The sweet potato
by Franklin W. Martin
Why grow sweet potatoes?
Sweet potatoes are already the 6th or 7th most produced food crop in the world, surpassed only by wheat, rice, corn, potato, barley, and possibly cassava. Among the reasons that sweet potato is a great crop is that it is relatively easy to grow, relatively free of pests and diseases, has relatively high productivity, and is always good food, principally starch, some protein and vitamin C, and, in orange varieties, rich in vitamin A. In addition, the young leaves, rich in protein and most vitamins, are also good food. Furthermore, the sweet potato is an excellent animal food. Its ability to produce in poor soils makes the sweet potato an especially good crop for poor tropical soils where fertilizer is not available. If the leaves are also used as food, sweet potato will probably produce more nutrients per acre than almost any other crop under those conditions. (The other tropical crop which produces well on poor soils and also has both edible roots and leaves is cassava. It has an advantage over sweet potato in drought tolerance, but sweet potato has the advantage in nutrients. That is because substances called polyphenols in the cassava leaf combine with protein during cooking and reduce the amount of protein that is digestible.) Nevertheless, like all crops the sweet potato must be produced with understanding in order to obtain maximum yields. It should never be treated with neglect.
Principle uses of sweet potatoes, and techniques
Leaves. The sweet potato plant can be harvested for leaves during the 2nd and 3rd months of production. Only the tender stem and young, not fully developed leaves, which constitute the distal 2-4 inches of the growing stem, should be taken. The leaves and stems are boiled for 15-20 minutes, washed, seasoned, and served.
Boiled sweet potato. The sweet potato is washed, peeled and trimmed, cut into 1 inch thick slices or cubes, and boiled 18-20 minutes. The boiling water is then discarded. The sweet potato can then be served as is, mashed, or combined in many dishes (casseroles). The mashed pulp can be used as a partial substitute for wheat flour in baked products such as pancakes, cakes, flat breads, cookies, fritters, or even bread.
Baked sweet potato. The entire sweet potato is wrapped and then baked in a modern or primitive oven until soft (one hour at 350 degrees C). During baking of most sweet potatoes, part of the starch is converted to the reducing sugar, maltose, thus increasing sweetness.
Osmotically modified boiled sweet potato. The peeled and trimmed sweet potatoes can be cut into thin (1/8") slices, placed in water 2 hours (moved once in a while) and then boiled. The products will be clearer, less sweet, and milder than those made from untreated sweet potatoes. (What is happening chemically is that the enzymes and substrates responsible for polyphenolic oxidation are partially lost, as well as some of the sugars).
Sweet potato flour. The flour of sweet potato is much more difficult to make than that of potato because the reducing sugars readily released from the starch combine with free amino acids to produce disagreeable colors, odors, and flavors. To avoid this the peeled sweet potato can be shredded, and the shreds immersed in water 2 hours. This process works better if the water is changed 2-3 times. The shreds are drained and then dried, first in the shade (with air movement or wind) and later in the sun (in some cases, drying over the stove or in an oven will be necessary). The brittle shreds are easily crushed to flour, or this can be done rapidly in a household blender. The flour can be stored for 6 months or more in sealed containers. It can be used as a substitute for wheat flour in the following amounts: 100% in white sauces, 25-50% in cookies, cakes and flat breads, and 15-20% in breads. From the water, starch can be recovered (see below).
Starch production. The peeled sweet potato is ground in a mill or blender as finely as possible, and mixed with 5-10 times its weight in water. The starch settles out, and the water is carefully poured away (can be used as pig feed). The starch is then mixed with water 1-3 times more and the process is repeated. After the last settling the water is carefully drained and the starch is dried on a metal surface in the sun. It can be used as is any starch, such as corn or potato starch, and can be stored in sealed containers for a year or more.
Breakfast cereal. A breakfast food similar to "cereal" can be made from any sweet potato. The sweet potato is grated (not as finely ground as for starch), suspended in water, and filtered through a cloth. The liquid is saved for starch, the residue is suspended 1-3 times more in water, and filtering is repeated. The portion of the sweet potato that does not
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