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.
Mechanical Composition of the Potato
| Per Cent | |
| Unpeeled potatoes | 100.0 |
| Outer, or true skin | 2.5 |
| Inner skin, or fibro-vascular layer[A] | 8.5 |
| Flesh | 89.0 |
Chemical Composition of the Potato
| Carbohydrates | ||||||
|---|---|---|---|---|---|---|
| Water | Crude Protein | Fat | Nitrogen- free- 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 |
[A] Including a small amount of flesh.
[B] From an unpublished compilation of analyses of American food products.
[C] König, "Chemie der Nahrungs-und Genussmittel," 3d ed., II, p. 626.
36. Sweet Potatoes contain more dry matter than white potatoes, the difference being due mainly to the presence of about 6 per cent of sugar. There is approximately the same starch content, but more fat, protein, and fiber. As a food, they supply a large amount of non-nitrogenous nutrients.
37. Carrots contain about half as much dry matter as potatoes, and half of the dry matter is sugar, nearly equally divided between sucrose and levulose, or fruit sugar. Like the potato, carrots have some organic acids and a relatively small amount of proteids. In carrots and milk there is practically the same per cent of water. The nutrients in each, however, differ both as to kind and proportion. Experiments with the cooking of carrots show that if a large amount of water is used, 30 per cent or more of the nutrients, particularly of the more soluble sugar and albumin, are extracted and lost in the drain waters.[[12]] The color of the carrot is due to the non-nitrogenous compound carrotin, C26H38. Carrots are valuable in a ration not because of the nutrients they supply, but for the palatability and the mechanical action which the vegetable fiber exerts upon the process of digestion.