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Washington, D. C. July 3, 1923
PRODUCTION OF SIRUP FROM SWEET POTATOES.
By H. C. Gore, Chemist in Charge, Fruit and Vegetable Utilization Laboratory,
b H. C. REESE, Assistant Development Engineer, and J. O. Rrep, Assistant
Development Engineer, Office of Development Work, Bureau of Chemistry.
CONTENTS.
Page. Page.
Sweet-potato by-products__________ 1 General properties of sweet-potato
Commercial possibilities for sweet- fy PEER tee ee 2
PaLhOm HIRU— =. 8 ee 2 ee 3 | Composition of sweet-potato sirup__ 32
Experimental work on sweet-potato Composition of sweet-potato pomace_ 32
DS De eS oe ee eee APPS ummeary + 3 Se ee oa 33
Production of sweet-potato sirup on
Eansmereial scale. - s—=-= Ss 20
SWEET-POTATO BY-PRODUCTS.
a
No sweet-potato by-products industry exists at present in the United
States where sweet potatoes rank second in importance of all vegeta-
bles grown. Consequently a large part of this crop is lost each year
from decay. When good growing conditions prevail up to harvest
time, a great many potatoes become overgrown, forming so-called
“jumbo” potatoes. These oversized potatoes, which often constitute
40) per cent of the entire crop, have the same composition as roots
of the standard market sizes, but they are of less value for shipping
to distant markets and for canning. The development of profitable
methods for converting such potatoes into useful products is there-
fore of great importance. While the production of oversized po-
tatoes is large, the production of undersized potatoes is usually
small. The small potatoes are used largely for seed purposes.
Sweet potatoes are richer in carbohydrates, especially starch, than
any other vegetable commonly grown. For this reason they are a
possible source of many products containing or derived from starch,
such as potato flour, dehydrated potatoes, starch, sirup, alcohol,
1 The work reported in this bulletin was conducted cooperatively by the fruit and vege-
table utilization laboratory and the office of development work. The laboratory process
for producing sirup from sweet potatoes was worked out by H. C. Gore, who also assisted
in the production of sirup at the experimental plant installed at Fitzgerald, Ga. The
equipment for the production of the sirup at Fitzgerald was designed and installed by
the members of the office of development work, who also determined the production cost
and the market value of the sirup. This part of the project was conducted under the
supervision of H. E, Roethe. The Union Cotton Oil Co., Fitzgerald, Ga., gave the use of
its plant for making the sirup on the commercial scale, and C. A. Newcomer, secretary
and treasurer of the company, cooperated with the investigators during the field work.
The States Relations Service of the Department of Agriculture and various State exten-
Sion directors submitted helpful comments and tested the sirup produced. ‘ Manufacturers
and jobbers of sirups, confectionery, soft drinks, cookies, biscuits, etce., tested samples of
Sweet-potato sirup and gave their opinions on its quality and possible use,
37996°—23 1
2 BULLETIN 1158, U. S. DEPARTMENT OF AGRICULTURE,
vinegar, breakfast foods, and various kinds of feeds. Dr. G. Wea
Carver, of Tuskegee Institute, has prepared all of these products,
as well as many others,? from sweet potatoes.
6, 1870, to Charles Delamarre, of New Orleans. In his process the —
peetoes were cooked and then reduced to a pulp by being passed —
etween wooden rollers. Eight pounds of crushed malt to each 100
sweet-potato flour, which sold readily at satisfactory prices, was made
there by the following method: F reshly harvested sweet potatoes
were thoroughly scoured by brushing machines which removed prac-
tically all of the peel. They were then sliced and conveyed to a drier
which consisted of a series of special drying floors placed one above
another. Here the slices were dried in an ascending current of hot
air. The slices were fed in at the top of the drier and were dropped
through from floor to floor as they were dried, by opening succes-
sively the shutters composing the floors. The dried product was
ground while still hot and was very finely bolted. Production ceased
about 1906, owing to the lack of démand for the flour,
The Williams Co., of Greenville, S. C., during 1916 produced
three sweet-potato products—a crumblike product known as yam
nuts, a sweet-potato flour flake, and a sweet-potato flake. Manufacture
was protected by United States patents 1238371, 1238372, and 1238373.
The general method employed in these three processes was to mix
the steamed peeled potatoes with flour, salt, and yeast, allow this
dough to ferment, add a large quantity of cooked potatoes to the _
sponge thus produced, and let it ferment. It was then made into _
loaves and baked. The loaves were allowed to cool, after which they
* Tuskegee Institute, Ala., Buil. 38, 1922; “Two great money crops of the South,”
address at the Voorhees Farmers’ Conference, Feb. 16, 1921, Denmark, S. C., published by
the Voorhees Press,
Se ee eee ee YS eee
; PRODUCTION OF SIRUP FROM SWEET POTATOES. 3
were worked up into the crumblike product, flour, or flakes. Produc-
tion, however, was unprofitable and eventually ceased.
The manufacture of sweet-potato flour by the flake process was
taken up by Mangels and Prescott of the Bureau of Chemistry dur-
ing the spring of 1919.2 Their method was as follows: The potatoes
were washed, steamed under 15 pounds pressure for 15 minutes, and
then dried in a drum drier of the type extensively used abroad in
treating Irish potatoes. The drier consisted of two revolving drums
supplied with steam at from 35 to 53 pounds pressure. The cooked
potato was spread out on the cylindrical surface of the drum by
means-of smaller auxiliary rollers, dried rapidly, and then removed
by scraper knives. The flakes thus obtained were ground into flour.
This process could not be applied to sweet potatoes on account of the
difficulties encountered in securing a product of attractive color and
flavor and because the flour obtained was so hygroscopic that it
hardened upon storage, forming gummy cakes.
The study of McDonnell* on the manufacture of sweet-potato
starch has great general technical value. Despite his satisfactory
results, the simplicity of the process, and the favorable reports on the
market value of starch, commercial development of sweet-potato
starch manufacture has never been undertaken.
COMMERCIAL POSSIBILITIES FOR SWEET-POTATO SIRUP.
_ From 5 to 6 gallons of sweet-potato sirup produced by the Bureau
of Chemistry was sent to each of many manufacturers of sirup and
various products made with sirup to obtain their opinions as to the
quality and commercial application of the sirup. Most of these
manufacturers submitted a report.
The consensus of opinion was that the sirup produced during 1922
was superior to that which had been produced during the preceding
year. There was, however, a wide variation of opinion as to the
quality of the sirup. Some claimed that the sirup was not sweet
enough; others that it was too sweet. Some thought the color was
too dark; others that it was good. Some reported that the sirup
had a sweet-potato taste, which was objectionable; others stated that
the sweet-potato taste in the sirup was hardly noticeable, and even
if present would not be objectionable. Some said the sirup was
either too thin or too thick; others that its consistency was about
right. This difference of opinion was to have been expected. Each
manufacturer measured the quality of the sirup by the requirements
of his product or of his particular method of manufacture.
In several cases the sirup fermented readily. This may not be
a serious objection, as it probably may be avoided by concentrating
the sirup to a certain degree Brix. This, however, would have to
be determined by a series of tests. The claim has also been made
that the sirup will scorch when cooking on a fire before it reaches a
temperature of 250° F.
The rephes of the manufacturers show that the qualities of the
sirup limit its field of use. Because of its color the sirup could not
be used generally in the manufacture of candies other than colored
ge eo tzctare of sweet-potato flour by the ‘ flake’ process,’ Chemical Age, 29 (1921):
4“The manufacture of starch from sweet potatoes,” S. C. Agr. Expt, Sta., Clemson
College, S. C., Bull. 136, April, 1908.
h
4 BULLETIN 1158, U. S. DEPARTMENT OF AGRICULTURE.
candies, such as caramels, taffy, and kisses, and those which do no
require a grain. The same holds true in baking. Doubtless the
sirup would make a good cooking and table sirup in the household.
It probably will never come into general use as a table sirup because
of its characteristic taste, although it might find favor with those
who like that taste.
Sweet-potato sirup has qualities which make it a valuable blend
for other sirups. Like glucose, at present the chief material for
blending, sweet-potato sirup prevents crystallization of cane sirup.®
The commercial application of this sirup is limited by its cost of
manufacture as compared with that of other sirups. If sweet-potato
sirup could be manufactured at a lower cost than:the other sirups,
uses would readily be found for it. With the present method of
manufacture and with the present market value of the other sirups,
however, this is impossible. It may be that in times of emergency,
when the price of sugar and sirups soars, sweet-potato sirup can be
manufactured at a profit. It is doubtful whether sweet-potato sirup
could be used as a substitute for glucose at its present low cost.
EXPERIMENTAL WORK ON SWEET-POTATO SIRUP.
PRELIMINARY WORK.
During the fall and winter of 1920 the Bureau of Chemistry ©
experimented with the sirup making from sweet potatoes in the
laboratory. The ease with which sirup could be prepared was dem- —
onstrated and the nature of the equipment necessary for its pro-
duction was determined.® The process for the production of sirup
from sweet potatoes thus developed gave indications of possessing
promising commercial possibilities. Field operations were therefore
begun in the spring of 1921 and continued during the first part of
1922, with a view to studying the commercial application of the
process, special attention being given to the determination of the cost —
of manufacturing, the market value, and the possible uses of the
sirup. Fitzgerald, Ga., was selected as the most satisfactory place for
the experimental plant.
A few hundred gallons of sirup of fair quality was manufactured
there in 1921. Small lots of the sirup were distributed among the —
larger manufacturers of sirups, confectionary, soft drinks, cookies,
biscuits, etc.; for expression of opinions on its quality, market value,
and possible uses. It then became evident that a still better sirup
was needed to meet trade requirements. With this in mind, further
laboratory work was done.
At the conclusion of these laboratory tests the experimental work
at Fitzgerald was resumed during the early months of 1922. The
plant was remodeled and numerous improvements were made in the
installation. About 300 gallons of sweet-potato sirup was produced.
5 Recent laboratory tests have shown that sweet-potato sirup can be used to prevent
crystallization in cane sirup. In the tests crystallization was much retarded by adding
20 per cent by weight of sweet-potato sirup (75.86° Brix) to the cane sirup and entirely
prevented by the use of one-third by weight of sweet-potato sirup, the composition of the
resulting mixture being three-fourths cane sirup and one-fourth sweet-potato sirup. Two.
samples of cane sirup were used in the tests. One was Georgia cane sirup of 75.02°
Brix, apparent purity 61.35; the other was Louisiana cane sirup of 73.8° Brix, apparent
purity 72.66, The tests were made by mixing the cane sirup with different quantities of
sweet-potato sirup, inoculating each mixture with a trace of powdered sucrose and letting
it stand in cold storage for several months.
6“ Preparation of sweet-potato sirup,’’ Chemical Age, 29 (1921): 151.
PRODUCTION OF SIRUP FROM SWEET POTATOES. 5
This sirup was distributed in much the same manner as that pro-
duced during the preceding year.
WORK AT FITZGERALD, GA.
BUILDINGS.
The apparatus used in conducting the experimental work on the
production of sweet-potato sirup on a semicommercial scale was in-
stalled in a frame building covered with corrugated sheet iron and
provided with a cement floor (Fig. 1). The engine room had the
Fic. 1.—Plant at Fitzgerald, Ga., which was used for conducting experiments on
the manufacture of sweet-potato sirup on a semicommercial scale.
same construction. The boilers for generating the steam were built
out in the open, but were covered with a roof made of wooden rafters
covered with corrugated iron sheathing. The potatoes were stored
in a frame building mounted on wooden piles. The walls were made
of slab boards over which pebble-finished tar paper was spread verti-
cally, battens being used to cover the laps. The roof of the building
was made of rafters covered with slab boards overlaid with pebble-
coated tar paper.
PLANT LAYOUT.
The layout of the experimental sweet-potato sirup plant (Fig. 2)
shows the apparatus necessary for the production of the sirup, the
floor space required by the various pieces, and their placing. A plant
of this size is large enough for an average daily production of 50
gallons of finished sirup. Its capacity could be greatly increased,
however, by the addition of a few other pieces of equipment.
APPARATUS.
A detailed statement of equipment used is given in Table 1.
6 BULLETIN 1158, U. S. DEPARTMENT OF AGRICULTURE.
TABLE 1.—Apparatus used in the manufacture of sweet-potato sirup.
Revo-
Item |[Num- Fl Hors scene) atk
em . oor ‘ TSse- * ermin-| Uni
No. ik Kind. space. Weight. ower. Capacity. Pate of | value,
vat i driving
pulley
Feet. |Pounds.
1 til aWashers. tae eae 64 by 11--. 850 1 | 150 bushels per hour. .. 90 $300
2 E-|-Mashitank== ND Yale oes 1, 200 Sy il 4200 eallons ste eecee 72 350
3 | 1 | Hydraulic press. ...- 4by11....| 2,800 14 | 17 bushels per hour. ..- 120 375
4 1 | Evaporator. -......- ARDY 182.2) “Aso0nlssaoee 40 boiler horsepower...|......-- 350
5 1°] Cooling: tank. 2222.2 53 by 63..- 175 4°.) 7550 'gallons ss. s2ee 90 55 3
6 1 | Filter press. ........ 4} by f4=5-| °2,800) |e2 sc ese- pe plate, 18-inch: |22s2--2— 400
| rame.
7 | 1| Flavoring and bot- | 74 by 7}... OO | reas 50-gallons . 532: 35 >=} eee 125
tling kettle.
8 1) aD rierse: a eee 144 by 113.| 1,500 14 | 60 square feet (area)...| 2,000 225
INSPECTION TABLE
CAPPING
« QOHP LNGInNE
Lncine Room
FINISHING
80H P BOILER
OWL RNC AD
TANK
ty
S
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&
x
8
Q
: (iP
eS
FH Ip
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eS
Fig, 2.—Layout of experimental sweet-potato sirup plant, Fitzgerald, Ga.
PoMACE DRIER
PRODUCTION OF SIRUP FROM SWEET POTATOES. 7
Fic. 3.—Sweet-potato washer in operation.
WASHER,
A standard-make, rotary-screen cylinder washer (Fig. 3) was used
to remove the dirt, sand, etc., from the sweet potatoes before they
were placed in the mash tank for steam blanching.
PRODUCTION OF SIRUP FROM SWEET POTATOES. 15
Sand all decaying potatoes were removed. Very little decay, only
_ from 1 to 2 per cent, was originally present, and this was almost
entirely eliminated as the result of the inspection given.
Pale distillers’ malt of high diastatic power was employed. The
kieselouhr which served as a filter aid was of California origin.
_ Before being added to the mixing tank it was ignited at a low red
heat in cast-iron pans in the fire box of the boiler.
LABOR.
The jabor force at the experimental plant, which was entirely
adequate, consisted of one mechanic and two laborers. A run of
_ erude sirup was usually made on two consecutive days. The next
_ two days were then required for the finishing of the sirup. Thus in
_ four 10-hour days approximately 140 gallons of finished sirup was
produced. By running the apparatus at full capacity approximately
_ 200 gallons of sirup could have been produced, or an average of 50
gallons a day. The mechanic received 40 cents an hour and the two
laborers each 20 cents an hour.
_ The authors acted as the supervising force in directing the opera-
tions. In commercial practice a plant superintendent would be
necessary.
METHOD OF MANUFACTURE.
A flow diagram of the method of manufacturing sweet-potato
sirup (Fig. 11) shows the steps in the operation and the points at
which steam and water were used. It also shows how the different
materials were handled and at what points they were put into the
operation.
WASHING THE POTATOES,
The sweet potatoes were given a visual inspection at the storehouse
and all those showing signs of rot were rejected. As the potatoes
were inspected they were placed in bushel crates. Upon the comple-
tion of the inspection each batch of potatoes was hauled in an auto
truck from the storehouse to the washer.
The washed potatoes were placed in bushel crates. The action of
the washer wore off the ends of the potatoes, thus effectively remov-
ing the withered ends, a possible source of off flavor.
The potatoes were weighed on the platform scales, four crates at a
time, and were then carried to the mash tank. Approximately 50
bushels of potatoes to a run or batch were used.
MASHING,
The potatoes were leveled in the tank and the tank covers were
closed. Live steam was then admitted at such a rate that gentle
streams of vapor passed out between the upper edge of the tank
and the covers. The condensate was allowed to drain off as fast as
it formed through the gate valve, which was opened slightly for
this purpose. This treatment removed from the outer portions of
the potatoes certain extractives which if retained impart a dull green
color and an off taste to the sirup.7?. One and one-half hours’ steam
7 Previous laboratory work (Chemical Age, 29 (1921): 151) had shown that the loss
of sweet-potato solids in the condensed water is very small.
16 BULLETIN 1158, U. S. DEPARTMENT OF AGRICULTURE.
cooking was given the potatoes. This softened them so that they
could be easily mashed into a uniform pulp by the stirrer, thus
breaking down the cells sufficiently to secure a good yield of sirup.
Immediately after cooking, water of the same weight as the pota-
toes was dumped into the tank and the stock was reduced to a pulp —
by stirring. With the stirrers still going, the temperature of the
mash was raised to 140° F. by running in live steam. The tempera-
ture was carefully determined by withdrawing large samples of the
mash through the gate valve and testing by thermometer. Readings
of the tank thermometers screwed into the sides of the mashing vat
were not reliable.’ When a temperature of 140° F. (the point at —
which the saccharifying action of malt is most active) had been —
reached a quantity of ground malt, equivalent to one-fifth of 1 per —
cent of the weight of potatoes taken, made into a thin mush with
cold water, was added. The stirring was continued during the addi- |
tion of the malt and for a few minutes thereafter so that it would be
thoroughly incorporated.
The laboratory work had shown that one-fifth of 1 per cent of |
malt was sufficient for the conversion of the undigested starch still
present after heating the sweet potatoes for 14 hours, in the case of
several standard varieties, including Big Stem Jersey, and that one-
tenth of 1 per cent was sufficient for the Porto Rico and Nancy Hall
varieties. In the field work one-fifth of 1 per cent was used. Diges-
tion of the starch was complete in from one-half to three-quarters
of an hour, the end-point taken being when no color was given. when
a drop of a portion of the filtered liquid was added to a dilute solu-
tion of iodin in potassium iodid. The mash was then ready for
pressing.
ee ee
PRESSING.
In pressing the hot pulp was made up in the form of layers from
2 to 3 inches thick between racks and cloths. The press cloth was
just large enough to inclose the pulp completely.
The procedure in laying up the pulp was as follows: The press
truck was wheeled under the quick-opening valve of the mash tank
and a rack was placed on the pressure platform. On this the form
was placed and over it was spread the press cloth, with its edges
parallel to the adjacent sides of the form. The center portion of the
cloth was then filled with hot pulp so as to fill the form, and the edges
of the cloth were folded over, forming the first press cake. The
form was then removed, a rack was placed on the press cake, and
the form was placed on the rack. Another layer was then built.
From 10 to 14 layers constituted a cheese.
When loaded the truck was pushed into the press. Cooked sweet-
potato pulp drains so readily that a large proportion of the liquid
(technically termed “ wort”) flowed out of the cheese before the appli-
cation of any pressure except that caused by the weight of the layers
of the cheese. As it flowed off the wort was collected in a galvanized
iron tank on the floor between the mash tub and the press, from which
Ly pe coal hy Alene ea ARMAS
an oe ene
8 This was due, in part at least, to the effect of the temperature of the tank walls on
the armored thermometer bulb. The tank walls were thicker than was anticipated when
the thermometers were ordered, and the bulb did not project far enough into the tank to
eliminate the influence of the tank-wall temperatures.
a
>
:
PRODUCTION OF SIRUP FROM SWEET POTATOES. 17
it was pumped to the supply tank of the evaporator. The pressure
_ was applied gradually.
In pressing the racks and press cloths tended to slip out of place,
owing in part to the lack of guides to make sure that the ram
ascended vertically, and in part to the fact that the galvanized wire
racks did not maintain their position as well as wooden ones. It is
believed that this difficulty could be overcome by the use of wire
cloth of coarser weave. It was overcome in the experimental work
by placing a series of wooden frames around the cheese as it was
being formed. Three frames, with sides from 6 to 8 inches high,
were superimposed, fitting one on another, by the use of cleats. Dur-
ing pressing the uppermost, and finally the middle, frame was re-
moved. Three cheeses of 10 to 13 cakes each were required for each
50-bushel lot of potatoes.
After pressing the pomace was shaken out of the cloths, which
were then ready to be used again. At the close of each day’s work
the cloths were washed and hung out on lines in the open air to drain
and dry. A batch of 2,500 pounds of potatoes yielded 200 pounds of
dried pomace and required 19 hours to dry. The wort received from
pressing, collected in the catch tank, was pumped to an overhead tank,
which acted as a reservoir, from which it was drawn by gravity to
the evaporator as needed.
EVAPORATION TO CRUDE SIRUP.
The wort contained about 15 per cent of solids. It was pale
yellow, with a faint, sweet taste. It was evaporated to approximately
60° Brix (cold). The most convenient method for operating the
evaporator was to fill it to a mark previously determined by experi-
ment, turn the steam into the coils, and evaporate until the sirup
became so thick that a test portion would. just begin to fall in blobs
or “flake” from the paddle or dipper used in testing. This usually
required from 15 to 20 minutes. The steam was then turned off and
the sirup was discharged. A new batch of the wort was admitted
and the process was repeated. It was expected that the evaporator
could be made to operate continuously. This, however, was imprac-
ticable, as it was impossible to control the feed and steam supply with
sufficient exactness.
The crude hot sirup thus prepared from the potatoes was received
in a collecting tank under the evaporator and was later pumped into
the cooling tank. Here it was allowed to cool and stand until ready
to be converted into finished sirup.
FILTRATION OF CRUDE SIRUP.
The crude sirup is turbid, containing albuminous matter coagu-
lated by the boiling, particles of sweet potato tissue, extraneous mat-
ter, and a small proportion of mineral matter of unknown composi-
tion which separates upon cooling and standing. After the crude
sirup had cooled and stood for about 40 hours, thus allowing time
for the salt to separate, it was mixed with enough cold water to dilute
it to 45° Brix or below. A quantity of ignited kieselguhr equivalent
to 2 per cent of the weight of potatoes taken was first mixed with
this water in the catch tank below the evaporator. The same volume
37996 ° —23—_3
|
}
|
18 BULLETIN 1158, U. S. DEPARTMENT OF AGRICULTURE.
P, WATER
SWEET 7) R
POTATOES Poraro i U ]
rel ae pie
J WaREHOUSE| CRATE UU SCALES —oayrl | Neri eee eee ee
a ea 8 k= See Pe oS. (Gebel pees | Ee AR A re
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ao eee ee an aw em eaten
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Cous|Q _, Scapes j }
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Nu Up ey Ue pe te 2 ©: _ 10a
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FREIGHT
t= Mes oer oa iene: BSR ay
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Fic. 11.—Flow diagram for sw
of the crude sirup was then added and thoroughly mixed, and this
mixture was pumped into the cooling tank in which the crude sirup
was stored. The sirup was then pumped into the filter press. The
filtrate was usually clear from the start. A small belt-driven pump
was attached to the delivery tank, with valves so arranged that the
filtrate could be sent either to the cooling tank or to the evaporator,
as desired. The pressure used in filtering was allowed to build up
gradually during the filtration to about 25 pounds. .
Fifty pounds of kieselguhr proved to be nearly enough to fill the
18 frames of the filter press. The filter cake was washed by pumping
cold water in through the feed ports. Laboratory experiments, using
a small filter press of the same type, showed that it was practicable to
reignite and reuse the kieselguhr. This, however, was not done in the
field trials at Fitzgerald.
EVAPORATION OF FILTERED SIRUP.
After being filtered, the sirup was evaporated to the desired
density, using the evaporator employed for crude sirup, thoroughly
cleaned, of course. As the dilute filtered sirup in boiling foamed
more than the crude wort, a special procedure was evolved to over-
come this difficulty. Unless the level of the sirup was kept rather
low in the evaporator, it had a tendency to foam over every time
the freshly filtered sirup was added. When a low sirup level was
maintained, however, the steam gauge at the head of the evaporator
coils became a convenient guide in indicating when to add a por-
tion of the freshly filtered sirup. As the contents of the evaporator
approached the density of the finished sirup, the steam pressure in
the coils gradually increased to 40 or 45 pounds. As the stream
of filtered sirup was slowly added by means of the pump, this pres-
Sa
; = oan GUHR | CONTAINERS
PRODUCTION OF SIRUP FROM SWEET POTATOES. 19
a PPING
ACHINE Ay ore
: y : |
i ~ Ea ] 1) Borr. Le j
; a) foes tears |
Caren Tare RRA ARM Jo Poa Dee
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a5 SH/PPING
“ae j FREIGHT
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perimental plant, Fitzgerald, Ga.
sure fell promptly to 30 pounds or less, owing to the greater rate
of heat transference from the steam coils to the sirup as it became
diluted. When the gauge indicated 30 pounds the supply of fresh
sirup was cut off. The pressure was then again gradually built up
to 40 or 45 pounds when the operations were repeated, the cycle of
operations requiring about 10 minutes. The sirup was discharged
at frequent intervals, care being taken, however, never to let the
steam coils become exposed to the air.
As in the case of the crude sirup, the approximate concentration
at which to stop evaporation was shown by the flaking of a test por-
tion poured from the testing dipper. This procedure gave a sirup
more dilute than desirable, the Brix when cold being only about
62.5, so that the flaking indication, although very convenient, is by
no means as exact an indicator for the density of the hot sirup as
the Brix spindle recommended by Dale® and others. The sirup
should be concentrated to about 70° Brix (cold).
CANNING AND BOTTLING THE SIRUP.
The finished hot sirup was either allowed to flow from the evapo-
rator directly into the cans or bottles and sealed or it was trans-
ferred to a steam-jacketed copper kettle. Here it was reheated to
the boiling point and drawn off into cans or bottles. The containers
were washed immediately before use and the bottles were heated in
hot water before being filled. When filled they were capped with
the aid of a foot-power capping machine and placed on their sides
to cool, the object being to sterilize the inner surfaces of the seals
by keeping them in contact with the hot sirup.
®U. S. Dept. Agr. Cir. 149, 1920.
BULLETIN 1158, U. 5. DEPARTMENT OF AGRICULTURE.
Dp jh ta L ; .
es
WAREHOUSE| CRATE
cos) a=
EXHAUST
| S70¢K FEED
te Hee mee
[a tere
PRODUCTION OF SIRUP FROM SWEET POTATOES 19
= — — IK ESELGUHR
1 yneHouse
SCALES
CAPPING
MACHINE
CASES
1] Borree
EVAPORATOR | 55), pe
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S/PPING
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Iie. 11.—Flow diagram for Sweet-potaifsiup experimental plant, Fitzgerald, Ga.
of the crude sirup was then added and thoroughly mixed, and this
mixture was pumped into the cooling tank in which the crude sirup
was stored. The sirup was then pumped into the filter press. The
filtrate was usually clear from the start. A small belt-driven pump
was attached to the delivery tank, with valves so arranged that the
filtrate could be sent either to the cooling tank or to the evaporator,
as desired. The pressure used in filtering was allowed to build up
gradually during the filtration to about 25 pounds. ,
Fifty pounds of kieselguhr proved to be nearly enough to fill the
18 frames of the filter press. The filter cake was washed by pumping
cold water in through the feed ports. Laboratory experiments, using
a small filter press of the same type, showed that it was practicable to
reignite and reuse the kieselouhr. This, however, was not done in the
field trials at Fitzgerald.
EVAPORATION OF FILTERED SIRUP.
After being filtered, the sirup was evaporated to the desired
density, using the evaporator employed for crude sirup, thoroughly
cleaned, of course. As the dilute filtered sirup in boiling foamed
more than the crude wort, a special procedure was evolved to over-
come this difficulty. Unless the level of the sirup was kept rather
low in the evaporator, it had a tendency to foam over every time
the freshly filtered sirup was added. When a low sirup level was
maintained, however, the steam gauge at the head of the evaporator
coils became a convenient guide in indicating when to add a por-
tion of the freshly filtered sirup. As the contents of the evaporator
approached the density of the finished sirup, the steam pressure in
the coils gradually increased to 40 or 45 pounds. As the stream
of filtered sirup was slowly added by means of the pump, this pres-
|
4
sure fell promptly to 80 pounds or less, owing to the greater rate
of heat transference from the steam coils to the sirup as it became
diluted. When the gauge indicated 30 pounds the supply of fresh
sirup was cut off. The pressure was then again gradually built up
to 40 or 45 pounds when the operations were repeated, the cycle of
operations requiring about 10 minutes. The sirup was discharged
at frequent intervals, care being taken, however, never to let the
steam coils become exposed to the air.
As in the case of the crude sirup, the approximate concentration
at which to stop evaporation was shown by the flaking of a test por-
tion poured from the testing dipper. This procedure gave a sirup
more dilute than desirable, the Brix when cold being only about
62.5, so that the flaking indication, although very convenient, is by
no means as exact an indicator for the density of the hot sirup as
the Brix spindle recommended by Dale® and others. The sirup
should be concentrated to about 70° Brix (cold).
CANNING AND BOTTLING THE SIRUP.
The finished hot sirup was either allowed to flow from the evapo-
rator directly into the cans or bottles and_sealed or 1t was trans-
ferred to a steam-jacketed copper kettle. Here it was reheated to
the boiling point and drawn off into cans or bottles. The containers
were washed immediately before use and the bottles were heated in
hot water before being filled. When filled they were capped with
the aid of a foot-power capping machine and placed on their sides
to cool, the object being to sterilize the inner surfaces of the seals
by keeping them in contact with the hot sirup.
®U. S. Dept. Agr. Cir, 149, 1920.
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20 BULLETIN 1158, U. S. DEPARTMENT OF AGRICULTURE,
YIELD.
Sweet-potato sirup made in this manner is bland and sweet, with
a distinctive flavor. It is not as sweet as cane sirup, cane-sugar
sirup, maple sirup, honey, or high-grade molasses.
a storage house is used. The potatoes begin to be available about
August 15 and can be kept in good condition up to April 15, when the
rapid rotting caused by high temperatures makes longer storage
‘impracticable. The loss is very small in the early part of the
season, but becomes noticeable during the last two months of the
period. Under favorable conditions the loss from rotting has been
as low as 2 per cent but may, even with normal care, exceed 10 per
cent under less favorable circumstances. An operating season of 192
_ days (8 months of 24 days each) and a shrinkage of 4 per cent have
been taken as a basis for the cost calculations.
It is fully realized that the estimated costs for various items will
vary with different sections and local conditions. Elaborateness of
_ buildings, possible elimination of the curing house, desired capacity
of the plant, and utilization of labor-saving equipment are im-
portant considerations.
LAND AND BUILDINGS.
A lot 100 by 200 feet would provide ample space for a plant of
_ the size here discussed. This would allow for the required build-
ings, for driveways for handling materials and the finished product,
and for a railroad siding.* It is assumed that the plant would be
_ built in a small town, in which case $1,000 would be a liberal esti-
_ mate for the cost of the land.
The following buildings are necessary: A sweet-potato storage
house, the factory proper, an office, and a boiler and engine house.
The size of the buildings can be determined from the plant layout
(Fig. 12). The cost of the buildings is determined by the type of
construction, which in turn is dependent upon the geographical
- location of the plant, as well as upon the amount of money which
is available. Clearly a plant buiit in a southern State need not
_ be as substantial as one built in a northern State. Since most of
the sweet potatoes are raised in the South, however, the plant was
considered as being located in one of the southern States. The type
of construction of the buildings would be the same as that described
on page 5. The office was taken as being made of weather boarding
with a tar-paper roof, the inside of the office finished with ceiling.
The approximate cost of such buildings would be:
SL ag a aa a $3, 500
LLCS pl tela i fet a EY Ee ee ee ee 3, 500
ye En Ra ae 900
i rnereer sansgne! fed ph 900
2S see eS eR Se ei 8, 400
_
rl nN t
LAYOUT FOR PLANT AND APPARATUS.
From the experience and information secured from the experi-
ments conducted at the experimental plant, a plant for the manu-
facture of sweet-potato sirup was designed. The number and size
of the buildings required by a plant of the given capacity may be
determined from the layout (Fig. 12). The type and number of
pieces of apparatus necessary for the manufacture of the sweet-
22 BULLETIN 1158, U. S. DEPARTMENT OF AGRICULTURE.
potato sirup on a commercial scale, as well as the floor space required
by the various pieces of equipment and the arrangement of the dif-
ferent pieces 1n their relation one to another, are shown in Figure 13.
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Fig. 12.—Layout for proposed sweet-potato sirup plant.
BELT Conveyor
STORAGE HOUSE
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PRODUCTION OF SIRUP FROM SWEET POTATOES. 23
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STORAGE FOR
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Finish LVAPORATOR N @)
2 ACHING AND
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Fig. 13.—Arrangement of equipment in proposed sweet-potato sirup plant.
APPARATUS.
- Some of the apparatus used in the proposed plant is of the same
_ size and design as that used in the experimental plant. These are
_ the potato washer, mash tank, hydraulic press, evaporators, filter
_ press, and flavoring kettle (pp. 7-12). Two larger cooling tanks are
_ substituted for the smaller cooling tank, and an additional evap-
orator is provided for handling the filtered sirup. The following
- additional apparatus is necessary in the construction of a plant of
_ the capacity of the one proposed:
TABLE 2.—Additional apparatus in proposed plant.
Num- per =e
Ttem . Floor : Horse- - ! Unit
Kind. space. Weight. power. Capacity. aoe value.
Feet. Pounds.
> i 1 | Endless belt con- | 24 by 55..- 550 1 | 23feet per minute ..... 15 $17
a veyer. bad
~ 2 1 | Hopper scales....... 3 hy! >. 5: VET aes ZO DUSNOISS se saat oo ofa oo 200
a 3 iy Pe OVALOT = se. 3 by 10.... 275 1 | 104feet per minute..... 20 215
a 4 i} measunne tank -.--.|....-..2..-- (SS Ge ee oe ee Aer 75
. 5 2 | Cooling tanks. ...... 7 by 74...- 200 apa mallons-* 2.) 24... | 90 225
6 tol Maxam hank: -2 22) 3,by4s-2.. 40 Nene oat 125 gallons. .....--.... Seeeeece 12
7 1 | Evaporator. ........ Se DY18-5<)) 1,950. |. 222... 2 40 boiler horsepower -...|......-- 350
8 1 OUGE 2 ses fic tea: Sia Soya er. ee: Rit tek a |=» DEEN eS eee ee 500
9 1 | Electric motor. .....| 14 by 2.. 550 0D Rs ae oe eee Oe 675 150
ENDLESS BELT CONVEYER.
The endless beit conveyer for carrying the sweet potatoes from the
storehouse to the potato washer is a standard type. The belt is 24
inches wide and is approximately 55 feet long. The shape of the
supporting idlers gives the belt a concave surface. Power is secured
from an electric motor and countershaft. The driving pulley makes
15 revolutions per minute, giving the belt conveyer a speed of 23
_ feet a minute, using a 6-inch conveyer pulley.
ttt a ——— ee
94 BULLETIN 1158, U. S. DEPARTMENT OF AGRICULTURE.
HoprPer SCALES.
A standard-type beam scale, with a sheet-iron hopper suspended, —
is used for weighing the potatoes as they come from the potato
washer. The hopper is 5 feet long by 3 feet wide by 2 feet deep,
which is large enough to weigh 20 bushels, or 1,000 pounds, of pota-
toes at a time. It is mounted on a pivot.
ELEVATOR.
From the hopper scales the sweet potatoes are dumped into a chute
which discharges them into the boot of a standard-type incline ele-
vator, equipped with a few variations to meet the needs in this par-
ticular case. The belt, which is 3 feet wide, is equipped with metal
buckets that form a sort of pocket in which the potatoes rest and are
carried to the top of the mash tank. Here the potatoes are dis-
charged into a chute which dumps them into the mash tank. The
elevator is 16 feet between pulley centers. The driving pulley makes
20 revolutions per minute, giving with 20-inch head pulley a belt
speed of 104 feet a minute.
MEASURING TANK.
After the sweet potatoes have been steam blanched water of the
same weight as the potatoes must be added before the mashing oper-
ation is started. To facilitate the measuring and handling of the
water, an elevated measuring tank is used. The tank is made of
24-cauge galvanized iron and is 4 feet in diameter and 6 feet deep,
with a capacity of approximately 4,600 pounds, or 550 gallons, of
water. Beer anetes ak 25 pe Fa. = Soe ee ee a ee ee eae Se 3-11
ee eee ee SE EE 2S oe 5 ee ee = ie ay Pe eee PAS ee 7S
888 labels at $0. _ SEL Sti Se eee Pa eee oe | 1.47
0 LEEPER DLS i ETE Sie es ee ee ee een en Se pe Se oe 22.94
| 3.77 sacks of cottonseed hulls at $0.80 per sack...217 122277222222. Slee feast Fi | 3.02
a TU OD Bad A eR Se | 131.76 | 141.14 144.19
4 eae et aie poe es Sa lee | 1.318 1.411 | 1.299
> J
= PRECAUTIONS TO BE OBSERVED.
Although the method used in making sirup from sweet potatoes
_ is simple, the following precautions must be taken to insure the pro-
_ duction of sirup of fine quality:
_ Use pale distillers’ malt in the proportion of from one-tenth to
_ one-fifth of 1 per cent of the potatoes taken.”
_ Use washed potatoes, free from decay.
> Use kieselguhr as a filter aid.’
: In cooking, reject the condensed water flowing from the mashing
tanks.
Use no wooden equipment in the pores of which potato juice has
- been allowed to decay.**
Clean equipment often enough to avoid all danger of spoilage.
Arrange equipment so that it can be cleaned readily.
Once the operations are started, complete the process without
delay.
: aeant letting the sirup thicken or bake on the evaporator ** coils.
2 Investigation of the action of the malt showed that to avoid the strong flavors
which some kinds of malt impart it was desirable to use a light-colored distillers’ malt
and to reduce the malt requirement to a minimum. The quantity of commercial pale
distillers’ malt required was only from one-tenth to one-fifth of 1 per cent of the weight
of the potatoes. The principal reason for this very small requirement was that the
sweet potato has a very active diastase which digests a large proportion of the starch
while the potatoes are being heated.
# The laboratory results indicate that in filtering the kieselguhr (diatomaceous earth),
“5 as a filter aid, should be ignited at a low red heat as recommended by Caldwell
(U. S. Dept. Agr. Bul. 1025) as a satisfactory method for eliminating the peculiar flavor
. which the untreated material seems to impart to the sirup. The kieselguhr, if reignited
between each use, could be used repeatedly.
14 The laboratory work also showed the necessity of taking care that the hot wort or
sirup never comes in contact with the wooden surfaces of racks, pressure platform, or
other equipment, in the pores of which potato juice has been allowed to decay, as such
contact is likely to cause contamination in flavor.
145 The laboratory work showed that color and flavor were injured by allowing the sirup
to bake on steam-heated surfaces, this work leading to better evaporating practice.
Pay ee Tee eT Mar PS
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*
32 BULLETIN 1158, U. S. DEPARTMENT OF AGRICULTURE.
To produce clear sirup, evaporate crude sirup to 60° Brix or above,
let it cool and stand for about 40 hours, then dilute it to 45° Brix
with cold water containing kieselguhr, filter, and reevaporate.1®
GENERAL PROPERTIES OF SWEET-POTATO SIRUP.
Sweet-potato sirup is clear and has an amber color. The taste is
sweet, with a slight distinctive flavor and a slight after taste. It
is not as sweet as cane sirup, honey, etc., which are characterized by
high proportions of sucrose or invert sugar.
COMPOSITION OF SWEET-POTATO SIRUP.
The analysis of a sample of the sirup made at Fitzgerald is given
in Table 11.
TABLE 11.—Chemical composition of sweet-potato sirup.
7a ine rt ee
etermination. z ree
analyzed. aie
Sohds (hy specific gravity) (per ednt)_iis-2e este es ee eee 62.50! | 244 S52-
a CER GEHL) - <5 25.2 stank ea ae Smes ain Se eee oe eg ee eee ae ae ee ee 1.02 1.63
IPTOLetn CN 20 6:25) (percent) eee. os ee Cee ea ee ee ene 1.03 1.65
Direct: polarization (°V..):23) 3.2 Se fe eo Pea eee ee a eee |}; 10454 hv = Saeees
In Vere POlArWwaAliOn: (LV Jian eee Se ee | eat eee 9252: tee ee
reduce Suzaras invert (percent)! jis: eee ea ee eee 2h | See
Reducing sugar. as maltose (percent) 2-08 sae ee eee ee ee 36. 23 57.94
Motalsupar assnyerb-Cper. Cent) 2235 8a2 ae ee ee eee oe ee ee ee = oe ae 31556 ee cae
Sucrose by polarization (percent). -..22 35 es en ee ee ee cree 9.40 14.96
Sucrose by reduction (percent) &. 55s 22 eae ee ae eee ee eee ae ee 9.30 14.96
Dextrini(per.cCent)' 8: cone ashes Ses (Se ee a ee ee 12.47 19.95
1 Analysis made in the nitrogen laboratory of the Bureau of Chemistry.
2 Munson and Walker method. The cuprous oxid on ignition gave over 98.5 per cent of cupric oxid,
showing that it was nearly pure. ;
Ee, fermentation method described in J. Assoc. Official Agr. Chemists, Methods of Analysis (1916),
p. 179.
Sweet potatoes contain very little dextrose or levulose, and practi-
cally no inversion of the sucrose present occurs during sirup making.
For these reasons a much better idea of the composition of the sirup
is given by the application of the usual methods of analysis than
would otherwise be the case. Thus, this analysis shows that the car-
bohydrates of the sirup are mostly maltose (57.94 per cent of the
sirup solids), dextrin (19.95 per cent of the sirup solids), and
sucrose (14.96 per cent of the sirup solids).
COMPOSITION OF SWEET-POTATO POMACE.
The chemical analysis of the dried sweet-potato pomace, deter-
mined by the cattle-food laboratory of the Bureau of Chemistry, is
given in Table 12.
16 One of the most difficult problems was the development of a simple method whereby
a sirup which would be clear and would remain so on cooling and standing could be
produced. The substance which gave most difficulty was a salt of unknown composition
which separated out in minute quantities in the finished filtered sirup, thus rendering it
turbid. This difficulty could be overcome by concentrating the wort to about 60° Brix
and letting it cool and stand for at least 40 hours. During this time enough of the salt
separates in the sirup so that by diluting it with cold water to about 45° Brix, filtering,
and reevaporating to final density the sirup will remain clear on cooling and standing.
PRODUCTION OF SIRUP FROM SWEET POTATOES. ae
TasLe 12.—Chemical composition of dried sweet-potato pomace.
Determination. Per cent. Determination. Per cent.
LS 50D. ee SSeS eee eee ANSOE | MELOvCINpertnter a. chee cast Peete oe 23.06
Dl. .cs. 42 23 Bees as 3 ee ee ere a A AGe | ROT e fier sa. St ct oa cok Ba Ack ane ees 13.50
DEPMIIO SUT C Ue ie ao. ncn ccs Coe cine ec cuhs 30) Nitrogen=free extract)... .<--2-....2502..- 50.09
This analysis indicates the probable high feed value of this pomace.
SUMMARY.
A method for the commercial manufacture of sweet-potato sirup,
based on laboratory and plant experimental work, has been developed
by the Bureau of Chemistry. The following important points were
brought out in the course of this work:
| Conversion of starch is accomplished satisfactorily by using malt
equivalent to one-fifth of 1 per cent of the weight of the potatoes,
by maintaining a temperature of 140° F., and by allowing the sirup
to stand for 45 minutes after the addition of the malt.
_ The drying of the pomace proved to be uneconomical, although it
may have a high value as a feed.
he use of the atmospheric or open-kettle evaporator was prac-
ticable in concentrating the crude and filtered sirups.
To produce a permanently clear sirup it was practicable to let the
_ crude sirup cool and stand for 40 hours and then filter it, in order to
remove certain salts which caused turbidity.
Satisfactory results were obtained in the filtration of the sirup by
using ignited kieselguhr as a filtering aid, 2 per cent of the weight of
_ the potatoes being employed, and filtering cold at 45° Brix.
| The yield obtained varied. During the spring of 1921 it was 1.55
_ gallons of sirup to a 50-pound bushel of potatoes. An average yield
of 1.87 gallons of sirup to a 50-pound bushel of potatoes was obtained
_ during the spring of 1922.
Adding 10 per cent of other sirups with a sucrose or invert sugar
/ content increased the sweetness of the experimental sirup.
. From the standpoint of quality the sweet-potato sirup has possi-
_ bilities for use as a table sirup, for cooking purposes, and in the
manufacture of colored and short-grain candies, such as taffy, kisses,
and caramels. For baking purposes it might find use in dark
products, such as ginger snaps. It also has properties suitable for use
in blending with other sirups to prevent crystallization.
For the proposed plant of 100 gallons per day capacity the cost of
manufacture of plain sweet-potato sirup, exclusive of containers, is
estimated to be 64.3 cents per gallon.
The commercial possibilities of sweet-potato sirup are limited by
_ the high cost of manufacture under the present method.
ORGANIZATION OF THE U. S. DEPARTMENT OF AGRICULTURE.
Secretary of Agriculture 3. 28). i ss HENRY C. WALLACE.
Assraiant Secretary: -- 2-2". ee C. W. PUGSLEY.
Ahrecto? of Scientific Werk —- 22k Ses HD. BACK.
Director of Regulatory Work
Weather Buréauls as Sesh ee CHARLES F. Marvin, Chief.
Bureau of Agricultural Economics_______- Henry C. TAytor, Chief.
Bureau of Animal Industry_____-____-~_- JOHN R. MouHter, Chief.
Bureau of Plant Industry. —_2 = See WittiAmM A. Taytor, Chief.
POPES. SCTUICE: 2 4! a ie ae W. B. GREELEY, Chief.
Bureaw of Chemistry. =22 5 Sis ee ee WALTER G. CAMPBELL, Acting Chief.
Puree of Sotts 2 Fe eh eee MILTON WHITNEY, Chief.
Bureau of Entomology__.-—--— = L. O. Howarp, Chief.
Bureau of Biological Survey___-___-~--__- E. W. NELson, Chief.
| Bureau of Public Roads___________-______ Tuomas H. MacDonat.p, Chief.
| Fixed Nitrogen Research Laboratory_____ F. G. Corrre.y, Director.
F| Division of Accounts and Disbursements_. A. ZAPPONE, Chief.
4 Dinsion of Publications__2_- —* Epwin C. PowELL, Acting Chief.
} DT Gig nd Et SU ee Pe eet CLARIBEL R. BARNETT, Librarian.
Sidtes Relations -Servicej__.~ = 2 ss A. C. TruE, Director.
Federal Horticultural Board____________~ C. L. Maruatr, Chairman.
Insecticide and Fungicide Board_____~ —___. J. K. Haywoop, Chairman.
Packers and Stockyards Administration__| CHESTER MorRIL1, Assistant to the
Grain Future Trading Act Administration__| Secretary.
Onice ofthe Solicitor 222228 2) te bass R. W. WiLiiAMs, Solicitor.
This bulletin is a contribution from—
Bureau of Chemistryc_ sy 23 fe ee WALTER G. CAMPBELL, Acting Chief.
Fruit and Vegetable Utilization Lab-
OTOLOT Ys oi. = 3: ee eR are en Be H. C. Gore, in charge.
Office of Development Work_____- D. J. PRICE, in charge.
34
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V