35. Potatoes contain about 75 per cent of water and 25 per cent of dry matter, the larger portion being starch. There is but little nitrogenous material in the potato, only 2.25 per cent, of which about half is in the form of proteids. There are ten parts of non-nitrogenous substance to every one part of nitrogenous; or, in other words, the potato has a wide nutritive ratio, and as an article of diet needs to be supplemented with foods rich in protein. The mineral matter, cellular tissue, and fat in potatoes are small in amount, as are also the organic acids. Mechanically considered, the potato is composed of three parts,--outer skin, inner skin, and flesh. The layer immediately beneath the outer skin is slightly colored, and is designated the fibro-vascular layer. The outer and inner skins combined make up about 10 per cent of the weight of the potato.
[Illustration: FIG. 9.--TRANSVERSE SECTION OF POTATO. (After COWDEN and BUSSARD.) _a_, skin; _b_, cortical layer; _c_, outer medullary layer; _d_, inner medullary layer.]
A large portion of the protein of the potato is albumin, which is soluble in water. When potatoes are peeled, cut in small pieces, and soaked in water for several hours before boiling, 80 per cent of the crude protein, or total nitrogenous material, is extracted, rendering the product less valuable as food. When potatoes are placed directly in boiling water, the losses of nitrogenous compounds are reduced to about 7 per cent, and, when the skins are not removed, to 1 per cent. Digestion experiments show that 92 per cent of the starch and 72 per cent of the protein are digested.[12] Compared with other foods, potatoes are often a cheap source of non-nitrogenous nutrients. If used in excessive amounts, however, they have a tendency to make the ration unbalanced and too bulky.
================================================ |Per Cent Unpeeled potatoes | 100.0 Outer, or true skin | 2.5 Inner skin, or fibro-vascular layer[A] | 8.5 Flesh | 89.0 ================================================
================================================================ | | | | CARBOHYDRATES |-----|-------|---|------------------------ |Water| Crude |Fat|Nitrogen-free-| | | |Protein| | extract |Fiber|Ash | % | % | % | % | % |% ---------------------|-----|-------|---|--------------|-----|--- Outer, or true skin | 80.1| 2.7 |0.8| 14.|6 |1.8 Inner skin, or | | | | | | fibro-vascular | | | | | | layer | 83.2| 2.3 |0.1| 12.6 | 0.7 |1.1 Flesh | 81.1| 2.0 |0.1| 15.7 | 0.3 |0.8 Average of 86 | | | | | | American analyses[B]| 78.0| 2.2 |0.1| 18.|8 |0.9 Average of 118 | | | | | | European analyses[C]| 75.0| 2.1 |0.1| 21.0 | 0.7 |1.1 ================================================================