November. 1931
*
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
1205
Production of Starch from Sweet Potatoes’’z Including Comments Regarding Manufacture of White - Potat o Starch R. T. Balch and H. S. Paine BUREAUO F CHEMISTRYA N D
SoIl.5,
L’.
s.
DEPARTMENT OF
.kGRICULTURE, W A S H I N G T O N ,
D
c.
The sweet potato is the second-largest vegetable crop grown in the United States. It has been variously estimated t h a t 10 to 30 per cent of the sweet-potato crop is graded as culls owing t o the rigid requirements for market potatoes. These culls are now wasted or inadequately utilized. The sweet potato being primarily a starch producer, the possibility of utilizing the cull and excess crop as a source of commercial starch has been investigated and is herein reported. A procedure has been devised for manufacturing starch whereby objectionable color can be largely eliminated from the starch, regardless of the color of the potato flesh. The method involves utilization of sulfur dioxide in the water used in grinding the potatoes. This treatment apparently keeps the pigments carried by the starch in a reduced form until they are extracted later in the process by a dilute sodium hydroxide solution, which is lthoroughly washed from the starch before it is dried. The other
operations-namely, the grinding of the potatoes, the separation of the starch from the pulp, purification of the starch by settling, and drying of the starch-are essentially the same as for production of white-potato starch. Photographs of laboratory and semi-commercial equipment suitable for production of sweet-potato starch are shown. Sweet-potato starch has been produced, which, for textile purposes, is equivalent to imported high-quality white-potato starch. Attention has also been given to the utilization as a stock feed of the starch factory byproducts, consisting of the spent pulp, which contains a substantial percentage of starch, and the effluents, which contain valuable sugars and proteins. Further studies pertaining to starch extraction, the physical properties of sweet-potato starch, the nature of t h e pigments present in the sweet potato, and the utilization of the byproducts are being conducted.
T
which is wasted, or very inadequately utilized, constitutes a serious problem which must be solved before the sweet-potato crop can realize its economic possibilities. At present, practically the only means of utilizing culls is for canning and for feeding to hogs and other livestock. Utilization by canning has not made the progress anticipated, since in 1929 a total of 368,693 cases with a value of $847,318 were packed in 37 factories in the United States, as compared with 768,951 cases, having a value of $2,121,653 in 1925 (24) With regard to feeding of cull and surplus sweet potatoes, it has been found that sweet potatoes can be successfully substituted for one-half of the corn in the ration, 3 pounds of sweet potatoes replacing 1 pound of corn (17). It has also been stated that when hogs are selling for 10 cents per pound, cull sweet potatoes can be marketed through them a t approximately 40 cents per bushel, but best results are obtained when sweet potatoes do not constitute the entire ration (2). It has been estimated that not more than 20 per cent of the total amount of culls is ever used for feeding. There appears to be no definite policy in various sections of the country with regard to feeding surplus sweet potatoes (17), and a study of recent data relative to production of sweet potatoes and the number of hogs on farms in various states, especially in the South, does not indicate any high degree of coincidence of areas of large hog production with areas of large sweet-potato production ( 2 2 ) ; hence it does not appear that canning and feeding to livestock offer a t present an adequate solution of the problem of utilization of cull sweet potatoes. For reasons which will be discussed more fully later, it is advisable, in considering means of utilization of cull sweet potatoes, to include also the problem of utilization of cull and surplus white potatoes. It has been estimated that, over a series of years, an average of 10 per cent of the white-potato crop in the United States consists of culls and unsalable stock (culls and No. 2 grade), and this figure is regarded as probably being low rather than high. The same authority also estimates that, over a series of years, the loss due to diseased and frozen potatoes and storage shrinkage has been about 10 per cent of the crop. Thus the average total loss which could be
HE sweet potato (Ipomoea batatas), which is related to
the common morning glory and the edible portion of which is an enlargement of the root resulting mainly from the storage of starch, is one of the most important vegetables, from the standpoint of value and production, grown in the United States, being second only to the white potato. It is grown most extensively in the Middle Atlantic and Southern States, and from the standpoint of soil requirements is particularly Fell adapted to a moist but well-drained, light, sandy loam, such as is found in the coastal region extending from Kew Jersey to Texas along the Atlrtntic Ocean to the Gulf of Mexico. The total production of sweet potatoes in the United States in 1929 was 84,521,000 bushels, with a farm value, as of December 1, of $79,819,000 (63). A total of 821,000 acres (63) was devoted to growing sweet potatoes in 1929. The importance of the sweet potato from an economic standpoint is particularly to be stressed in view of the high degree of suitability of this crop to a large area of light sandy loam land which has been deforested of pine during recent years, and which a t present is being utilized only to a limited extent. It has been estimated that not more than 2 per cent of the total area mentioned is now being used for that purpose ( I ) . Measures which would make profitable an extension of sweet-potato production would therefore be of great importance to a large section of the Middle Atlantic and Southern States. Sweet- and White-Potato Culls The rigorousness of grading requirements for market grades of potatoes and the fact that t’hesweet potato is very irregular in size result in rejection of a large proportion, variously estimated a t from 10 to 30 per cent, of the total sweet-potato crop as culls, including both over-size and under-size potatoes ( 2 7 ) . The large proportion of the total crop of sweet potatoes I Received June 30, 1931. Presented before t h e Division of Sugar Chemistry a t t h e 81st Meeting of the American Chemical Society, Indianapolis, Ind., March 3 1 t o April 3, 1931. 2 Contribution S o . 111 from Carbohydrate Division, Bureau of Chemistry a n d Soils.
1206
I S D U S T R I A L A S D ESGI,VEEECI,VG CHEMISTRY
saved by utilization is between 10 and 20 per cent and, in all probability, is a t least 15 per cent of the crop.3 ilside from very limited feeding to livestock, the only systematic means of utilization of cull and surplus white potatoes in the United States is for production of starch, this being confined to starch factories in Aroostook County, Maine, and to a few factories in Minnesota. Means of Utilization of Sweet- and White-Potato Culls
Considerable attention has been given by various investigators to possible means of utilization of cull and surplus white and sweet potatoes in the United States. Since starch is the principal valuable constituent, the production of alcohol or some derived saccharine product has naturally been considered, It is now generally agreed, however, that use of potatoes for alcohol production is not feasible in competition with blackstrap cane molasses (the principal raw material now used in the United States). The production of sirup from sweet potatoes, which was proposed several years ago (6), has not proved feasible so far. Although production of starch from cull white potatoes in Maine has proved profitable (S), this means of utilization in other sections of the country has generally been considered impracticable, largely because of insufficient constancy of supply and inadequate market for potato starch. Most of the white-potato starch consumed in the United States is used in the textile industry for the sizing of warp and the finishing of certain classes of textiles. The production of starch from sweet potatoes has been confined almost exclusively to Japan, with some small production in Burma and Brazil. In 1919 there was produced in Japan a total of 276,993,931 pounds of starch, approximately 75 per cent of which was white-potato starch, whereas 22 per cent was sweet-potato starch, and 3 per cent consisted of other starches (8). Sweet potatoes are preferred to white potatoes as a source of starch in some sections of Japan, because of their greater content of starch and the fact that the plant requires less fertilization than white potatoes ( 2 7 ) . The variety generally used is Shijunichi-imo (40 days’ sweet potato), which has a white or light-red skin and milky-white flesh; because of lack of desirable sweet flavor, it has limited use as food and is comparatively cheap. The trend of consumption,, production, and importation of white-potato starch in the United States over a period of years is shown in Table I. Table I-Production Importation and Consumption of White-Potatd Starch in th; United States DOMESTIC YEAR PRODUCTION (25) IMPORTS (26) CONSUMPTION‘ Pounds Pounds Pounds 32,147,438 27,709,400 4,438,038 1904 40,291,674 15,418,259 1909 24,873,415 37,475,213 13,934,741 1914 23,540,472 3,194,595 19,671,781 1919 16,477,186 14,134,454 1920 6,100,577 15,025,504 1921 8,924,927 7,295,077 1922 11,981,565 16,671,316 1923 4,689,751 11,440,594 1924 10,714,747 20,842,303 1925 10,127,556 19,166,890 1926 34,350,473 27,272,048 1927 7,078,425 17,216,209 1928 42,822,106 28,282,097 1929 14,539,709 a Exports of starch from the United States are classified by the Bureau of Foreign and Domestic Commerce, U. S . Department of Commerce, as “corn starch” and “other than corn starch.” Since the latter consists very largely of wheat starch, and exportation of potato starch is neg!igible, consumption of potato starch has been estimated by adding domestic production-to imports.
These figures show that the total consumption of white-potato starch in the United States has been steadily increasing since
* White potatoes constitute the largest vegetable crop in the United States. The average acreage for the years 1924 t o 1929, inclusive, was 3,364,500 acres. with an average estimated farm value, as of December 1, of 8314,148,833 (calculated from data in U. S . Department of Agriculture Yearbooks, covering the period in question).
Yol. 23, No. 11
1921, and that domestie pr.ducti in has been decreasing for the last 10 years as compared with the previous 15 years, whereas imports have been increasing. This situation is not due to the lower price of imported starch. In fact, imported potato starch (now obtained principally from Holland and Germany) has been selling a t l/4 to 1 cent per pound above domestic potato starch. From July 2, 1928, to Sovember 19,1928, the price of domestic potato starch was 53/4to 61/4cents per pound as compared with 6 to 6’/2 cents for the imported; during 1929 and 1930 the general price range was 5l/2 to 6 cents for domestic and s3/4to 6l/1 cents for imported potato starch (10). Imports of potato starch during 1927 were about four times the quantity produced in the United States, and during 1929 (a very favorable year for domestic production) they were about twice the quantity. The reason for the higher price of imported potato starch is its superior quality, particularly in color, domestic potato starch being frequently gray and off-color in comparison. By producing domestic potato starch of quality and color equal to the imported potato starch and selling on a competitive basis, there is an opportunity to increase the production of domestic potato starch to as much as five times the recent output without any increase in total consumption, i. e., by simply displacing imported starch. The recent increase in the import duty on potato starch from 13j4cents to 21/2 cents per pound (18,19) is a favorable factor. However, the increase in tariff alone will not bring about this result, since certain consumers must have a high quality and white color and will pay a premium for it, although an excessive premium may reduce consumption. Total potato-starch consumption in the United States in 1929 (see Table I), including potato starch used as such and dextrin made therefrom, amounts, on a potato basis, to a p proximately 1.65 per cent of the domestic white-potato crop for that year, whereas domestic potato-starch production amounts to only about 0.56 per cent of the domestic white potato crop for 1929. These figures have been calculated on a low starch-yield basis, by using the ratio of 1 ton of starch obtained from 100 barrels of white potatoes, which is fairly typical of results obtained in Maine (9). It has been estimated that means for an average utilization of 10 per cent (comprising culls and surplus) of the white-potato crop are required in order to stabilize this crop and provide for years of overproduction. While the utilization of 1.65 per cent of the white-potato crop, which could be achieved by merely displacing imported potato starch without increasing total consumption, would be only about one-sixth of that required for stabilization of the entire crop, it would be quite sufficient to stabilize the potato crop in certain important areas. It must be remembered that potato districts located too far from points of consumption of potato starch would be handicapped by excessive freight costs. Thus it appears that, under certain conditions, an extension of the potato-starch industry in this country might be profitable and might assist materially in stabilizing the potato crop. The outstanding difficulties with regard t o utilization of farm wastes have been as follows: excessive cost of assembling low-grade material, usually of high-water content, a t one point in sufficiently large quantities to justify large-scale operation; and excessive overhead cost per unit of production because of insufficient supply of raw material to justify factory operation over more than a limited period of the year. For these reasons the authors have considered the possibility of production of starch from culls of white and sweet potatoes in the same factory. Such a plan might be feasible in certain sections, for instance, in the states of Virginia and North Carolina, which have the geographical advantage of being situated near the southern limit of substantial white-potato production and near the northern limit of large sweet-potato
November, 1931
ILVDCSTRIL4LA X D ESGIAYEERISG CHEJIISTRY
production. The availability of inland coastal waterways for affording cheap transportation is also of importance. Having decided that certain economic factors relating to production and consumption are reasonably favorable, provided starch of suitable quality can be produced, investigations have been conducted on sweet-potato starch from the standpoint of its properties and quality and of the technology of production. The technology of the production of whitepotato starch is well developed in E u r ~ p e ,and ~ there is probably no particular problem involved in applying existing procedures to the conditions in this country, except possibly the need for some modification, such as use of sulfur dioxide to prevent fermentation in case production of white-potato starch is practiced further south and in warmer periods of the year 'than is now customary. The authors have regarded production of sweet-potato and of white-potato starches as being possibly carried on together in any further industrial develop ment, inasmuch as they are similar in properties and, although this may not be indispensable, production of both in the same factory might serve to extend the annual period of operation. Problem of Color in Sweet-Potato Starch Production of starch from sweet potatoes has received the attention of a number of investigators ( I G ) , and a review of their work reveals two outstanding technical difficulties. One was the relatively low extraction of starch (50 to 57 per cent of the starch in the potato), which can be considerably increased, however, by means of pulping machines of more recent design and by regrinding the pulp after the initial extraction. The cell membranes enclosing the starch granules of the sweet potato apparently resist rupture to a greater degree than do those of the white potato. Fortunately, granules of sweet-potato starch are considerably smaller than those of the white potato, thus making it possible to subject the pulp to more rigorous grinding and yet separate successfully the finely divided pulp from the starch by the use of a finer refining screen (250 mesh). The second difficulty experienced by previous investigators concerns the color of the starch. Although starch having a fair degree of whiteness was obtained a t times, the results were not consistent. Earlier experiments of the authors fully substantiated these results. In order to compete successfully with prime white-potato starch in the textile industry, i t is essential, aside from the question of price, that sweet-potato starch be equal to it in whiteness, that it yield colorless pastes, and that the viscosity of these pastes be sufficiently high and sufficientlystable when kept hot for several hours. Of the two problems mentioned it was believed that the color of the starch should be given first consideration, for if a solution were found to be impossible or impracticable, there would be little chance for the development of an industry of sweet-potato starch. The pigments of the sweet potato have received very little investigation, and the information recorded in the literature relative to the identity and properties of these pigments is very limited, In the first series of experiments pertaining to the study of color, the influence of the color of the potato flesh upon that of the starch was determined in a general way. By the customary process for white-potato starch, the starch was prepared on a laboratory scale from sweet potatoes of the following groups and varieties: Spanish group-Yellow Spanish, Red Spanish, Triumph, Porto Rico, and an unknown variety, probably the Purple Yam; Florida g r o u p N a n c y Hall, Florida, and General Grant; Southern Queen group-Southern Queen; Pumpkin group-Norton; Jersey group-Big-Stem Jersey @1).
The white-flesh varieties of the Spanish group yielded the
* The German procedure
i n greatly abridged form is used in Maine.
1207
lightest-colored starch; but, with a single exception, the starch was not quite equal in color to prime white-potato starch. Contrary to what might have been expected, the yellow-flesh potatoes yielded starch which not only varied considerably in color, but which also, in many instances, was not as white as that produced from a potato such as the Nancy Hall having a more highly colored flesh. I n these experiments the Southern Queen variety gave the most inferior starch. The Southern Queen and the Porto Rico varieties contain unusually large quantities of milky latex which, after grinding, soon becomes dark and sticky, probably as a result of oxidation. Later experiments indicate that this latex does not have any influence on the color of the starch but does have a tendency to adhere to the screens used to separate the starch from the pulp, thereby reducing the size of the screen openings. These experiments indicated that it might be possible to produce a fairly satisfactory starch from light-flesh sweet potatoes. This conclusion is in agreement with results obtained in Japan (@,where such varieties are selected for starch production and are little used for culinary purposes. However, to use these varieties for starch in this country would entail growing them specifically for this purpose, since the varieties that are ordinarily grown for market are those with more highly *lored flesh. Consequently, the use of the lightflesh varieties for starch production would not solve the problem of the utilization of the culls which result from the grading of the market varieties. It was found in preliminary experiments that the starch obtained from peeled sweet potatoes was somewhat superior in color to that obtained from unpeeled potatoes. However, i t did not appear that the expense of peeling was justified by the improvement in quality of the starch. The solution of the problem of color depended upon devising a procedure somewhat different from that used in producing white-potato starch and upon including in this procedure carefully controlled chemical treatment. In experiments made with this end in view, the first results which appeared a t all promising were obtained by steeping the sliced potatoes in a dilute sulfurous acid solution for several hours before grinding. Further experiments indicated that results equal t o steeping could be obtained by applying sulfur dioxide-water in a continuous stream to the grinder while pulping the potatoes. The use of sulfur dioxide is of value also in controlling fermentation during warm weather. The starch resulting from the use of sulfur dioxide in the manner indicated remained very white as long as its reaction was slightly acid. Upon applying the ammonia test6 to this starch, it turned brilliant yellow, and the color was much more pronounced than that obtained in the absence of sulfur dioxide treatment. It was evident that the use of sulfur dioxide during grinding materially altered the pigments with great benefit to the color of the starch, provided it did not acquire an alkaline reaction. Although the application of sulfur dioxide in the grinding process was a distinct advance toward a solution of the color problem, i t was still necessary to devise means of eliminating the pigments which respond to the ammonia test. This test is not always applicable, however, since the pigments become altered, probably by oxidation, and fail t o give a yellow color with ammonia after the starch has stood for several weeks, although the starch, if originally off-color, remains so. Regardless of the starch process used, dilute alkali solutions dissolve a varying proportion of certain contaminating substances from sweet-potato starch and yield a highly colored yellow or green extract (depending upon the concentration of alkali). Addition of a drop or two of concentrated ammonium hydroxide solution to a suspension of starch in cold water. This is a useful test frequently employed in the starch industry.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1208
In most cases, however, the color of the starch made by the white-potato process was not greatly improved by such a k a line treatment, even after washing the starch free from alkali. The pigments were very resistant to the action of various chemical reagents, including bleaching agents, which i t a p peared possible to use without materially altering the properties of the starch.
Vol. 23, No. 11
tration of sulfur dioxide mentioned above does not reduce the p H to the isoelectric point of the protein unless an abnormally large proportion of sulfur dioxide-water is used a t the grinder. Addition of sulfur dioxide to the water used a t later stages of purification of the starch is not to be recommended for the reasons indicated, Although an alkali is used a t a subsequent stage of the process, it is desirable that its function be limited
SWEET POTATOES SULFUR
WATER-WASHER
I STOVL
/ I
WATER1
SCALES
su$:$ous-
I
\
DISCARD
I ST WLPER
1
I JT SCREEN-WATER
P N D PULPER WATER-PND
1
SCREEN
I I
MIXING
DRIER MIXER
I I I WAREHOUSC SAGS
SCALES
NaOn SOLUTION
I
DISCARD EFFLUENT
STARCH
I
WATER /
\
EFFWENT DlfCARD I
STARCH +&O :N MIXING TANK
I C%?F%-
WAXCR
h R c H
WFL&
I
I
DIXARD
DRIER
t
WAREHOUSE
Figure I-Flow
Diagram for Manufacture of Sweet-Potato Starch
It was noted, however, that alkalies, such as ammonium hydroxide and sodium hydroxide, almost completely dissolved the interfering pigments from starch produced by the sulfur dioxide extraction process, provided the treatment was sufficiently thorough. Starch which gave only an extremely faint ammonia test was obtained. This starch was fully equal in color to imported white-potato starch and yielded a gelatinized paste (1 part starch to 15 parts water) of equally satisfactory color. I n some instances the starch was somewhat superior in color to high-quality Dutch or German potato starch. Moreover, the physical properties of the starch were not adversely affected by the chemical treatment when properly controlled, as will be shown later. The complete process as finally developed was then tested on both laboratory and semi-industrial scales with various lots of sweet potatoes, stressing particularly the Nancy Hall, Porto Rico, and the Southern Queen varieties, which previously had yielded inferior starch from the standpoint of color. Excellent results were obtained in every instance. For the steeping of grains, as in the production of corn starch, a 0.2 to 0.3 per cent sulfurous acid solution is customarily used, but for sweet potatoes, in spite of their higher content of moisture, a content of about 0.15 per cent sulfur dioxide in the water used for grinding was found to give satisfactory results. An important factor controlling the concentration of sulfur dioxide to be used is the fact that sweet-potato protein has a fairly definite isoelectric point a t a pH of about 4.0, which must be avoided owing to flocculation of protein a t this pH value and cont,amination of the starch with this material during the process of settling. Since the constituents of sweet-potato juice have a strong buffer action, the concen-
to the removal of substances responsible for color and that it should not include the elimination of protein. In the first experiments very dilute ammonium hydroxide solution was used for treating the starch, and although it gave satisfactory results, its odor may prove objectionable in commercial practice; also, its cost is greater than that of sodium hydroxide. A solution containing 0.5 per cent caustic soda (70 per cent sodium hydroxide) based on the weight of dry starch has been recommended in certain starch industries which employ an alkaline treatment. Numerous tests showed that this concentration is also sufficient for extracting the pigments from sweet-potato starch following sulfur dioxide treatment, provided sufficient time is allowed. With constant stirring, the pigments responding to the ammonia test are almost completely extracted within 3 hours, but, in order to allow ample time, it is recommended that the total time of contact by stirring be a t least 5 hours, after which the starch may be allowed to settle overnight. The supernatant liquid may then be drawn off and the starch purified further by tabling and washing. As it is important to eliminate the alkali completely, the starch from the tables should be stirred in very slightly acidified water (using sulfur dioxide or acetic acid), filtered or centrifuged, and washed further with water. Any residue of free sulfur dioxide or acetic acid remaining in the starch will be volatilized without detrimental effect in the operation of drying the starch. Having developed a practical procedure for the production of starch of prime grade from sweet potatoes of any variety, as was demonstrated on both laboratory and semi-industrial scales, a proposed scheme of factory operation was devised as shown in Figure 1. It is believed that the procedure outlined
November, 1931
IA'D G S T R I A L 41VD ENGIA'EERIA'G CHEMISTRY
is well adapted to such modifications as may appear desirable in the light of further experience. Although the diagram is practically self-explanatory, there are several features of the process which require some discussion. Various Features of Sweet-Potato Starch Process
HANDLIKG SWEETPOTATOES-The manner in which the sweet potatoes are to be handled depends upon a number of factors, such as the proximity of the starch factory to the source of supply and the question as to whether the potatoes are obtained direct from the field or from storage. For maximum starch yield it is essential that the period between digging the potatoes and extracting the starch be reduced to a minimum, since during storage of sweet potatoes a portion of the starch undergoes transformation as the result of enzymic hydrolysis ( 7 ) . If sweet potatoes are grown specifically for starch production,6 it is possible to stagger the plantings and defer the harvestings so as to insure a steady delivery of freshly dug potatoes a t the factory for a period of about 60 days in the northern sweet-potato belt and about 90 days in the Gulf region. If the factory operating period is to be extended further, some provision for low-cost storage must be made. If a sufficient quantity of cull white potatoes is available, it would be preferable to use these, so far as possible, after the supply of sweet potatoes is depleted, as white potatoes undergo less deterioration in storage. WATERSUPPLY-An ample supply of water of good quality is a prerequisite for the manufacture of starch. The quality of the water is especially important if it is desired to make starch of the finest grade. It has been claimed that distilled water has been used in the purification of white-potato starch. Water of this degree of purity, however, is not essential if care is taken to insure its cleanliness and freedom from dissolved salts which might affect the quality of the starch. The quantity of water required for the manufacture of sweetpotato starch probably will not exceed that used for whitepotato starch, which has been variously estimated a t from 9 to 24 times the weight of potatoes (12, 15). Owing to the fact that sweet potatoes usually contain less water than white potatoes and that more extensive purification of the starch is required, the water consumption will be nearer the larger than the lower estimate. GRIXDING-Since the effectiveness of starch extraction depends primarily upon the extent to which the walls of the cells that contain the starch granules are ruptured, particular attention must be given to this step in the process. Two-stage grinding is necessary for maximum recovery of starch. For the initial grinding, mills of a number of different types, such as those operating on the saw-blade, swinging-hammer, or squirrel-cage principles, undoubtedly fulfill the requirement There is a t present no finality of opinion in the potato-starch industry regarding the type of mill which is most effective for the second grinding. It is desirable that the cell walls be simply ruptured with as little disintegrating action as possible, since too great disintegration of the pulp increases the difficulty of separating the starch. Regrinding tests with a stone buhr mill conducted a t the experimental starch factory of this division a t the Department of Agriculture Experiment Farm a t Arlington Farm, Va. (near Washington, D. C.), gave favorable results. SCREENING-The separation of starch from the pulp, which is accomplished by thorough washing on screens, presents no serious problem. Screens of any of the types developed for manufacture of white-potato starch will serve the purpose. Screens of finer mesh than those used for white-potato starch 6 In 1928 the LdfOurChe Starch and Refining C o , Thibodaux, La., produced starch from sweet potatoes (Triumph variety) grown specifically for this purpose, but operation was discontinued later, largely because of failure t o produce starch of satisfactory quality.
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may be used for sweet-potato starch, on account of the smaller size of the granules of the latter. A 100-mesh screen is satisfactory for the first stage, but the refining screen should be of a t least 200 mesh. When fine pulping by regrinding is practiced, it may be found desirable to screen the starch suspension a t later stages of the purification process through an even finer screen, such as 250 mesh [size of opening, 0.0024 inch (0.0061 cm.)]. iilthough such screens are rather expensive, their extra cost is justified by the improved quality of the starch. STARCH PURIFICATION-PUI'ifiCatiOn Of Sweet-pOtatO starch really begins a t the grinding stage, for it is here that the sulfur dioxide treatment is applied. Sulfur dioxide-water of the proper concentration (0.15 per cent) should be added in a continuous stream. In addition to having a beneficial effect upon the color of the starch, it apparently retards oxidase action and fermentation. The fruit water separated from the starch does not become nearly as highly colored in the presence of sulfur dioxide as in its absence. Screening is the second step in purification, and the third step consists in separating the starch from the contaminated water in which it is suspended as it leaves the screens. This separation may be conducted by settling in tanks, in troughs, (starch tables), or in centrifugal separators. A quick separation of the dark-colored fruit water is conducive to the production of high-quality starch. If tables are used, the effluent is run into settling tanks in order to recover the small quantity of fine-grain starch which is always present. When a sufficient quantity of this second starch has accumulated, it may be refined in a manner similar to the purification treatment used for first-grade starch. Tank-settling processes of p~rification,~ such as are sometimes used for white-potato starch, do not function well with sweet-potato starch, because the rates of deposition of the starch and of the suspended impurities are so nearly the same that it is not possible to effect a high degree of separation. When starch tables are used, the continuous flow of the liquid a t a suitable gradient carries away a large proportion of the material which would settle with the starch in a tank. Smallscale tests indicated that three settlings of the starch on the tables (with sodium hydroxide treatment applied between the last two settlings) are sufficient t o free the starch from suspended potato fiber. The additional washing of the starch (with slightly acidified water, and then with plain water) may conveniently be conducted on leaf-type vacuum filters, such as are used in the corn-starch industry, or in centrifuges, with the additional advantage that the starch discharged from the filter or centrifuge is in suitable condition for drying. STARCH DRYIiw-This stage of starch manufacture is highly important, for improper heating may readily injure the quality of the starch. Any one of the drying systems employed in the starch industry is applicable to sweet-potato starch. However, in designing a strictly up-to-date plant, preference would be given to recent models of continuous driers with accurate temperature control. By-products of Sweet-Potato Starch
The waste materials resulting from potato-starch production consist principally of the residual pulp and the waste waters from the various stages of starch separation and purification. Suitable utilization of these materials is desirable in order that the operation as a whole may yield a satisfactory profit. The greater sugar content of sweet potatoes (especially after storage) as compared with white potatoes is a factor contributing to the value of the waste waters, and salvage of this sugar requires careful consideration. Purification by settling in tanks is used exclusively in the white-potato starch indiistry in Aroostook County, Maine.
A proniising by-product is a stock feed&prepared from tbe spent pulp, possibly mixed with a sirup rich in sugar@and protein, prepared by evaporating the more concentrated portions of the waste water. The pulp can be dried economically by directly heated tunnel driers, such 89 are used for drying spent-beet cossettes in beet-sugar factories. In the flow sheet (Figure 1) only tlic utilization of the pulp is indicated. I t is possible to Pegregate the niore concentrated portion of the waste wat,er. This portion CRII he Concentrated
Sweet potatoes (2200 prams) were grouiid with 1100 grams of water. The pulped mass was squeezed by hand in a cheesecloth bag, and t h e pulp was then treated and squeezed with several successive portions of watcr of 1100 grams each. The starch was allowed to settle from each portion, and the aqueous solutions were analyzed. The data are summarized in Table 111. ?'able III---Compo~ifionof Successive Fraetione from Scareh Table
1
50
2
50 50 50
a
4
.. 3.7
6.10 2.R0 1.35 0.65
1.8 0.9 Lost
.. 60:s
62.4 66.1 Lost
0.%0 0.201
11.1
0.115 0.048
8.5 7.j
7.2
About 90 per cent of the soluble material of sweet potatoes i s extracted by the use of only 150 per cent of water (based on weight of potatoes); from the standpoint of utilization i t wonld hardly be practicable to concentrate any fraction of effluentbeyond the third in the series shown in Table 111. The water extracts from this experiment were combined and evaporated to a heavy sirup, which w&s added to the spent pulp in the ratio of aiiout 1 part by weight of sirup to 4 parts of air-dried pulp. The resulting product, after being airdried and coarsely ground, presented an attractive appearance as a stock feed, and the C O I I ~ ~ I I S UofS opinion of severa,l feed manufacturers and specialists regarding it was very favorable. The behavior of this product with respect to
Figure 2-Laboratory Grlndcr and Tanks
I
of EWzzent
Q"*NT*rY
econoniically by multiple-effect x.acuiim evaporation if starch production is conducted on a sufficiently large scale. The approximat,e ccmposition of spent air-dried saectpotnto pulp produccd on a laboratory scale is shown in Table 11. This composition repre:ents tlrc results obtained by single grinding and low-starch extraction. Wit11 efficient operation and regrinding of the pulp, its starch cont.ent would, of course, be reduced considerably. In the semi-industrial plant used in these experiment.6, dried pulps (with a11 equivalent moisture content) have been procured containing 52.6 per cent of starch. Tabla 11-Compeition of Air-=led Spent Sweet-Potato Pulp (Single Grinding) V A Y l B T Y OPw'*ran C.""II T.niew PII"T-3.N Swhxr POTATO CO\TFNT STARCW Finrn EXTRAIT Asrr (N x 6 25) 40 % ?z c% V " 1 '," . .. ," ," 9.24 9.09 8.23 12.31 7.41
66:& 72.62 68.19 66.27 64 .BO 67.15 64.45
6.'& 6.41 5.72 5.84 7,4x 5.94
9.13 8.33 5.20 Determined by acid~hydrolydrmethod. a lnciuded ior cOmphrironwith rwret-potato
0.65 1 27 0.14 (1.22 0.98 0.40 0.27
1.34 1.08 1.54 1.333
La8
1.75 1.23
2.10 1.58 2.84
2.71 6.41
2.98 4.20
Figure 3 Lahuremry Starch Tables ~~
The constituents of the fruit water and other effluents which are most valuable from the standpoint of utilization in a sttick feed are the sugars and protcins, which are present in the sweet potato to the extent of from 2 to 9 per cent and from 1 to 2.5 per cent. respectively. In several laboratory experiments the effluent from the first starch table (about 300 per cent water was used based on weight of potatoes) was found to have an average solids content corresponding to about 2.2" 13rix, and 50 to 67 per cent of the solids consisted of sugars. Since it is not practicalile to utilize effluentsof such low density, the following experiment was eonductrd in which theeffluentn-asanalyzed at various stages of starch ext,raction to detennine a t what point a separation could be made without uiidue loss of sugars and protein:
-
Such a feed would be cornplrabie to the by-product feed produced by grain-alcohol distilleries from meat erains and concentrated bCn-sfiil slop and otlirr ~ . m s i e h
hygroscopicity is very similar to that of commercial molasses feeds, and it may, therefore, be regarded as acceptable from the standpoint, of stability, provided its initial moisture eontent is properly controlled. The composition of this product, show11 in Table IV, indicates that it should be supplemented with additional protein and fat to make a Inore balanced feed. This could readily be supplied by adding cottonseed, peanut or soy-bean meal (products generally available in the reginn where sweet potatoes are grown). It is possible that, by using a continuous mechanical expeller or dewaterer, such for instance as is used for pressing sugar-beet pulp, fruit water might be pressed from the pulped potatoes, thus reducing dilution with watcr and restricting the cost of evaporation te sirup. An experiment was made to determiiie whether or not the sulfur dioxide applied earlier in the process vould he retained in sufficient quantity to preclude the utilizatioo of the effluent
INDUSTRIAL A N D ENGINEERING CHEMISTRY
November, 1931
as an ingredient of stock feed. A quantity of sweet potatoes was ground with about 200 per cent of water containing 0.21 per cent sulfur dioxide (by titration). After extracting and separating the starch, the effluent was analyzed for sulfur dioxide a t two stages of evaporation in a vacuum of about 29 inches (73.7 cm.). These results are shown in Table V. Table IV-Composition
of Sweet-Potato Sirup and Pulp Feed
( N o sulfur dioxide used)
% Moisture Ash Total sugars Starch (by diastase) Crude fiber Ether extract Protein (N X 6.25) Undetermined
11.63 2.57 14.32 49.59 3.51 0.44 3.56 14.38
Table V-Analysis of EfRuent of Sweet-Potato Starch Containing Sulfur Dioxide TOTAL PROTEIN CONCENTRATION AT WHICH SO& SUOARS, ( N X 6.25). BASE^ ON BASEDON BASEDON EFFLUENT SAMPLE WAS TAKEN SOLIDS SOLIDS SOLIDS SOLIDS Original After partial evaporation Final sirup
%
%
4.32 47.97 79.78
2.16 0.61 0.37
%
%
70:k2
6102
...
..
An additional loss of sulfur dioxide may be expected in commercial practice if the sirup is added to the pulp before subjecting the latter to the final stages of drying, and dilution of the sirup by mixing with pulp would cause a further reduction in the percentage. It is believed, therefore, that the use of sulfur dioxide in the quantities specified will not seriously interfere with utilization of effluent as an ingredient of mixedpulp feed. Production of starch and cattle feed from sweet potatoes would constitute a completely integrated southern industry with its operations confined to a limited area, thereby reducing all transportation costs, The starch could be utilized by southern cotton-textile mills, and the feed could, in all probability, be consumed advantageously by the southern dairy industry, which apparently has distinct possibilities of expansion. Cottonseed and peanut press cakes, which could be used to increase the protein and fat contents of the feed, are also products of the same region in which sweet potatoes a r e extensively grown. Other means of utilizing wastes of sweetpotato starch are also under investigation, particular attention being given to certain fermentation products in view of the large content of carbohydrates. Starch Extraction and Refining on Laboratory Scale I n the laboratory experiments, each run was made with about a bushel of potatoes as raw material. The equipment used is shown in Figures 2 and 3. The grinder (Figure 2) is a commercial-size, hand-driven cocoanut shredder, which was converted to a motor-driven machine and mounted on an iron pipe stand. If the potatoes
-Total
Table vI-AnalYSiH
1211
are previously sliced or coarsely ground, a bushel may be pulped within l l / z hours. Water was supplied to the grinder by siphon from a reservoir, and the rate of flow was controlled by means of a stopcock. The pulped potatoes were caught in large precipitating jars, from which they were transferred to cheesecloth sacks, and the starch was extracted by intermittently squeezing out the starch suspension and kneading in several changes of water. It is possible by this simple means to liberate the free starch completely from the pulp, as was demonstrated by microscopic examination. The crude starch suspension was strained through a 200-mesh screen into a 15-gallon (56.9-liter) tank equipped with a motoroperated stirrer and an outlet at the bottom (Figure 2). This tank was mounted upon a rigid frame equipped with casters so that it could be readily moved. Separation of the starch was effected by tabling in two wooden troughs, 2 inches X 3 inches X 10 feet (5.08 cm. X 7.62 cm. X 3.05 meters), lined with tin plate and superimposed one above the other (Figures 2 and 3). The gradient of the troughs could be varied as required. Although constancy of flow of the starch-bearing water to the table, such as could be accomplished through a weir box, is desirable, it is not essential. Refining of the starch consisted in resuspending it in water, rescreening (250-mesh screen), and retabling a t least twice. In the experiments in which the starch received sodium hydroxide treatment, this operation was performed between the second and third tablings, the starch being washed further on a vacuum-leaf filter. The starch was air-dried to about 10 per cent moisture content. The percentage extraction of starch with the laboratory equipment was low, amounting to approximately 50 per cent of the starch in the potato. This is somewhat less than can be obtained from white potatoes under the same conditions. The starch content of the sweet potatoes ranged from 14.9 per cent to 25.7 per cent. Starch Production on Semi-Industrial Scale I n order to study the technology of production of sweetpotato and other starches on a larger scale which would permit a high percentage of extraction, complete semi-industrial starch production and refining equipment was installedgin the Color and Farm Waste Division Building of the Department of Agriculture Experiment Farm a t Arlington Farm, Va. This experimental starch plant is unique because of its compactness and extreme flexibility of operation, which was the keynote in designing the installation. It permits extraction and purification of starch from any plant source whatever, and the various units are so interconnected that they can be operated in any sequence desired. General views of the equipment are given in Figures 4, 5, and 6, showing the grinders, shaking and brushing screens, storage tanks, settling troughs, continuous leaf-type vacuum filter, and centrifuge. The equipment also includes a potato washer, starch driers of two types (Proctor truck and Stokes vacuum), a ball mill and grinders of three types. 9 This starch plant of semi-industrial scale was designed by and installed under the direction of the late L. E. Dawson, formerly in charge of starch investigations in the Carbohydrate Division.
of White-Potato and Sweet-Potato Starches
HCI.
STARCH
IKSOLUBLE
ASHWATERMATERIAL MoistureWaterHCISOLUBLE PROTEIN (RASP; Momfree soluble insoluble MATERIAL (NX6.25) METHOD) ' r u m Original basis
%
%
%
%
%
%
%
0.308 0.365 0.247 0.258
0.026 0.143 0.024 0.019
0.056 0.133 0.022 0.025
0.385 0.205 0.265 0.080
0.100 0.175 0.138 0.100
0.193 0.200 0.159 0.083
11.87 14.64 16.83 17.33
0.396 0.315 0.278 0.173 0.113 0.182 0.116 0.115
0.035
0.064
0.110
0.069
o : k
o:iis
olio3 0.105 0.000 0.150 0.100 0.260
o:ik
0.116 0.110 0.094 0.100 0.063 0.168 0.105 0.103
13.21 15.S5 8.26 8.02 8.09 12.09 11.85 10.36
%
SOURCE OF SAMPLE
%
WHITE-POTATO STARCH
86.36 84.12 80.10 81.44
97.99 98.55 96.31 98.51
Prime domestic (Minnesota) Superior domestic (Maine) Pearl domestic (Maine) Imported (Holland)
SWEET-POTATO STARCH
0.019 0.017 0.F12 Nil Nil
0.054 0.043 0.114 0.063 0.063
0.249 0.306 O:i06 0.018
85.57 81.30 89.62 89.79 89.39 86.34 86.39 86.96
98.59 96.61 97.69 97.62 97.26 98.21 98.00 97.40
Commercial production, Louisiana, 1928-9 Produced in Japan SOr-NaOH process, laboratory scale Porto Rico variety SOZ-NHIOHprocess, laboratory scaie, Porto Rico variety Son-",OH process, laboratory scale, Southern Queen variety SOz-NaOH process, laboratory scale, Nancy Hall'variety SOz-NaOH process, plant run, Nancy Hall variety S0z-NaOH process. plant run, Nancy Hall variety
T h e tanks. which caii be used fur steiping, starch settling, or storage, are made of a noncorrosive rrlloy designed to withstand the action of chemical agents, SUCII as caustic soda or sulfurous acid. One of the tanks is equipped with a large draw-off valve which may S C T V ~for discharging steeped material into a scroll elevator leading to the hopper of the grinders. A spray of water over the scroll permits washing of the matcrial being elevated. T h e shaking or jig screen is divided along the center, and each half is covered with silk holting screen of different
mesh.
Figure 4-Semi-lnduatrisl
Starch Plant
Genrisd view looking r.bichhas becn here proposed for the manufacture of sweet-pottito starch, provision is made for double grinding and screening, and table-settling and chemical treatment, uhich may add approximately 0.5 cent per pound of starch to the cost (if inaniifaeture.
sweet-potato starch, and it is estimated that the cost of a factury of this capacity would be approximately 50 per cent greater. It is probable that in regions of high density of sweet-potato production a sweet-potato starch factory of considerably greater capacity than the foregoing would ha feasible. I n the estimate just given, tlic capital expenditure required for manufacturing by-products w&s not included, since the most profitable method of utilization has not been finally determined. If i t is decided that production of stock-feed is the logical solution, cost estimates may be based on the production of sugar-beet pulp f&, the equipment for which would he satisfactory for handling sweet-potato pulp. Acknowledgment The autliors are indebted to F. 11. Thurber of this division for measurements of the comparative viscosity of paste made from sweet-potato starch, and to F. W. Coleman and J. B. Snider of this division for analyses of starch made from white and sweet potatoes.
.
Literature Cited
Some cariipcns;ttiiig advantages will accrue, however, by making a prime rather than a second-grade starch, by locating starch factories in the southern cotton-textile manufacturing sections of the country, which would reduce freight charges on the starch and also afford large supplies of readily accessible meet potatoes, and by utilizing to advantage the by-products of the starch factory, which a t present is not practiced in the domestic vhitepotato starch industry. The value of highgrade sweet-potato starch is expected to be practically equal to that of imported prime whiie-potato starch, quotations on which a t the present time are 5 S / rto SI/, cents per pound (10). At this price, there u d d appear to be suiticicnt margin for profitable manufacture of sweet-potato starch and a price for cull srveet potatoes which would represent, a satisfactory salvage value to the grower. The present cost. of a white-potato starch factory of sinall capacity (200 tons of starch annually) and of the type which exists in Maine and Xinnesota has been estimated a t about $10,000. A more elaborate process is recommended for
neattie,P ~~ ~~ ~~ ~ ~~ ~~ ~~ i I ~ ~ ( 2 ) Clemson (South Carolina) Agr. Cullege, Bull. 84 i l ! W l i 13) Corbett, C ~ l d w e i l ,Stiiart, Bratlie, and I'iohr. U. 5. T l q , t . .Ax'. Y c u book, p. 61s (1025). (4) Corbett, Caidwrll, S t u s i f , Beatfie, and Flohr, I,%., pi' til8 -21. 16) Corbett, Caldwrll. Sturrt, Beattie, and I'lohr. I b i d . , p . 6 2 0 . i G i Gore et PI..U. S. Itept. Agr., BuU. 1158 (1023). ( 7 ) Ililsselbrins and Ifaivkinr, J. A g r . Rmcorrh. 3. 881-42; 6. 648-60 (1015). i R J iap;iiiesc U c i i l ~ A f r i r i i l i i i r c arid Commerce, 35fli Statistical Rest.. 1,. 2LI i l W 0 ) ; 1'. S. 'I'hrltf Commission, Tariff Information Survey, "Sfarcli a i d Kclated Msferlaln," 1921. (9) Morrii"gsf*r, Nicol, IOC., New YWk, PErroual Co,".nanicnlioe, 110) O i l , Pain:. Drug Replr., June 8, 11131. (11) I'aror, "Hnndbuch der Stirkcfabrikntion:' Zzid rerired ed., Paul Pnrey, Berlirr, 1028. (1%) Rehwald, "Starch-Making and the Manufacture ui Dextrin, Starch, Surar. .. Svrui, and Suxm Coloutina." tiandated from 6th rerired Germen cdilion by Salter. P. 07, Seott. Greenwood. 1026. (13) Rehwald, I&&, P . 108. (14) Reiehert, "The DiRerentiitiun arid Sprdhdtg of Starches i n Relation t o Genera. Species, cic.," Part 11, 1., 886. the Caiiregie Institution of Whrbington, 1913. 115) S-e, "Pabrikation dcr Knrtotfelsluke," p. 460. Springer, 1897. ( 3 0 ) South Carolina Agr. Exnt. Sfa., nidi. 136. 117) Summaiv of reulies to oiiesti~nnnireaddressed fo Am. Exnt. Stations ~. of several southern stater. (18) T h t f Act of 1922, Schedule 1, paragraph 85. (19) Taritf Art of 1930, Sehediiie 1. Paragraph 81. (20) l'aritf Readjustment, 1929. Herrings before Coiiimittee O n Ways ;arid Means. i l ~ u i eof Renreseetatives. 70th Conmess. 1, SQa. (21) Thomproa and Heattie, U. S. Uept. A s . . Bull. 1041, X-LO (1!121) (22) U. S. Depf. Agricuitiiir Yezibook, p p 8911,1001 (1027,. (23) U. S . Dq'f. Agriculture Yearbook. p. 780 (19311. (24) U. S . Depi. Commercc, Census Bur,, Biennial Ceiiciir 01 Manufacturas, 1925; l'ieliiinary Ilepi., 1B29. (25) U. S.Dept. Commerce, Census Bur., Bieniiial Ceiisiis of hliiiufacturerr. 1004, 1909, 1914, 1919, I!I21, 1923, 1025, 1Y27. 1929. (28) U. S. Dept. Commerce, Bur. I'orrign and Domestic Commerce, "For. elm Commerce and Navi~aliunof the United Strtcr," 1804, 1909. 1914, 1'119, 1!120-1920. ( 2 7 ) Zasshi, .I. C'hrm. Ind. (JanawI. 16, 1288-1313 (1912)
Portable Equipment Patented for Quick-Freezing FoodsPatcntshave beeiigrantcdcoveririgth~inventionof portableequipmcnt for quick-freezing perishable foods near their source on cattle ranges, in truck gardens. orchards, and on fish piers, according to a statemetit by General Foods Corporation. The equipment was designed a t the Birdseye IAmratorics, Gloucester, Mass., for use under the basic patent rights of the original Birdseye process. The new apparatus is being used this fall adjacent to harvest fields a t several places in the East for quick-freezing various kinds of vegetables and berries, and for freezing sea-foods. Its advantages over equipment now in use arc said to be increased freezing capacity per unit of space, faster loading and unloading, quicker freezing action, and smaller capital investment. It is
an enclosed series of refrigerated nletal platcs, arranged one above the other, between which packaged foods are sandwiched for quick-freezing. The new apparatus is known as the Birdseye multi-plate froster. While having similar elasticity and simplicity of operation, and utilizing the Same direct-contact and controlledpressure principles of the double belt machine, the multi-plate appafatus sets new standards of low first cost, efficiency of operation, and economy of floor space. It is an all-purpose machine, valuable for both small- and large-scale production; and will liaiidle either packaged or bulk products. I t costs less to install and operate, and requires less floor space than even a sharp-freezer of equal capacity.
(1)
Pisure 6
Semi-Induatrirl starch Piant
(irorral Yiew iron, corner near grindini: rquipmcot
~~
i
i
~
~
