Sweetpotatoes: More Than Starch

P. R. Dawson, L. H. Greathouse, W. O. Gordon.

Starch, the chief component, aside from water, of sweetpotatoes is the only one that has been separated, purified, sold, and utilized commercially. However, the more important nonstarch portions have been isolated and identified in the laboratory. They affect the nutritional value of sweetpotatoes, as well as the conditions of processing the crop for starch, fermentation products, feed, and food products. They add up to so much less than the starch and water, however, that processing sweet-potatoes to recover them, except possibly as byproducts, is generally considered impractical.

Sweetpotatoes vary in composition with the variety, the soil and climate where they grow, and the duration and conditions of handling and storage after harvesting. At the time of digging, water and starch make up about 90 percent of most sweetpotatoes. The Moisture ranges from 75 to 60 percent; the starch, from 15 to 30 percent. The rest consists of 2 to 3 percent of sugars, 1 to 2 percent of protein, 1 to 3 percent of pectic substances, about 1 percent of mineral matter, and smaller percentages of fat, cellulose, hemicelluloses, resins, coloring matter, organic acids, enzymes, and vitamins.

As soon as sweetpotatoes are separated from the vines, the sugar content begins to rise as much as twofold to threefold, with a corresponding decrease in starch, in 10 to 30 days. The rate and extent of change in the sugar and starch content during that time and in later storage vary with the variety, temperature, and humidity.

INVESTIGATORS AT the Southern Regional Research Laboratory have explored the possibilities of recovery and utilization of some of the nonstarch components as byproducts, or coproducts, of sweetpotato-starch manufacture. The investigations had the dual aim of increasing the byproduct credit (extra income from byproducts) and of simplifying the waste-disposal problem.

We found that sweetpotatoes of the improved high-starch L-5 variety, later named Pelican Processor, had approximately the following average composition when ground for starch in the factory at Laurel, Miss. (in percentages) :

In the starch-extraction process, sweetpotatoes are ground with about 3 parts of limewater, and the liberated starch is separated from the pulp by washing over a series of fine screens. The suspension of crude starch thus obtained is allowed to settle or is centrifuged to separate the starch from the process liquor.

The liquor, often called fruit water, is actually a limewater extract of ground sweetpotatoes. It contains most of the sugar in the original potatoes at a concentration of 0.75 to 1.0 percent and 80 to 85 percent of the nitrogenous matter at a concentration of about 0.4 percent, or as high as 0.7 percent if the potatoes have a higher protein content. The fruit water also contains small amounts of other organic substances and mineral matter, which are in solution or so finely suspended that they are not separated in settling or centrifuging. Often it also carries a little fine starch, the amount of which depends on the efficiency of the sedimentation or centrifuging system.

In the commercial sweetpotato-starch plants that have operated so far, the fruit water has gone to the sewer, along with the effluent (waste) waters from other stages of starch refining ox plant clean-up. The waters carry variable amounts of very-fine-grain or contaminated starch, the recovery of which is impracticable. The effluents have constituted the major waste-disposal problem in the manufacture of sweetpotato starch. The pulp remaining on the screens after washing out the free starch has not gone to waste.

When commercial-scale sweetpotato-starch manufacture was undertaken at Laurel, the workers recognized that the rapid spoilage of the wet pulp would create an intolerable nuisance if it were dumped in the waste and would prevent its disposal as feed unless immediately dried. They also recognized that the potential byproduct value of the pulp would offset the cost of converting it to stable form. Hence, the spent pulp has been pressed and dried and sold for feed. The dried pulp contains 40 to 50 percent of starch and 20 to 30 percent of other digestible carbohydrates, about 2 percent of crude protein (representing about 10 percent of the protein in the original potatoes), and about 11 percent of crude fiber. It makes a good carbohydrate feed and has readily sold at a price that has afforded a fair byproduct credit in the manufacture of starch.

The fruit water is the most troublesome waste. The water from typical sweetpotatoes contains 1.5 to 2 percent of organic matter, mostly in dissolved or highly dispersed form. In a factory that grinds 100 tons of sweetpotatoes every 24 hours, the daily fruit water, about 70,000 gallons, carries 5 tons or more of such organic matter. In it is more than a ton of nitrogenous matter and 2 tons or more of sugars on a dry-substance basis.

Such an effluent cannot be tolerated in the smaller watercourses. It can easily overload small municipal sewage systems. Recovery of the protein and sugars that comprise most of the organic matter in the fruit water and their conversion to some usable product would help eliminate the waste-disposal problem and might reduce the costs of starch manufacture by the revenue from byproducts. Such possibilities have been under investigation at the Southern Laboratory since 1942.

THE PROTEIN remains in solution, or is very highly dispersed, in the fresh fruit-water effluent, which is alkaline from the limewater. With other organic material, it separates out as a flocculent precipitate if the liquor becomes acid through fermentation on standing.

Previous investigators had found that sweetpotato protein is largely precipitated from water solution if the acidity drops to about pH. 4.0. Our recovery process is based on this behavior. We found that most complete precipitation takes place at pH 3.8. If hydrochloric acid is added to the alkaline fruit water until the pH is 3.8 and the liquor is heated to 176 F., about three-fourths of the nitrogenous matter in the fruit water is coagulated into a light flocculent precipitate. Heating to this point does not increase the proportion of the total nitrogen actually coagulated, but it causes a denser precipitate, which is more easily separated by sedimentation and filtration.

Half to two-thirds of the dry substance in this precipitate is protein, as computed from the total nitrogen by use of the conventional factor 6.25. The remaining dry substance is a mixture of other organic material, dissolved or finely dispersed in the fruit water and thrown down with the protein coagulate. Hence the precipitate is really a crude protein concentrate.

Because of the high dilution, it is impracticable to filter or centrifuge the coagulated concentrate directly from its mother liquor. If it is precipitated under the best conditions, however, the coagulate will settle in 6 hours to about one-tenth the volume of the total fruit water, and 90 percent of the water can be removed by drawing off the overlying clear liquor. The settled suspension still contains up to 95 percent water. Further water can be effectively removed by filtration under carefully controlled conditions. The coagulate gives a highly compressible filter cake. As long as the mass is fluid, therefore, only a very light force must be applied through the filter. An effective procedure is to allow the settled concentrate to spread over a rather open filter in a layer not exceeding 2 inches in depth and let it drain until the mass begins to show definite plasticity.

For successful settling and filtration, the starch content of the original fruit water must be reduced to 0.2 percent or less before coagulation. Otherwise gelatinization of the starch during 1 9 heating causes the liquor in the settled concentrate to have enough viscosity to retard settling and to make filtration Very slow. Also, agitation during coagulation must be kept to a minimum just enough to effect even distribution of acid and heat. Otherwise, the curd structure of the coagulate is broken up and gravity filtration is impeded. A new kind of filter cloth, made of synthetic plastic fabric, facilitates this type of filtration.

After the filter cake begins to show Plasticity, a suction of not more than 5 centimeters of mercury may be applied until a puttylike consistency is reached. The vacuum may then be increased to 70 centimeters of mercury. At that stage the cake must be covered or pressed down to prevent cracking and consequent loss of vacuum. The concentrate is removed from the filter as a stiff paste, which has 80 to 85 percent moisture.

The material may then be dried directly to low moisture content for preservation. For use as a feed supplement, however, the most practicable prose= dune is to mix the wet protein cake from the filter with the wet pressed residual pulp from the starch-extraction process and dry the mixture in one operation. If the two components are combined in the proportion in which they are recovered from a given quantity of sweetpotatoes, the byproduct feed contains 10 to 12 percent of crude protein, rather than only about 2 percent where the protein is not recovered.