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VEGETABLES, FRUITS, AND NUTS
SINCE it is difficult to draw any logical line of demarkation between vegetables, fruits, and nuts, because of the many important characteristics common to articles belonging to more than one of these categories, they will here be discussed in one chapter. The number of fruits and vegetables used as food being so large and so likely to grow, it would seem impracticable to attempt to treat them all in the same way in this chapter. What is attempted, therefore, is (1) to discuss adequately a few which are both typical and of large practical importance, (2) to mention briefly as many other important and interesting members of the group as can be included without making the chapter too much like a catalogue, and (3) to include at the end of the chapter references to discussions which will put the reader in touch with as much additional literature on fruits and vegetables as may be desired.
Among fruits and vegetables there are wide differences in protein content and in energy values but practically all are important sources of mineral elements or vitamins or both. In fact the chief nutritional significance of the fruits and vegetables as a group now appears to be in their mineral and vitamin content.
Vegetables Of the food materials commonly known as vegetables, some are leaves, some flowers or fruits, some seeds, some stems, bulbs, tubers, or roots.
Leaf vegetables include representatives of several botanical groups, different kinds of leaves being preferred in different countries. In America and Europe, cabbage, lettuce, spinach, chard, and beet greens are the leaves most used as food; in the Orient a much wider variety of leaves is commonly eaten, among which one of the most popular in parts of China is young tender clover.
Green leaves may be ranked next to milk, butter, and egg yolks as sources of vitamin A. It is in green leaves that vitamin A is formed, and in general they contain a higher concentration of this vitamin than is contained in any other part of the plant. From the leaves more or less of the vitamin A passes to the other parts of the plant, where in general it is found in higher concentration in the actively functioning parts, such as the embryos of seeds and the tips of growing shoots, than in storage organs such as the endosperms of seeds and fleshy roots or tubers. It is chiefly to McCollum that we are indebted for this general view of the relation between the functional activity of the parts of plants and their richness in vitamin A. Steenbock, on the other hand, has pointed out that the storage organs of some plants contain more vitamin A than others and that a high concentration of this vitamin often accompanies a high concentration of yellow or green coloring matter. While there are exceptions to both“ rules,” these suggestions of McCollum and of Steenbock are useful in helping one to remember the vegetables which may be expected to be rich in vitamin A. Among leaves, the thinner and greener the leaf the richer it is apt to be in vitamin A. Spinach is the richest in vitamin A of the leaves thus far studied, containing (notwithstanding its high water content) about as much of this vitamin as an equal weight of butter or of egg yolk. It has been found in the case of cabbage, and is doubtless true of lettuce also, that the loose green leaves are relatively rich and
the inner white leaves are relatively poor in vitamin A. In fact the most careful quantitative comparisons yet made have shown white cabbage to contain only about as much vitamin A as an equal weight of white potato while spinach showed at least fifty times as much. Head lettuce showed an intermediate amount. Green string beans (young seed pods) showed at least as much
String Beans as Source of Fat Soluble Vitamin
Weight in Grams
204 Fig. 28. — String beans as source of vitamin A. Weight curves of three rats which
at first were fed a diet deficient in vitamin A. All grew well for some time by virtue of the vitamin A which they had previously stored, but finally began to weaken and lose weight and all developed the characteristic eye disease due to lack of vitamin A. Upon adding string beans to the diet the eye trouble soon disappeared and the animals all quickly recovered in weight and strength and again grew at a rapid rate showing that the string beans furnished liberal amounts of vitamin A.
vitamin A as head lettuce and several times more than the inner leaves of new raw cabbage. As regards vitamin A, therefore, the emphasis should be placed upon green rather than merely upon leaf, vegetables. (See Fig. 28.)
Probably all the leaf vegetables when raw are fairly rich in vitamins B and C. Raw cabbage and raw spinach rank with orange and lemon juice and with tomatoes among the richest sources of vitamin C. In the case of vitamin B there have been fewer attempts at strictly quantitative comparison, but the available evidence warrants the belief that the leaf vegetables as well as most other vegetables contain vitamin B in relative abundance.
So far as can be judged from the few experiments thus far recorded, neither the vitamin A nor the vitamin B of vegetables is destroyed to any serious degree by ordinary cooking. In the case of vitamin C, however, the losses in cooking are very different for different vegetables. Tomatoes retain their high antiscorbutic value when cooked or canned, while in cabbage this value is very greatly diminished. Thus in the experiments of Eddy and his co-workers cabbage was found to lose from go to 95 per cent of its vitamin C (antiscorbutic value) when cooked either by the pressure cooker or by the open-kettle method.
Flowers and Fruits as Vegetables Cauliflower is the only important example of a flower used as a vegetable; but several important vegetables including tomatoes, pumpkins, squashes, and cucumbers are botanically fruits. Of these, tomatoes occupy much the largest place in the American food supply, and the popularity of the tomato, fresh and canned, is fully justified by our present knowledge of food values, for the tomato is rich in all three vitamins and retains them well when cooked and canned. Over 200,000,000 cans of tomatoes are put up commercially each year in the United States besides the large quantities that are canned in the household for home use. The natural acidity of the tomato makes it possible to sterilize it in canning with greater certainty and at a lower temperature than in the case of most other foods. While the canning of tomatoes is a very important industry, it is so simple, the results are so certain, and the product is so nearly uniform and so universally familiar that no description of the industry seems necessary here.