specimens of 300 and 860 p.p.m., respec-
tively. Further work is being done on this phase of the study.

The Favorable Gas Environment. — From a total of 75 field tests,
results comparable to those illustrated in figure 4 indicate that farm
practice should be directed to securing the sulfur dioxide environment
obtained by burning the quantities of sulfur and exposing the fruit for
the periods given in table 8. The favorable absorption data indicated in
the table, for freshly sulfured fruit, were secured in field trials under
reasonably good farm practice ; they would appear capable of material
improvement. An individual grower attempting to improve his sulfur-
ing practice should have an analysis made of his freshly sulfured fruit
and from these results modify the pan area, amount of sulfur, or ex-
posure period to secure the desired absorption. The analysis of freshly
sulfured fruit is used here to avoid the variable of sulfur dioxide loss
in the drying yard.

Laboratory tests using liquid sulfur dioxide at various concentrations

Bul. 636]

Sulfuring of Fruits for Drying

27

indicated that the higher concentrations may result in a more rapid ab-
sorption by the fruit. However, "fixation"13 of sulfur dioxide by the fruit
tissues also requires time; and a minimum exposure time — as yet unde-
termined but believed to be dependent on such factors as variety, matur-
ity, and composition — is essential to avoid excessive sulfur dioxide losses
in the drying yard.

Somewhat higher concentrations than those indicated in figure 4 can
be secured in burning sulfur by using burner pans of larger diameters.
Longer effective sulfuring periods are obtained by using larger quanti-

TABLE 8

Conditions of Good Farm Sulfuring Practice

(Using 10-inch-diameter pan burner and tight house)

Fruit

Pounds

sulfur burned

per 1 ,000

pounds
cut fruit

Sulfuring
period,
hours

Maximum SO2

concentration

in house,

per cent

Probable SO2

content of

fresh fruit after

sulfuring,

p. p.m.

Apricots

Nectarines

Peaches

4
5
5
13
2

3
4
4
30
2

2.5
2.5
2.5
1.0
2.5

4,000
3,200
3,200
2,000

1,500

ties of sulfur, in a burner pan of any given size. The sulfur dioxide con-
centration in the gaseous atmosphere drops to an ineffective low point
within about 1 hour after the sulfur has burned to completion and there
is little advantage in holding the fruit in the compartment longer if dry-
ing-yard conditions are favorable.

In the majority of the farm structures tested the gas environment
values were materially less than the optimum for good practice indicated
in table 8. The major causes for this were the use of earthen burning pits,
ineffective or baffled burners, poor-burning sulfur, improper ventilation,
and leaky houses.

TYPES OF SULFUR AND FACTORS AFFECTING
THEIR USE

The most commonly used source of sulfur dioxide gas is sublimed sul-
fur, burned within the sulfur compartment, although rock and granu-

13 By "fixation" is meant the result of those changes either physical or chemical, or
both, which in effect cause a bonding between the fruit substance and sulfur dioxide.
When "fixation" takes place, sulfur dioxide is more firmly held by the fruit in contrast
to the "free" sulfur dioxide that readily escapes from the fruit immediately after
sulfuring.

28

University of California — Experiment Station

lated sulfurs are occasionally used. In a few cases gas obtained from the
evaporation of liquid sulfur dioxide has also been tried.

Variability in Sulfur Burning. — During the harvest of 1936 unusual
difficulty was reported from several districts in getting the sulfur to
burn satisfactorily. Early the next spring about two dozen samples of
sulfurs remaining from the previous season, and stored over winter in
typical farm storage sheds, were collected. When these were burned in
clean metal pans in an empty sulfur compartment one group burned to

Fig. 5. — Illustrations of slag resulting from incomplete burning: A, Light-
colored slag indicates that the fire was smothered through insufficient ventila-
tion; B, dark-surfaced slag indicates that the fire was smothered by carbon or
carbonaceous matter or other impurities which floated to the surface of the
molten sulfur.

completion, or between 90-100 per cent; the remainder, comprising
about two-thirds of the samples collected, burned to only 10-40 per cent.
Invariably, the sulfur slag from the latter group was covered with a
black film or scum as shown in figure 5,5. Unsuccessful attempts were
made to correlate this incomplete combustion with the use of metal sul-
fur burners, insufficient ventilation, moisture absorbed from the air,
and texture and acidity of the sulfur. An admixture of the slag from a
poor-burning sample, however, was found to materially reduce the burn-
ing percentage of a sulfur which previously burned to completion.

Investigations pertaining to the composition of the black film and fac-
tors contributing to its formation and influence on the completeness of
combustion of sulfur were conducted in the laboratory of the Chemistry
Division by H. W. Allinger and C. S. Bisson. The information resulting
from their studies is summarized in the remaining paragraphs of this
section.

BUL. 636] SULFURING OF FRUITS FOR DRYING 29

Chemical tests on the slag obtained from some poor-burning sulfurs
showed that the black surface film consisted largely of carbon or car-
bonaceous matter with small amounts of iron compounds and silicious
material. Further experiments indicated that the film probably origi-
nated from the interaction of the hot molten sulfur (or hot sulfur va-
pors) with traces of certain organic impurities present in the sulfur or
the burner. Extraction of samples of poor-burning sulfurs with solvents
to remove oily contaminants increased the amount of sulfur that would
burn.

Laboratory tests were therefore directed toward determining the type
of impurities which contributed to this black film formation. The addi-
tion of very small amounts of different organic substances to samples of a
high-quality sulfur was found to decrease the percentage of sulfur
burned and to produce a film or scum similar in appearance and effect
on burning to that observed when contaminated sulfurs are burned. Of
the many organic substances added, the petroleum oils were found to
favor the formation of the black film or scum, and to decrease the per-
centage of sulfur burned. Small amounts of iron oxide and ignited dust
showed little or no effect on the percentage of sulfur burned. Samples
of a high-grade sulfur when stored for several days in an atmosphere of
vapors arising from fuel oil at room temperature were found to absorb
sufficient amounts of volatile carbon compounds to cause black film
formation, and to decrease the amount of sulfur burned. This would
probably have an important bearing on defining proper conditions for
the storage of sulfur.

By raising the temperature of samples of poor-grade commercial sul-
furs, which increases their volatility, such sulfurs can be made to burn
almost completely.

Later, in the field trials it was observed that farm practices were some-
times the source of sulfur contamination with resultant difficulties in
burning. Attempting to burn sulfur in oily pots or storing sulfur on oily
floors gave ample opportunity for contamination. In one instance sulfur
stored over winter in a farm garage burned poorly, owing presumably
to the automobile exhaust gases which had been absorbed.

Liquid Sulfur Dioxide. — In a comparison trial with liquid sulfur di-
oxide, 10 pounds of the liquid was used in a relatively tight house to
sulfur 1,000 pounds of Tilton and Royal apricots. A maximum gas con-
centration of 5.75 per cent sulfur dioxide was attained, and a concen-
tration of 2 per cent or more maintained over an exposure period of 3
hours. Chemically, 10 pounds of the liquid sulfur dioxide is the equiva-
lent of 5 pounds of burning sulfur. Theoretically, the gas formed from

30 University op California — Experiment Station

either would result in approximately a 33 per cent concentration if it
were all contained in the available gas space of about 180 cubic feet in
the one-car compartment under the standard temperature and pressure
conditions stated in the section on experimental procedure. In practice,
however, leakage and absorption reduce the theoretical values mate-
rially.

To compensate for the convection currents normally induced by the
burning of the sulfur, three 6-inch electric fans were used in the liquid
sulfur dioxide trial, but the distribution so secured was very poor. Tem-
perature on the trays ranged between 70° and 75° Fahrenheit. Analyses
of the fruit so sulfured averaged 4,600 p.p.m. in the freshly sulfured
samples and 450 p.p.m. in the dried product. The parallel check run in
which 4 pounds of sublimed sulfur was burned, and the exposure main-
tained at 3 hours, resulted in analyses of 4,050 p.p.m. and of 500 p.p.m.
At current market prices the sublimed sulfur is the more economical
source of sulfur dioxide gas.

Fruit treated with fumes from liquid sulfur dioxide showed little
"juicing" when removed from the sulfur house, but juiced as much as
the check lot after standing in the drying yard for a short period of time.
No other differences were noted in the two lots of fruit.

SULFUR BURNERS

In the majority of farm sulfur houses the sulfur is burned in an earthen
pit excavated just inside the door and between the car track rails. Nu-
merous trials with various burners demonstrated the desirability of
limiting the burning area of the molten sulfur so as to secure gas con-
centrations similar to those indicated in figure 4. This avoids dissipation
of the heat of burning, and also protects the burning sulfur from con-
tamination. Moreover, restricting the burning area also permits design-
ing a ventilation system which will provide an adequate supply of air to
maintain combustion without excessive loss of gas through venting.

Metal Pan Burners. — A tin pan, of 10-inch diameter and 3-inch depth,
makes a very satisfactory, easily cleaned sulfur burner. Setting this on
the floor in the open space between the front of the tray stack and com-
partment doors brings it close to the air inlets and gives it sufficient space
to avoid a fire hazard. The sulfur should be leveled in the pan, and ig-
nited with a match which is subsequently discarded to avoid carbona-
ceous contamination of the sulfur. The pan confines the molten sulfur
during the initial stages of burning and aids in maintaining the tem-
perature necessary for combustion. There appears to be little practical
value in insulating the pan burner to prevent the loss of heat, except in

Bul. 636

SuLFURING OF FRUITS FOR DRYING

31

the case of poor-burning sulfur. By raising the temperature of such sul-
fur through insulation, or by the use of regenerative burners, or of su-
perimposed pans, it may be possible to secure complete combustion in
spite of contamination. Such measures, however, are not recommended
with good-burning sulfur because of the danger of sublimation of the
sulfur and subsequent deposition on the fruit. If the floor of the house
consists of damp soil, a layer of gravel beneath the pan burner will mini-
mize chilling. The pan may be placed in a shallow pit, if desired, for fire
safety.

3.0
Ul

2

3 2.5

TE

:st

61

TEST

62

/

\

/

\

/

/

/

/

30 60 90 120

150 180 210 240 0 30 60
TIME IN MINUTES

90 120 150 180 210

Fig. 6. — Average results of two successive tests of a two-car parallel compartment
made on the same day. In test 61, 6x/2 pounds of sulfur were divided and burned in
two earth pits 21 inches deep. In test 62, the only change was to burn the same amount
of sulfur in two pans set in the pits. Under the higher gas concentration of the sec-
ond test the apricots absorbed 2,670 parts per million of sulfur dioxide, as compared
to 2,010 for the first.

Earthen Pits and Other Burners. — Several comparative trials showed
unmistakably the advantage of pan burners over the earthen pits com-
monly used. The results of one such trial are shown in figure 6. In the
earthen pits the molten sulfur is dissipated and cooled among the clods,
almost invariably leaving some slag. In one instance 69 pounds of waste
slag were removed from a burner pit after one season's operation, esti-
mated at thirty burnings (fig. 1). Hearths or shallow pits of concrete
give excellent results as sulfur burners, provided they are kept clean.
Pits lined with concrete pipe are difficult to clean. It is important that
the burner be so shaped as to concentrate the molten sulfur to a burning
surface area comparable to that of a 10-inch-diameter pan if comparable
gas concentration characteristics are desired. Stove burners outside the
compartment appear to offer little advantage other than a saving of in-
terior space.

Burner Baffles. — It is important in the natural-draft system that the

32

University of California — Experiment Station

surface of the burner not be constricted if free burning is desired. Plac-
ing a shield close above the burner, as is commonly done when the burner
is beneath the tray stack, materially reduces the rate of burning and con-

6.0
5.5
5.0

i 4.0

m 3.5
<?3.0

1

■EST

43

/

\

/

\

/

V

/

\

'"]

-\

/

/

5 ">

o
a 0.5

bJ

a.
0

TE

ST

50

p —

L-^

/

V

/

/

1

TEST 48

^^•^

-■*-

—■

*»«■

"

'~y

\

7

t

_TE

ST 49

*""

——"

30 60 90 120 150 180 210 O 30 60 90 120 150 180 210 240 270 300 330

TIME IN MINUTES
LEGEND

S02 CONCENTRATION

TEMPERATURE

u

60 I

!c

<r

50 id

a.

5

30

Fig. 7. — Tests showing the effect of burner baffles and fruit on the gas concentra-
tion and temperature in the sulfuring compartment. Baffles retard the rate of burn-
ing, and the fruit by absorption, buffers the sulfur dioxide to a lower concentration
in the compartment. The compartment vents consisted of two track inlet openings
and a 1-inch vertical outlet flue at mid-height of the rear wall. Test 43 shows the
concentrations in an empty compartment ; test 48 was made in a similar empty com-
partment but which had a sheet-metal burner baffle raised only 2 inches at the rear
edge of the burner. Test 50 corresponds to test 43 and test 49 to test 48 except that
in these two the compartments were filled with fruit. Wind directions were similiar
for the four tests; wind velocities ranged from 300 to 600 feet per minute for tests
43 and 48, and 0 to 125 feet per minute for tests 49 and 50. The apricots used in the
two latter tests absorbed 4,420 and 4,570 parts per million, respectively, more in 180
minutes under the higher gas concentration than in 300 minutes under a gas concen-
tration half as high.

BlJL. 636] SULFURING OF FRUITS FOR DRYING 33

sequently the gas concentration in the house (fig. 7). A preferable cor-
rection in such instances is to fasten a sheet-metal shield permanently
against the underside of the car, leaving the burner as unobstructed as
possible (fig. 10).

DESIGN OF THE SULFUR HOUSE

The ideal sulfur house is a compartment of suitable size and economical
construction made relatively tight against air infiltration. It may hold
one or more tray stacks, but must provide space for the sulfur burner
and channels for convection circulation about each stack and adjacent
to the open ends of the trays. For tight construction a ventilation system
will be required. A clean sulfur burner is essential.

In most drying yards the tray stack is piled on a flatcar with flanged
wheels, and run into the house on steel rails. The conventional farm sul-
fur house holds one stack of fruit trays piled as high as two men can con-
veniently reach, usually numbering about 25 trays. Some operators
prefer a lift car which can be removed, leaving the tray stack supported
within the compartment on horses or on ledges built along the side walls.
Judged by the effectiveness of sulfuring there appears to be no choice
between the two arrangements. Several types of houses used in commer-
cial practice are shown in figure 8.

House Capacity. — Smaller operators generally prefer the one-car
house as the more convenient ; larger operators find some economy of
construction in multiple-car houses. The latter types follow different
arrangements. Two-car tandem and parallel, four-car tandem, four-car
tandem-parallel, and seven-car tandem (the latter with a crosswise place-
ment of the tray stack rather than the conventional lengthwise ar-
rangement) are some of the designs which have been studied in this
investigation.

Type of House. — Small operators and those with young orchards
sometimes use a movable "lift-over" or "hood-type" house which can be
placed over tray stacks on the cars or wooden horses. Those built of paper
or panel board on a light wood frame as described by Beekhuis (1935)
are relatively short-lived and inconvenient to operate, but may be eco-
nomically warranted for very small-scale production.

A unique tunnel type, continuous-operation sulfur house was the sub-
ject of one field test. Cars of freshly cut fruit entered at one end and
were discharged some hours later from the opposite end. Sulfur burners
were placed at both ends and at mid-length. The large fruit capacity and
mode of operation did not appear conducive to economical practice or
to accurate control, particularly when starting or ending the day's run.

34

University of California — Experiment Station

The discomfort of the sulfur dioxide gas to the operators, moreover, was
deemed a serious fault.

Convection Channels. — The general fundamentals of all such struc-
tures are that, for economy of sulfur to be burned and satisfactory opera-
tion, the compartment shall be tight, and no larger than necessary to
cover the tray stacks and provide for the convection currents which dis-

Fig. 8. — Typical farm sulfur houses of the better types: A, Well-built wood
frame house with sloping front and rear walls, asphalt paper neatly applied
over wood sheathing, and top-hinged doors, counterbalanced; B, wood frame
house with overhanging roof and plastered interior; C, concrete house with ver-
tical sliding doors ; D, Concrete house with side-hinged doors in a removable wood
frame ; E, large-capacity wood frame houses with top-hinged doors, such as used
in pear drying; F, cumbersome overhead counter-balances for top-hinged doors
on shed-roofed houses; G, four-car house in tandem-parallel arrangement; H,
stucco and plaster houses of tight construction except for excessive ventilation.

Bul. 636]

SULFURING OF FRUITS FOR DRYING

35

tribute the sulfur dioxide gas from the sulfur burner. "With tight com-
partments, proper size and placement of sulfur burner, and adequate
venting, it appears possible to secure adequate gas concentration and
uniform distribution in any design or size of compartment. Since most
of the sulfur houses in use are of single-car design, and most farmers
and investigators favor the small house, this discussion will feature the
permanent structure of that type.

ABOUT 2 SO IN.
INLET VENT AT
EITHER TRACK

Fig. 9. — Longitudinal section of typical, well-designed sulfur house showing pan
burner, convection channels and ventilation openings. The numerals on the tray stack
indicate the relative positions of the gas-sampling outlets. With a tight ceiling the roof
shape is unimportant. Arrows indicate direction of the gas flow. The numbers indi-
cate points at which experimental gas samples and temperatures were taken.

The depth of the sulfuring compartment must allow for the length
of the tray stack, which is frequently staggered 3 inches to provide hand
holds on the trays, and a space above the sulfur burner at the front and
one at the rear wall to provide for convection currents, as indicated in
figure 9. If the compartment provides for cars in tandem, a spacer should
be used to insure a 3-inch clearance between tray stacks for the same
reason.

If the compartment is constructed too short to permit a free air space
above the burner, a pit 18 inches deep may be excavated under the front
end of the car for the burner and a fire-protective sheet-metal plate fast-
ened beneath the car as shown in figure 10. The convection currents from
this arrangement, however, are less positive in direction and the gas dis-
tribution slower than with the free-standing burner, and the trays imme-
diately above are subject to heating. A second corrective arrangement is

36

University of California — Experiment Station

a small addition for the burner built against the rear wall of the com-
partment.

It is essential that the convection channel about the tray stack not be
restricted ; an opening of about 3 inches across the top, down the back
wall, and under the tray stack is necessary for air circulation from the
burner and return. The rear-wall convection channel may be maintained
by placing a 2 X 4 inch stud vertically at the center of the rear wall as
shown in figure 9.

Fig. 10. — Location of the burner : if the compartment is too short to permit a free-
standing burner, it may be placed in a pit under the car which is protected with a
metal shield. A second method of correction is to construct a tight addition at the rear
for the sulfur burner, such as shown in the illustration at the right.

A horizontal ceiling is recommended, in preference to leaving the
house open to the roof, as this reduces the gas volume and facilitates con-
vection about the tray stack. The shape of the roof itself is unimportant
except that some protection may well be given to the doors. The wide
overhang shown in figure 8, B, however, is probably more than is eco-
nomically justified.

The width of the compartment also should be reduced to a minimum ;
a clearance of 2 to 3 inches on either side of the tray stack should be
adequate to permit ready placement or removal of the cars.

Tight Construction. — A fundamental requirement of the compart-
ment is that it must be tight against air leakage which would cause drafts
and disturb the sulfur dioxide gas distribution about the tray stack ;
such construction is shown in figure 9. This requires tight sheathing and
that all construction joints of the structure be sealed, the larger ones
with mastic. Painting the interior surfaces of the structure with asphalt
or other acid-resistant paint seals small cracks. The light, wood-frame
structures covered with sacking as described by Jewell (1927a and
1927&) are much too porous to hold a desirable gas concentration. Lining
the compartment with asphalt building paper or plywood with sealed

Bul. 636]

SULFURING OF FRUITS FOR DRYING

37

joints is also practiced, and is one of the most satisfactory methods of
correcting badly deteriorated wood frame houses. The structure should
be so tight that no light sources other than the vents are visible to a per-
son within the closed compartment, and that fumes can be seen escaping
from only the vents of a house in operation. Since the door is difficult to
keep tight in its frame, it is necessary to use a replaceable weather strip-

120 150 180 0 30 60 90
TIME IN MINUTES

20 150 180

Fig. 11. — The tightness of the door can be a critical factor in an otherwise tight
house, as shown in the results of simultaneous tests on two compartments of the house
shown in figure 8, C. The superior gas concentration secured in the compartment as
illustrated by the graph at the right was achieved by plugging the 1% -inch- diameter
outlet vent at the top center of the rear wall and the ^4-inch crack behind the top of
the vertical sliding door. The door crack alone had opened about 10 square inches of
leakage area. The large amount of sulfur burned (8% pounds) accounts for the con-
centration curves being much higher than would normally be expected from the earth
burner-pits used. The sulfur dioxide absorption by the apricots in the untightened
compartment was 3,140 parts per million, and by those in the tightened compartment,
3,780.

ping or some similar pliant material along the door jamb to form a tight
joint when the door is closed. The results obtained by tightening the
doors of the house illustrated in figure 8, C are shown in figure 11.

Type of Door. — The type of door is often a matter of controversy. The
ordinary door hinged at the side is sometimes criticized as being in the
way of car movements, but the field experiments failed to substantiate
this claim, particularly if the house was set back far enough from the
cross track to permit free operation of the door. Advantages are sim-
plicity of construction and operation, and the ease with which it may be
weather-stripped. A door hinged at the top to swing up is in a con-
venient position when opened, but counterweighting and the horizontal
position subject the door to racking and early deterioration. Overhead

38 University of California — Experiment Station

door supports as illustrated in figure 8, G add to the life of top-hinged
doors ; a counterweight and cable balance as shown in figure 8, A causes
less racking of the structure than the overhead balances shown in figures
8, F and H. The vertical sliding door tends to rack the entire structure,
and is more difficult to weather-strip, particularly at the crack between
door and ceiling. Regardless of type, the door must be well braced to hold
its shape, and should have roof or flashing protection to prevent roof
water draining directly on the door. Eccentric or wedge fasteners, such
as refrigerator door fasteners, should be used to force the door to a tight
fit against the jamb weather-stripping. Door hardware should be painted
with bituminous paint to protect it against acid attack.

Natural Ventilation. — Since the amount of air within a closed com-
partment is not sufficient to burn the sulfur necessary to accumulate and
maintain the desired sulfur dioxide concentration, it is obvious that some
ventilation must be supplied. If it has been possible to tighten the com-
partment against leakage beyond the critical ventilation point and no
ventilation is supplied, the burning sulfur will smother. The color of
the sulfur slag formed under such conditions will be a bright sulfur yel-
low, in contrast to the continuous black coating over the slag resulting
from sulfur smothered by impurities as illustrated in figure 5.

Tests were made of several types of natural-draft ventilation for tight
houses, including outlet and inlet holes of various areas and locations,
and vertical outlet flues. Ceiling outlets similar to those described by
Cameron and associates (1929) gave satisfactory results. It was con-
cluded, however, that the simplest adequate system for the one-car house
using a 10-inch-diameter burner pan consists of inlets of not more than
2-square-inch area each at both track openings under the door, and two
outlets of 1-inch-diameter holes placed, respectively, at the top center of
the door and of the rear wall. When the burner pan is set in a pit, a 2-inch
pipe sloping down from in front of the door to the pan level may be used
as an air inlet instead of the track vents. These systems are relatively
free of backdrafting or any gas disturbance within the compartment,
regardless of the direction or velocity of the wind. If a strong wind is
blowing directly against either the front or rear wall, closing the outlet
on the windward side is desirable to reduce drafts on the interior. If
the wind is blowing against the front of the house, it is also advisable to
partially close the inlets. With a tight house and ventilation system of
this type the orientation of the house with respect to the wind has little
effect on the conditions within the compartment. The results to be ex-
pected from such a house under practical operating conditions will be
comparable with those illustrated in figure 4.

Bul. 636]

SULFURING OF FRUITS FOR DRYING

39

If it should be found desirable to increase the sulfur dioxide concen-
tration by more rapid burning from a larger-area sulfur burner, it will
be necessary to increase the size of the ventilation system in proportion.
An incomplete series of controlled tests on this phase of the problem indi-
cate that the total outlet area should be about %0 °f that of the free-
burning surface area of the sulfur. It is easily possible to get the

LEGEND

--• I. TOP FRONT

•— 2. TOP REAR

3. BOTTOM FRONT

— 4 BOTTOM REAR

CONDITIONS FOR THE TEST

f 300

t 200

U

O

w 100

30 60
TIME

90 120 150
IN MINUTES

180

^

■sil'si ^l^sa.

^L

WIND DIRECTION

yT

:mperature

/

x_

A

/

\

A

<-i

"A

WIN
VELO

° \

CITY)

V.,

< <r

a. i

eo o! 25

5

30 60
TIME

90 120 150
IN MINUTES

Fig. 12. — Excessive venting may cause drafts. The results shown above were se-
cured in the new, plastered house, shown in figure 8, H. Inlet vents of a 4-inch tile
under the door and four 1-inch diameter holes at the bottom of the door, and an out-
let vent consisting of a 1%-inch pipe through the rear wall at mid-height, were used.
In addition there was appreciable leakage between the door and jamb. The draft
sweeping through the lower part of the house resulted in a definite gas stratification.

ventilation system too large ; keep the area just adequate to completely
burn the sulfur. The results of excessive and improperly designed ven-
tilation in the house illustrated in figure 8, H are shown in figure 12.

Forced Ventilation. — A limited number of observations and tests were
made of forced-draft ventilation in which air was supplied under a slight
pressure to the sulfur burner by a mechanical system, with no provision
for outlets other than leakage. In no instance were the results superior
to those of an adequate and well-designed natural-draft system, when a
good-burning sulfur was used. Forced draft, however, permits the use
of rock sulfur and of the poorer grades of sublimed sulfur which do not
burn readily under natural-draft conditions. The economic advantage of
a fan to burn these cheaper grades of sulfur is questionable. Typical
forced-draft equipment is shown in figure 13.

Some tests were also made of a forced convection system in which small

40

University of California — Experiment Station

electric fans placed within the compartment augmented the natural
convection. The advantages secured with this system over those of
natural convection in a properly built house are so slight that they do
not justify the added expense.

Construction Material. — The material used in the construction of a
house must permit construction which will remain tight under the haz-
ards of use and weather. The use of large sheet materials with few

Fig. 13. — Fan installations for mechanical ventilation to the sulfur burner.
A, Large installation for multiple houses. B, Small-scale installation mounted
on the ground near the front of the house. C, installation for a single large house,
mounted on the roof. The installations tested did not give results superior to
properly designed natural draft; such equipment may, however, permit the
burning of the cruder forms of sulfur.

construction joints appears more favorable than that of those with
numerous joints. Concrete, brick, plywood, sheet steel, and matched
lumber sheathing covered with asphalt building paper on a wood frame
have been used successfully.

Many observations and tests made on sulfur houses constructed of
different materials, including the experimental houses shown in figure
14, failed to show an outstanding operating advantage for any one,
provided the construction was tight. Masonry structures have some
effect in maintaining a more uniform interior temperature, generally
tending to lower temperatures during the warmer part of the day and

Bul. 636]

SULFURING OF FRUITS FOR DRYING

41

higher at night. The plywood and steel structures permitted more ready
conduction of solar radiation, and their interior temperatures tended to
follow the diurnal atmospheric temperature. Under the test conditions,
however, these effects were not of sufficient magnitude to have any
appreciable effect on the temperature of the fruit ; heat from the burning
sulfur was a factor of greater importance in this respect.

Masonry and steel structures have the advantage of being fire-resis-
tant. Plastered structures as a class were among the tightest of the farm

.

«*»»««»*£

\

•▼"

1 "

f ■ '

$

; \

S"

h [U

m V •...-.■

vm

mm

Wmmmm^

Fig. 14. — Experimental sulfur houses of different materials erected near
Aromas. From left to right they are: a plywood house of shop-fabricated
panels ; an old farm duplex with one compartment remodeled with plywood ;
a shop-fabricated steel house ; and a concrete house made of precast panels.

structures tested ; heavy annual maintenance is required, however, ow-
ing to the destructive action of acid on the plaster.

Painting. — It is recommended that structures, regardless of the ma-
terial, be given two bituminous or other acid-resisting paint coats on
the interior to protect them against the severe acid conditions incident
to their use, and to decrease leakages. Steel and concrete surfaces, in-
cluding plaster and mortar joints in brick construction, are susceptible
to early attack by the acid and should be painted two coats with a good
grade of gilsonite asphalt or other acid-resistant paint. Houses with
wood siding should be painted on the exterior to prevent weathering
and the formation of shrinkage cracks. A good grade of exterior lead-
and-oil paint should be used ; plumbago paint has been used in some in-
stances as being more resistant to the sulfur dioxide gas.

Moisture Condensation. — When fruit sulfured in the evening was
permitted to remain in the experimental steel and plywood houses all
night, condensation formed on the walls and ceiling; no moisture was
apparent at any time in the concrete house. Such condensation may be

42

University of California — Experiment Station

a factor augmenting the destructive action of the sulfur dioxide gas
on the construction materials.

Remodeling Old Houses. — The house should be built on a concrete
foundation in preference to mud sills to gain greater rigidity and free-
dom from cracks. It is essential that the joint between foundation and

Fig. 15. — Sulfuring houses embodying the principles of good design. Plans and
specifications for these are available for a nominal fee from the Agricultural Exten-
sion Service, University of California, Berkeley, California. The upper picture
shows a gable-roofed three-compartment house, listed as plan C-173; the lower is a
three-compartment, flat-roofed plywood structure, plan C-194.

BUL. 636] SULFURING OF FRUITS FOR DRYING 43

sill be sealed with asphalt mastic to prevent air infiltration. A concrete
floor is an aid in maintaining sanitary conditions.

Steel rails used for car tracks are durable and more satisfactory than
wood rails lined with strap steel.

Old sulfur houses which are structurally sound can be reconditioned
by sealing the interior, tightening the door, and reroofing. For the in-
terior treatment, plastering, or lining with asphalted paper, plywood
or light-gauge sheet steel followed by proper painting will prove satis-
factory.

New Construction. — Plans for two designs of sulfur houses are avail-
able, as shown in figure 15. u Structures erected from these plans have
been built and tested under farm operating conditions, and found to
meet the standards outlined in this publication.

RETENTION OF SULFUR DIOXIDE BY THE FRUIT
DURING DRYING

Of the sulfur dioxide held by the fruit when it leaves the sulfur house
only a fractional part remains at the end of the drying period. The
greatest loss occurs during the first 24 hours of drying-yard exposure.
The graphs shown in figures 16 and 17 indicate the change in the pro-
portion of sulfur dioxide in the fruit at different periods during drying.
The sulfur dioxide retention has been plotted on a logarithmic scale so
that the relative slopes of the curves may better illustrate the rate of
change in this proportion for the different stages of drying. It has been
determined that the losses are minimized by rapid dehydration, and
by favorable drying-yard conditions. It is desirable to secure as rapid
drying as possible, especially during the first half day. The fundamental
conditions favoring rapid drying are heat and dry air passing over the
trays.

Relation of Loss of Moisture and of Sulfur Dioxide. — The observation
stated above is in keeping with the observations of Roleson and Nichols
(1933) and Hanus and Vorisek (1937) that the sulfur dioxide content
of fruit after it has been exposed in the drying yard for one day will
approximate the retention when dried. During the first 24 hours the
loosely held sulfur dioxide tends to leave the fruit rapidly. Heating of
the fruit by solar radiation and the drying of the fruit surface appar-
ently increase the fixation of sulfur dioxide and make its escape from the
fruit more difficult. After this initial period the rate of loss of sulfur
dioxide decreases, the rate being greater from the moist fruit and slow-

14 Plans available from Agricultural Extension Service, University of California,
Berkeley, California. (Price 25 cents per plan.)

44

University of California — Experiment Station

ing as the drying approaches completion. The rate of moisture loss tends
to follow a similar trend.

The sulfur dioxide content of fruit is expressed in parts per million
as related to the weight of the drying or dried fruit at a given time. In

3000

b 2000

z

ui
Id

a 1000
w 900

800

1

»

\

\

\

\
\

•

\ s1

FRUIT St

ILFURED WIT*
ULFURED Wll

LIQUID SOg
H BURNING

SULFUR

^

\
\

\

N

\

^"**"^«

•

__U

\

^

k

ie~ ■

^,

•«— -.

rnpfe

— 22».

""* — M

6A.M. 6 PM.

Fig. 16. — Five tests showing the loss of sulfur dioxide from drying apricots under
natural drying-yard conditions. Fruit sulfured with liquid sulfur dioxide for a short
time evidently had low fixation.

&

1000

900
800
700

•

\
\

\

\

•

•

\
\

v \

(

m LIQUID
TH BURNING

SULFUR

^

?Q

1,

26

K>-

____28__

sr

26

'1

29

"-"27

i

26

6A.M. 6RM.

Fig. 17. — Four tests showing loss of sulfur dioxide from drying pears under typical
drying-yard conditions. Fruit sulfured with liquid sulfur dioxide evidently had low
fixation.

Bul. 636]

SULFURING OF FRUITS FOR DRYING

45

most of the fruit-drying areas the loss of weight in water is sufficient
after the first 24 hours to tend to keep the ratio for sulfur dioxide con-
tent almost constant. In the Aromas apricots and in the Lake County
pears this ratio was not maintained constant but declined at a much

TABLE 9
Sulfur Dioxide Retention in Dried Fruit as Affected by Variety and Locality

Fruit and variety

District

Year

Number
of

tests

Sulfur
dioxide
in freshly
sulfured
fruit,
p. p.m.

Sulfur
dioxide
in dried
fruit,
p. p.m.

Retention
ratio

Apricots:

Blenheim

Blenheim

Blenheim

Blenheim

Hemskirke

Moorpark

Royal

Royal

Royal

Royal

Royal

Royal

Royal

Tilton

Tilton

Tilton

Tilton

Nectarines:

New Boy

Peaches (clingstone) :
Paloro

Peaches (freestone) :

Lovell

Muir

Pears :

Bartlett

Raisins (sulfur bleach)
Thompson Seedless.

Vacaville

Davis

Aromas

Hollister

Hollister

Hollister

Hemet

Upper Ojai Valley

Ventura

Suisun Valley ....

King City

Aromas

Aromas

Davis

Hollister

Aromas

Aromas

Gridley

Escalon

Brentwood

Esparto

Kelseyville

Kerman

1937
1937
1938
1937
1937
1937
1938
1938
1938
1938
1938
1938
1939
1937
1937
1938
1939

1937

1937

1937
1937

1938

1938

4,005
4,250
2,230
3,475
2,400
1,970
2,730
1,970
4,160
2,720
3,253
2,130
2,781
4,340
2,670
2,480
2,056

2,030

2,510

1,860
2,800

2,760

2,365

1,978

1,970

715

1,085

1,254

1,035

2,800

1,225

1,790

1,052

1,104

600

420

2,350

1,090

565

309

1,150

1,630

1,040
1,530

1,106

1,020

0.49
0 46
0.32
0.31
0.52
0.52
1.02
0.62
0.43
0 39
0.34
0.29
0.15
0 54
0.41
0.23
0.15

0.57

0.56
0.55

0.41

0 42

slower rate after the first day ; this is shown in figures 16 and 17. In the
Hemet district the moisture loss is sufficiently more rapid than the sulfur
dioxide loss to cause an apparent gain in the sulfur dioxide content, as
shown in the analysis and retention ratio of table 9.

Expressing the analyses as actual contents on a moisture-free basis
rather than as ratios would have shown a continual sulfur dioxide loss

46

University of California — Experiment Station

from the time the fruit left the sulfur house for both Aromas and Hemet,
the difference being a much smaller and slower loss for the latter. The
ratio expression is preferred for this investigation as it is commonly used
in the dried fruit industry. A more accurate picture of the various phe-
nomena concerned would be secured if the analyses were expressed on a
moisture-free basis.

Effect of Climatic Differences. — The climatic differences of the various
districts particularly with respect to temperature and relative humid-

TABLE 10

Comparison of Sulfur Dioxide Losses from Freshly Sulfured Apricots Held in

Tray Stacks Overnight with those Spread in the Evening; Aromas, 1939

Fruit and test conditions

Absorption,
p. p.m.

S02
content of
freshly
sulfured
fruit next
morning,
p. p.m.

Per cent

S02 lost

overnight

SO2

content of

dried fruit.

p. p.m.

Retention

ratio,
dried fruit

Royal apricots; kept in house overnight;
door left open from 8 :00 p.m ; night fog .

Tilton apricots; kept in house overnight
with door ajar from 8:30 p.m; night fog.

Tilton and Blenheim apricots; held on
car on track overnight from 7:00 p.m;
night fog

2,325

3,185

2,635
2,330

2,330

1,510
1,830

1,256
1,100

1,260

35
43

53
53

46

530
475

415
355

330

0.23

15

.16

Roya! apricots ; spread on drying yard at
6:00 p.m. as fog rolled in

Royal apricots (same lot as preceding)
spread at 9:30 a.m. after remaining in
opened sulfur house overnight

15
0.14

ity materially affect the retention ratios, as shown in table 9. A compari-
son of these ratios serves as a relative rating of the drying effectiveness,
or drying-yard efficiency, in the different districts. The drying condi-
tions in Aromas in 1939 were not so favorable as during 1938, and this
is reflected in the retention ratios for the two years. Considerable differ-
ence in retention ratio may be secured in any given district or between
districts of similar climate because of the variations in the initial sulfur
dioxide absorption by the fruit. The higher the initial absorption analy-
sis for any given drying condition, the lower the retention ratio. Hence,
it is obvious that the fixation of sulfur dioxide by the fruit is not directly
proportional to the absorption. Furthermore, it will be observed that
quickly sulfured fruit of high initial sulfur dioxide content loses this

Bul. 636]

SULFURING OF FRUITS FOR DRYING

47

at the greatest rate ; this is, presumably, attributable to the fixation-time
factor previously mentioned.

Placing of Fruit on the Drying Yard in the Evening. — Growers fre-
quently hold sulfured fruit overnight in the sulfur house rather than

TABLE 11

Effect of Location of Drying Yard Within a District on Retention

of Sulfur Dioxide; Aromas, 1938

Test no.

Location

S02

absorption,

p. p.m.

Absorption
factor

SO2 in

dried fruit,

p. p.m.

Retention
ratio

13a

1,550
2,120

1,060

2,380

31.0
10.9

10.1

13.2

670
940

830

650

0 43

14a

.44

15a

Notch between two hills, subject to

.78

16a

Protected hollow on river bank, in al-

0.27

TABLE 12

Effect of Ground Cover in the Drying Yard on Sulfur Dioxide

Retention by Apricots

Drying-yard condition

S02

content of

freshly

sulfured

fruit,

p. p.m.

SO2

retention,

p. p.m.

Retention
ratio

Green grass, 8 to 10 inches high . ,

Trays laid on grass

Trays supported 2 feet above

2,910
2,260
3,200
3.340

490
510
640
850

0.17

.23

Trays laid on 2-inch rails, flat
on the ground

Trays supported 2 feet above
k the ground

.20

0 25

spread it on the drying yard in the late afternoon or evening. While
considerable sulfur dioxide loss takes place from fruit so held in the
house it is not so great as that from fruit spread during a cool, humid
evening. In developing the individual test data for table 9 it was noticed
that invariably those samples spread after midafternoon had retention
ratios below the average for that district.

If the fruit is held in the tightly closed house overnight, it frequently

48

University of California — Experiment Station

juices enough to run out on the tray. This causes a loss in weight of the
dried product, impairs the quality grade, and increases the labor of
tray cleaning. Most operators follow the practice of opening the door
slightly, or of partially removing the car from the house, as they believe
that this reduces juicing, presumably because of the cooling of the fruit.

no

100

90

cr

x

£ 80

70

60

50

40

30

r

/

\

•

s

//

k s

V

\

\

A

\\

\\

Y

\

\\

/

\\

>*

[/

V

-^

A

r

*'

— —

' "~ ■■

FRU

AIR

IT
TEN

■EMF
1PER

ERA

ATUF

■URE
E A

r t

«AY

>URF/

,CE

i

12 I 2345676

P.M.

HOURS ON JULY 25

8 9 10 II 12 I 2 3 4

A.M. RM.

HOURS ON JULY 28

Fig. 18. — Typical Aromas drying-yard conditions for two days, showing the rela-
tive internal temperatures of the fruit at different stages of drying with relation to
adjacent air temperatures. The freshly sulfured fruit was placed in the drying yard
at noon. Evaporative cooling at the cut surface of freshly sulfured fruit holds the
internal temperature below the air temperature until the surface has dried. The di-
vergence between temperature of air and partially dried fruit on the 28th was min-
imized by clouds.

Comparative tests indicating the losses to be expected from this pro-
cedure and from evening spreading in the drying-yard, are summarized
in table 10. Although these results were from only one test for each
condition, it would appear desirable under such conditions to delay
sulf uring until about midnight and to open the house as early next morn-
ing as possible to prevent excessive juicing.

Securing High Retention Ratio. — Drying-yard factors favorable to
high retention ratios include : (1) spreading the fruit in the drying-yard

Bul. 63C

SULFURING OF FRUITS FOR DRYING

49

immediately after sulfuring to secure full sun exposure and (2) a mod-
erate, arid breeze to evaporate any juice from the cups and the surface
moisture in 3 hours or so. Shade, fog, or location near bodies of water
or green vegetation all retard drying and reduce sulfur dioxide retention.
Table 11 gives comparison data of one test sample each from four
privately operated drying yards in the Aromas district, located within a
1-mile radius. Those for tests 13a and 14a were hillside locations ; the
retention ratios of these two were almost identical. Test 15a was on a
drying yard in a "notch" between two peaks and subjected to strong air
currents. The high retention ratio, however, is believed to be partially

TABLE 13

Effect of Tray Color on Sulfur Dioxide Betention by

Tilton Apricots

Color

S02

content

of freshly

sulfured

fruit,

p. p.m.

SO2

retention,

p. p.m.

Retention
ratio

Black

White

3,165
3,165
3,165

400
270
420

0 13

.08

0 13

due to the fact that the test fruit was held overnight in the sulfuring
compartment and subjected to leakage from an adjacent compartment,
with a higher fixation resulting. Test 16a is from a lowland drying yard,
near a river. The humidity from the heavy vegetation along the river
and from the alfalfa covering the drying yard undoubtedly delayed
drying and so reduced the retention ratio. Similar undesirable results
from green vegetative ground cover in the drying yard were also noted
in one instance in the Hemet district. The high absorption ratio for test
13a is believed to be due to the exceptionally low gas enviroment main-
tained during sulfuring.

Further evidence of the influence of drying-yard environment on the
retention of sulfur dioxide by the fruit is given in table 12. These data
are from one test series of Aromas (1939) , hence the influence of humid-
ity close to the ground and above dried grass is more pronounced because
of the night fogs than would be the case in interior-valley yards. For
such climatic conditions, or on drying yards covered with green grass,
setting the trays on wooden horses 2 feet above the ground will prove
beneficial.

50

University of California — Experiment Station

Fruit Temperatures During Drying. — When fruit is first exposed to
the sun the ready evaporation of moisture maintains the internal tem-
perature of the fruit below that of the air at the tray surface. Later the
slower evaporation and darker color of the fruit (and consequently

TABLE 14

Comparison of the Effect of Sun-drying and Dehydration on Eetention

of Sulfur Dioxide by Apricots ; Aromas

Treatment

S02

retention

p. p.m.

Per cent in-
crease in SO2
retention of
dehydrated
over sun-
dried

Retention
ratio

Sulfured with liquid SO2 for 95 minutes at 4 per cent concentration

Freshly sulfured fruit

Sun-dried; spread 5:00 p.m.

Dehydrated

Sun-dried; spread next a.m.
Dehydrated

Sulfured under typical farm conditions

Freshly sulfured fruit.

Sun-dried

Dehydrated

Freshly sulfured fruit

Completely sun-dried

Sun-dried 24 hours

Dehydrated, after sun-drying 24 hours.

Freshly sulfured fruit

Sun-dried

Sun-dried, after dehydrating 1 hour —

Freshly sulfured fruit ,

Sun-dried

Dehydrated, after sun-drying 3 hours.
Dehydrated

Freshly sulfured fruit

Sun-dried

Dehydrated, after sun-drying 3 hours.
Dehydrated

greater absorption of solar radiation) result in internal temperatures
rising above the air temperature during the hours of solar radiation, as
shown in figure 18.

Effect of Tray Color. — The color and surface texture of the tray is a
factor in the heating of the fruit, the darker colors being more effective
in absorbing solar radiation and transmitting it to the fruit. This is

Bul. 636]

SULFURING OF FRUITS FOR DRYING

51

o

f-8

co

CM

CO

0

CM

-2 ^

0

O

« ,

S02
retentio
in sun
dried
fruit,
p.p.m,

O
co

CM

10

O
CM

**<

CO

d

8

SO2

orpti
tresh
Ifure
ruit,
.p.m

0

>o

O

"tfl

<M

lO

t-~

00

co

CM

d

|.a«"a

d

m

ca d a> g
o'>> 6 6?

10

>o

2

in

"

"

Moisture
content
in dried

fruit,
per cent

t^

CO

CM

O

CM

CM

d

0

'■+3 0

02

CO

CO

O

d +3
CD 03

CO

>o

CD

CD

0

O

C .'

S02

entio
deh.v
rated
ruit,
.p.m.

0

00
CM

0

CO

CO

O

O
IO
O

-5 d-d*" a

^"H

C >*.

q^

S02

orpti
fresh
Ifure
ruit,
.p.m

O

£

O
O

O

CM

CO

co

co

CM

'-I

03 3 t*"1 Q,

-a d w

ea—

,-h M - to

cj C a) ^

10

10

O

O

Tot
dryi
tim
hou

0

10

«o

CO

CM

"-1

co

2

03

listure
ntent
dried
ruit,
r cent

t-

co

0

CM

CO

CO

d

_o

§ °-S ft

"c3

.

>>

<n >>+>

,d

>."S d

O

10

10

0

<B

-d

'-£-d g

CM

CM

CM

Q

.2 d

,1,

J^

CD

IO

s- 0)

ft O

W^ft

bfi to

d -S

•S d

lO

«0

O

O

>> a>

a

CO

IO

UO

q"

i

0

•O

Ui

H

CD

<B >»-m

>.-d d

'42 "0 «

00

0

IO

0

-d
0

15 S <D

•*<

"*

co

co

ft co

tf^ft

2-§

O

U5

O

«o

d

CO

10

CD

■rt co

O

10

CD

CD

H

IO

CD

co

rd S

0) ^

•-1

"-1

'"H

S3

<N

CM

CO

CO

• - O

HJ3

52 University of California — Experiment Station

shown in the one test itemized in table 13. During the progress of this
experiment it was observed that air temperatures on the tray surfaces
were 12° to 18° Fahrenheit higher at midday on the black-painted sur-
faces than on the white. Those on the naturally weathered brown tray
surfaces were within 6° of the black-surface temperatures. Fruit tem-
peratures exhibited the same general differences, but tended to range
5° to 12° below their respective tray temperatures for the first 5 days
of drying because of evaporative cooling.

Dehydration. — The apparent effect of rapid drying in reducing the
sulfur dioxide loss, at least during the initial stages, led to a limited
number of dehydrater trials, all with favorable results. The data shown
in table 14 give the scope of one trial series designed to show the relative
effect of complete dehydration, and of partial dehydration taken either
as the initial drying period or delayed until after an initial sun exposure
intended to fix a more attractive color. The retention ratios of the de-
hydrated products in every case show an appreciable advantage, al-
though all initial sulfur dioxide contents were low. The moisture loss
in the dehydrater was sufficiently rapid to give an apparent gain in
sulfur dioxide content, but obviously lower total retention, compared to
that in the fruit put in from initial drying-yard exposures. This is
similar to the situation existing in the arid Hemet drying yards, pre-
viously described.

Table 15 lists the dehydrater data for a series of runs in which the
drying was expedited by preheating for 2 to 4 hours in the dehydrater
with the vents closed. This brought the fruit up to the dehydrating
temperature more rapidly and so reduced the time requirement for
drying.

Apricots immediately after dehydration appeared somewhat lighter
in color and with slightly less luster than the drying-yard product. In
the 1939 tests dehydrated fruit after three months' storage had attained
a very attractive color, markedly superior to the darkening drying-yard
fruit produced in the check tests.

SUMMARY

The results secured in this investigation on the sulfuring of fruits for
drying indicate that there are three phases of the problem: (1) The
absorption of sufficient sulfur dioxide by the freshly cut fruit to allow
for normal drying and storage losses and still maintain a high-quality
fruit; (2) the maintenance of favorable drying conditions with low
loss of sulfur dioxide ; (3) storage at cool temperatures and moderately
low humidities to retain the sulfur dioxide during storage. Only the

BUL. 636] SULFURING OF FRUITS FOR DRYING 53

first two of these are within the scope of the present investigations, since
they are of immediate concern to the grower. The conclusions and ob-
servations based on the survey and experiments are summarized in the
following paragraphs.

The sulfur house should be economical and durable and designed to
promote rapid, uniform sulfuring. In a house of sufficiently tight con-
struction to prevent drafts, vents will be required to provide air for
burning the sulfur. For the type of vents recommended, a fixed ratio
must be maintained between the vent area and the surface area of the
sulfur burner. Doors hinged at the side and secured with refrigerator-
type latches are preferable to vertical sliding doors or those hinged at
the top, when the door opening does not exceed 4 feet. For wider open-
ings a door hinged at the top and secured against the jambs with refrig-
erator-type latches is preferable. The interior surfaces of sulfuring com-
partments should be painted with acid-resistant paints, regardless of
the construction material, for durability and to tighten the structure
against air leakage.

An old, leaky sulfuring compartment giving unsatisfactory results
should be remodeled or replaced. Plans for new sulfur houses are avail-
able through the Agricultural Extension Service, University of Cali-
fornia.

Sulfur should be burned in a clean metal pan of 10-inch diameter.
Clean concrete hearths of equivalent area are satisfactory burners. Un-
lined earthen pits are unsatisfactory. Insulated, regenerative or forced-
draft burners may be desirable for burning the poorer grades of sulfur,
or those carrying contaminants.

Laboratory experiments have demonstrated that the black film or
scum from some low-quality sulfurs consists chiefly of carbon or carbo-
naceous matter. Of the various kinds of organic materials added to test
samples of a high-quality sulfur, petroleum oils were found to cause
the formation of a black surface film most rapidly, and thus decreases
the percentage of sulfur burned. By increasing the temperature of poor-
burning sulfurs to cause more rapid volatilization, it was shown that
poor-quality sulfur could be burned completely.

A good grade of refined sulfur is recommended as being more economi-
cal 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. If an appreciable amount of slag remains,
the reason may be surmised from its color. Proper handling and storage
of the sulfur supply is essential ; it must not be permitted to come into
contact with oily surfaces or vapors.

54 University of California — Experiment Station

The temperatures at various points in the sulfur house differ con-
siderably during sulfuring, commonly showing a difference of 20°
Fahrenheit between ceiling and floor. Sulfuring fruit at the relatively
high temperatures of 100°-120° tends to decrease absorption but in-
creases retention of sulfur dioxide. Fruits sulfured at high temperatures,
however, bleed more readily than when sulfured at lower temperatures.

The freshly sulfured fruit should be dried as rapidly as possible.
Every possible advantage should be taken in the location of the drying
yard, and in placing the fruit on the drying field, to maintain conditions
favorable to rapid moisture evaporation.

Dehydration offers definite advantages in coastal areas where climatic
conditions during the drying season are unfavorable to the production
of a high-quality product.

Size, variety, and maturity as well as the districts in which the fruit
is produced and dried are important factors in determining the absorp-
tion of sulfur dioxide during sulfuring and its retention during drying.

ACKNOWLEDGMENTS

The authors wish to acknowledge the assistance received from the
Pomology and Chemistry divisions of the College of Agriculture ; Dr.
loan Radu, Institute for Agricultural Research of Rumania, Bucha-
rest ; Messrs. H. A. Beekhuis, H. Bedford, B. L. Hatherell, and Farrell
Stone of the California Dried Fruit Association ; farm advisors R. D.
McCallum, V. W. De Tar, M. M. Winslow, V. F. Blanchard, and T. W.
Thwaits; F. E. Twining and Max Bartholomew; Dr. Maynard Joslyn;
the Douglas Fir Plywood Association ; the Portland Cement Association ;
the Soule Steel Company ; and numerous growers and drying-yard op-
erators.

BlJL. 636] SULFURING OF FRUITS FOR DRYING 55

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56 University of California — Experiment Station

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