20. CELLULOSE is a form of carbohydrate closely related to starch. It
helps to form the structure of plants and vegetables. Very little
cellulose is digested, but it should not be ignored, because it gives
the necessary bulk to the food in which it occurs and because strict
attention must be paid to the cooking of it. As cellulose usually
surrounds nutritive material of vegetable origin, it must be softened
and loosened sufficiently by cooking to permit the nutritive material to
be dissolved by the digestive juices. Then, too, in old vegetables,
there is more starch and the cellulose is harder and tougher, just as an
old tree is much harder than a sapling. This, then, accounts for the
fact that rapid cooking is needed for some vegetables and slow cooking
for others, the method and the time of cooking depending on the presence
and the consistency of the cellulose that occurs in the food.
21. IMPORTANCE OF A VARIETY OF FOODS.--Every one of the five food
substances just considered must be included in a person's diet; yet,
with the exception of milk, no single food yields the right amounts of
material necessary for tissue building and repair and for heat and
energy. Even milk is in the right proportion, as far as its food
substances are concerned, only for babies and very young children. It
will thus be seen that to provide the body with the right foods, the
diet must be such as to include all the food substances.
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