STRUCTURE, COMPOSITION, AND FOOD VALUE
9. STRUCTURE OF VEGETABLES.--Although vegetables vary greatly in composition and consequently in food value, they are similar so far as physical structure is concerned. In general, they consist of a skeleton framework that is made up of cellulose. Their digestible part is composed of tiny cells having thin walls that confine the actual food material in the form of a liquid or semiliquid. As the vegetables grow old, the cellulose material and the cell walls gradually toughen, with the result that old vegetables are less easily made tender than young ones and are not so agreeable to the taste as those which have not grown hard. The total food value of vegetables, as well as of cereals, meats, and, in fact, all foods, varies with the quantity of water and cellulose they contain. Therefore, the vegetables that contain the least coarse material are the ones that have the highest food value.
10. The green color that characterizes many vegetables is due to a substance called chlorophyl. This substance is essential to the normal growth of plants and is present in the correct amount in only those which are properly exposed to the sunlight. Sufficient proof of this is seen in the case of vegetables that form heads, as, for instance, cabbage and head lettuce. As is well known, the outside leaves are green, while the inside ones are practically white. Since it is exposure to the light that produces the green color, a vegetable or plant of any kind can be bleached by merely covering it in order to keep out the sunlight. This procedure also enables the plants to remain more tender than those which have been allowed to grow in the normal way and become green. For instance, the inside leaves of a head of lettuce are always very much more tender than the green outside leaves. In fact, the center of any kind of plant, that is, the leaves and the stem that appear last, are more tender, possess a lighter color, and have a more delicate flavor than the older ones.
11. PROTEIN IN VEGETABLES.--Taken as a whole, vegetables are not high in protein. Some of them contain practically none of this food substance and others contain a comparatively large amount, but the average is rather low. Vegetables that are high in water, such as lettuce, celery, tomatoes, and cucumbers, contain so little protein that the quantity is not appreciable. Such vegetables as potatoes, beets, carrots, etc. contain slightly larger quantities. Dried vegetables, such as beans, peas, and lentils, contain comparatively large amounts of this substance, and for this reason may be substituted for such high-protein foods as meat and fish.
12. The composition of vegetable protein is only slightly different from that of animal protein. In fact, the experiments of scientists show that animal protein may be readily replaced by vegetable protein. One of these proteins is sometimes called vegetable albumin, but the chief protein of vegetables containing the largest amount of this substance, namely, beans, peas, and lentils, is called legumin, from the term legumes, the name of this class of vegetables. It is generally agreed that vegetable protein is not so digestible as animal protein, but this disadvantage is offset by the fact that it does not bring about so much intestinal trouble as does the protein of animal foods and is less likely to cause disturbances that are usually attributed to foods high in this substance. Vegetable protein is affected by heat in much the same way as other protein.
When any of the dry vegetables high in protein are served at a meal, meat should be eliminated, or the result will be an oversupply of protein. As this condition is not only harmful but wasteful, it is one that should receive proper consideration from the housewife.
13. FAT IN VEGETABLES.--As vegetables as a class are low in protein, so are they low in fat. In the case of some vegetables, the quantity of fat they contain is so small that it is never considered in discussing the food value of these vegetables, while in others slightly larger quantities are to be found. However, on the whole, vegetables are so nearly lacking in this food substance that it is necessary to supply fat in their preparation and in the serving of meals in which they are included. This is done in a variety of ways, depending on the nature of the vegetable. For instance, in order that baked beans may take the place of meat entirely, fat in the form of salt pork is usually added when they are prepared. The pork, of course, also supplies a very small amount of protein, but it is not used with the beans for this purpose. Practically all cooked vegetables are served with butter or with a sauce that contains fat. Green vegetables that require no cooking but are served as a salad, are supplied with fat by the salad dressing that is used with them. The fat varies greatly, depending on the kind of dressing used.
14. CARBOHYDRATES IN VEGETABLES.--When the composition of vegetables is considered chemically, the most striking thing about them is the carbohydrates they contain. It is this that distinguishes this class of foods from animal foods. The carbohydrate of vegetables is found in both its forms, starch and sugar. It is in the form of sugar in many of the vegetables when they are young or immature, but it turns into starch as they mature. This change can be easily observed in the case of peas. As is well known, young green peas are rather sweet because of the sugar they contain, while mature or dried peas have lost their sweetness and are starchy. The sugar that is found in large quantities in such vegetables as peas, carrots, turnips, etc. is largely cane sugar. The starch that vegetables contain occurs in tiny granules, just as it is found in cereals, and is affected by cooking in the same way. The mature vegetables in which the starch has developed, although less tender and less sweet than young ones, have a higher food value. In fact, the carbohydrate that vegetables contain constitutes a large proportion of their food value.
One of the chief sources of starch among vegetables is the potato, in which the starch grains are large and, if properly cooked, easily digested. Irish, or white, potatoes contain very little carbohydrate in the form of sugar, but in the sweet potato much of the carbohydrate is sugar. In either of these two forms--starch and sugar--vegetable carbohydrate is easily digested.
15. MINERAL MATTER, OR ASH, IN VEGETABLES.--The mineral matter in vegetables is found in comparatively large quantities, the average amount being slightly over 1 per cent. The presence of this substance is of great value, because the mineral salts of both fruits and vegetables are essential in the diet of adults in order to keep their health in a normal condition. The mineral salts of vegetables render the blood more alkaline instead of more acid, as do those contained in cereals and meat. A large number of vegetables, particularly those low in food value, such as greens, celery, etc., are very valuable for their mineral salts. In reality, this substance and the cellulose they contain are the things that recommend the use of these vegetables in the diet. Minerals of all kinds are found in solution in the water contained in vegetables, but chief among them are calcium, sodium, iron, phosphorus, and sulphur. Greens and salad vegetables are particularly high in iron, the element that assists in keeping the blood in good condition. These minerals are easily lost if the method of cookery is not planned to retain them.
16. CELLULOSE IN VEGETABLES.--The special use of cellulose, as has already been learned, is to serve as bulk in the food containing it. In vegetables, the cellulose varies greatly as to quantity, as well as to texture and the amount that can be digested. In young vegetables, it is very soft and perhaps digestible to a certain extent, but as they grow older it hardens and they become tough. This fact is clearly demonstrated in the case of beets. Those which are pulled from the garden in the summer and cooked are tender and soft, but those which are allowed to mature in the ground and are then put away for winter are, when cooked in the late winter or early spring, so hard and tough that it is almost impossible to make them soft. The quantity of cellulose that vegetables contain therefore depends largely on their age and condition. Those low in total food value contain, as a rule, larger quantities of it than those high in food value. This is due to the fact that both water and cellulose, which are usually found together in large quantities, help to detract from the fuel, or food, value of foods.
Very young persons or those who are ill sometimes find it impossible to take in its original form a vegetable that contains a large amount of bulk, or cellulose. In such a case, the vegetable may be put through a colander or a sieve in order to break up the cellulose and make it easier to digest. Under ordinary conditions, cellulose should not be avoided, but should be included in large quantities in the diet through the vegetables that are consumed daily.
17. WATER IN VEGETABLES.--The majority of vegetables contain a large quantity of water. Such vegetables as lettuce, cucumbers, tomatoes, etc., which are low in total food value, contain the most water, the average percentage being about 95. The dry vegetables, which are high in food value, average only about 10 per cent. of water. The water that is found in vegetables, whether it is much or little, is contained in cell-like structures surrounded by cellulose, and it holds in solution the mineral salts and much of the nutriment of the vegetables. In addition, the water holds in solution to a certain extent the material that gives vegetables their distinctive flavor. When any of this water is lost in the preparation of vegetables, the substances that it contains are also lost. It is therefore essential that correct methods of preparation be chosen for the cooking of this food, so as to prevent the waste of valuable food materials.
18. DIGESTIBILITY OF VEGETABLES.--The digestibility of vegetables is largely an individual matter; that is, a vegetable that agrees with one person may not agree with another. The fact that there appears to be no apparent reason for such a condition would lead to the conclusion that it is due to the peculiarities of the person. Because of this, it is not fair to make the general statement that a particular vegetable is easy to digest and another one is hard to digest.
The chief cause for difficulty in the digestion of vegetables lies in their volatile oils, which give them their flavor, but which are irritating to many persons. Vegetables having a strong flavor, such as radishes, onions, cucumbers, cabbage, and cauliflower, are the ones that disagree most frequently with persons who eat them; but sometimes the way in which some of them are cooked has more to do with this than the vegetables themselves.
Vegetables containing considerable cellulose and water do not of themselves give trouble in digestion, because they contain practically nothing to digest; but they are sometimes responsible for interfering with the digestion of other foods. Vegetables that are extremely high in starch, such as potatoes, are easily digested by most persons, provided they are properly cooked. For instance, a plain baked potato is easily digested, but the same potato sautéd in fat is more difficult of digestion.
19. TABLE SHOWING COMPOSITION AND FOOD VALUE OF VEGETABLES.--As vegetables vary considerably in the amount of the food substances they contain, so do they differ greatly in their food value. This is clearly shown in Table I, which gives the percentage of the food substances of vegetables, as well as the food value per pound, in calories, that these vegetables contain. The figures in this table are taken from Atwater's Table of American Food Materials, and refer to the edible portion of the material. In the case of several vegetables, no figures are given by this authority, but in the table here presented the percentages and the calories for the vegetables most similar are used. For example, the figures for lettuce are used for endive, as the composition and food value of this vegetable are not included and it resembles lettuce very closely. Constant reference should be made to Table I as progress is make with the study of vegetables and their preparation. Noting the difference in the composition of the different vegetables, as well as the variation in their food value, will be not only interesting but instructive. For instance, when the housewife realizes that lettuce and celery furnish only 85 to 90 calories to the pound, while dried beans and peas average more than 1,700 calories to the pound, she will understand better the place that these foods occupy in the dietary.