Production of Sweetpotato Starch

F. H. Thurber, E. A. Gastrock, W. F. Guilbeau.

Sweetpotatoes are a big crop, and a highly variable one.

Of all the vegetable crops, they are second only to white potatoes. The fluctuations in production from year to year often have been enormous. In 1930 the yield was 54,517,000 bushels. In 1932 it was 86,594,000. In 1936 it was down to 59,765,000 bushels. Many factors influence the marketing of farm crops, but when such variations occur in supply it is difficult or impossible to establish a stable food market for a perishable crop. For that reason, the Department of Agriculture has had many requests for help in finding profitable industrial uses for sweetpotatoes to supplement the food outlets.

One approach was to develop procedures and equipment (which are now in commercial use) for drying sweetpotatoes for feed. Another was to investigate the possibilities in the fermentation industries, which use enormous amounts of carbohydrates in making industrial alcohol, lactic acid, and yeast. Sweetpotatoes have been successfully used for making all those products experimentally.

A third way is to isolate and purify the principal components and find profitable markets for them. It is with this third approach that we are here concerned. More than 72 percent of the solid content of sweetpotatoes grown for industrial utilization is starch. (Other materials are present in such small amounts that their recovery would be profitable only as byproducts.) To find an industrial outlet for sweetpotatoes, therefore, we concentrated on the determination of the properties, uses, and production of the starch although we were well aware that a tremendous amount of starch is used each year and that the production of cornstarch, the leader, could be expanded. Our point was that no two starches are exactly alike.

In cotton mills, pastes made by heating starch with water under controlled conditions are used for sizing warp and for finishing. The warp is passed through the hot paste and dried. The hardened film of starch on the fibers is tough enough to prevent breaking of the warp during weaving. Sweetpotato-starch paste was used for sizing cotton threads in the laboratory. After drying, the threads were found to be coated with a tough, rubbery film of gelatinized starch. Technologists, who tried out large samples of the starch, believed that it would be satisfactory for warp sizing. Later, when the starch was made in larger quantities, this rubbery gel and tough, film-forming characteristic proved to be outstanding. Besides warp sizing, the starch was found useful for finishing some types of cloth, for laundry work, and for many other purposes where a smooth, tough film is required, such as paper finishing, insulating fabrics, and coatings in dry batteries. The same qualities appear to carry over to dextrin made by heating the dry starch with acid under controlled conditions. The dextrins, with hot water, form clear solutions, which remain fluid when cold and thus handle well in spreaders used to apply them to postage stamps and other surfaces. They have excellent remoistening properties.

For food use, one starch should be about as good as another in actual food value, but the clear, stable gel with a high water-holding capacity formed with sweetpotato starch created a ready market for the starch as an ingredient in prepared food products.

Pioneer experimental work on sweet-potato starch in the United States was begun at the South Carolina Agricultural Experiment Station in 1895 and was continued for about 15 years.

Sweetpotato starch has also been produced under the name of Brazilian arrowroot. For many years it has been manufactured in Japan, where it is used in laundries, for sizing textiles and paper, for foods, and in cosmetics.

But the starch produced there like the starch first produced in the United States was gray and of poor quality; it did not meet the standards of United States buyers. If a market was to be developed for sweetpotato starch manufactured in the United States, we had to find procedures and equipment to produce a white, pure starch.

SWEETPOTATO STARCH granules are 1.633 times heavier than water. A few granules are large, but generally they range in diameter from about 27 microns (about one-thousandth of an inch) to 2 microns. Many samples average 12 microns in diameter. Some of the granules are spherical, some are ellipsoidal, and others are irregular. If we assume that all are spherical, the surface area, volume, and number of granules per unit weight can be calculated. Such calculations are not strictly accurate because the granules are not all spherical, but they give an idea of the volume and surface area of the particles to be washed in developing a starch-purification system. They show that a pound of the 27-micron-diameter granules would contain about 27 billion particles having a surface area of about 95,000 square inches, while the same amount of 2-micron-diameter granules would contain 66,000 billion particles with a surface area of more than 1 million square inches.

Those properties are used in devising means of extracting and purifying starches. Grinders ( such as saw-blade rasps, hammer mills, and attrition mills) are used to break the cell membranes which enclose the starch granules. The resulting mass is stirred with water, and the starch is separated from the pulp with silk, nylon, or metal screens. The starch granules are so much smaller than most of the pulp particles that they are washed through the screen along with the solubles, protein, gums, and other solids, such as fine pieces of fiber. The heavy starch granules soon settle; then the water, containing solubles and some of the lighter solids, can be siphoned off to leave a fairly pure starch. More solubles and more of the light solids can be removed by repeating the settling process. In cool climates, where the fermentation of the sugars does not interfere, the process has given excellent results.

Tabling, a refinement of the settling process, is one of the most efficient means of separating starch from solid impurities. Starch tables are long, smooth, flat-bottomed troughs that have a slope of about one-thirty-second inch per foot. For large-granule starches, the tables may be as short as 30 or 40 feet and for small-granule starches as long as 120 feet. Starch from the screening system or from settling tanks is pumped to the head of the table at a predetermined constant rate. The heavy starch granules form a constantly increasing semisolid layer on the bottom of the table that behaves almost like a heavy liquid and forces the lighter solid particles to the top. Water from which the starch has separated flows along the surface and carries the lighter particles to the lower end of the table, where they either flow off the end or are deposited on the lower sections. In refining small-granule starches, the starch content of the overflow from the table is too great to be discarded, and settling tanks are used to recover that starch.

In the Department's experimental work with sweetpotatoes, starch was separated by settling, by tabling, and by means of centrifugal separators. The tests showed that settling alone would not produce a satisfactory yield of high-grade starch. When tabled, starch from the centrifuge or from the settling tanks was relatively clean, but it still had a yellowish cast.

Exposure to the air causes darkening on the cut surface of a sweetpotato. The use of a reducing agent, such as sulfur dioxide, prevents the darkening. R, T. Balch and H. S. Paine, Department chemists, found that a white starch resulted when the grinding and screening process was carried out in a solution that contained a small amount of sulfur dioxide, after which sodium hydroxide was added to dissolve color-forming compounds. When iron equipment was used, however, the tannin-like substances in the sweetpotatoes produced dark compounds with the iron that was not removed by the alkaline treatment. Sodium hydroxide, together with a trace of sulfur dioxide, was finally used throughout the process to keep the iron from dissolving. With that procedure, high-viscosity white starch was produced in pilot-plant equipment in which iron tanks and iron pipe lines were used.

IN 1933, WHILE LABORATORY WORK was still in progress, many requests for aid in utilizing surplus sweetpotatoes were received. Surveys were made to determine the probable cost of producing sweetpotatoes and the probable selling price of sweetpotato starch. Cotton-mill technicians believed that the starch could be used in their work but that the price might be somewhat higher than that of cornstarch. The selling price, it seemed, might be about 4 cents a pound. In a small plant with a capacity of 2,000 bushels (60 tons) Of sweetpotatoes a day, the processing and sales costs were estimated to be 20 cents for each bushel of potatoes; that would leave only 20 cents a bushel that could be paid to farmers for sweet-potatoes that would yield one-sixth of their weight in finished starch.

Some growers believed they could make a profit on sweetpotatoes at 20 cents a bushel if they were assured a steady market for field-run potatoes. The crop was being produced almost entirely by hand labor, but at that time farm labor costs were computed at the rate of 10 cents an hour.

In 1934 the Federal Emergency Relief Administration established a small starch plant at Laurel, Miss., to ensure a market for sweetpotatoes in that area and so provide an income for farm families that would otherwise be on relief. Department engineers and chemists assisted in designing, building, equipping, and operating the plant at first. It was then turned over to a local cooperative for operation.

After experience was gained in the plant, some changes in equipment and procedures were made. The capacity of the screening system was increased; limewater was substituted for the sulfur dioxide-sodium hydroxide solution; sodium hypochlorite was used to remove the last trace of color in the starch; and settling tanks were added to increase capacity and starch recovery.