Human Foods and Their Nutritive Value | Page 6

preventing their being available as food. In the
earlier stages of plant growth a part of the cellulose is in chemical
combination with water, forming hydrated cellulose, a portion of which
undergoes digestion and produces heat and energy in the body.
Ordinarily, however, cellulose adds but little in the way of nutritive
value, although it is often beneficial mechanically and imparts bulk to
some foods otherwise too concentrated. The mechanical action of
cellulose on the digestion of food is discussed in Chapter XV. Cellulose
usually makes up a very small part of human food, less than 1 per cent.
In refined white flour there is less than .05 of a per cent; in oatmeal
and cereal products from .5 to 1 per cent, depending upon the extent to
which the hulls are removed, and in vegetable foods from .1 to 1 per
cent. The cellulose content of foods is included in the crude fiber of the
chemist's report.
9. Starch occurs widely distributed in nature, particularly in the seeds,
roots, and tubers of some plants. It is formed in the leaves of plants as
a result of the joint action of chlorophyll and protoplasm, and is
generally held by plant physiologists to be the first carbohydrate
produced in the plant cell. Starch is composed of a number of

overlapping layers separated by starch cellulose; between these layers
the true starch or amylose is found. Starch from the various cereals
and vegetables differs widely in mechanical structure; in wheat it is
circular, in corn somewhat angular, and in parsnips exceedingly small,
while potato starch granules are among the largest.[4] The nature of
starch can be determined largely from its mechanical structure as
studied under the microscope. It is insoluble in cold water because of
the protecting action of the cellular layer, but on being heated it
undergoes both mechanical and chemical changes; the grains are
partially ruptured by pressure due to the conversion into steam of the
moisture held mechanically. The cooking of foods is beneficial from a
mechanical point of view, as it results in partial disintegration of the
starch masses, changing the structure so that the starch is more readily
acted upon by the ferments of the digestive tract. At a temperature of
about 120° C. starch begins to undergo chemical change, resulting in
the rearrangement of the atoms in the molecule with the production of
dextrine and soluble carbohydrates. Dextrine is formed on the crust of
bread, or whenever potatoes or starchy foods are browned. At a still
higher temperature starch is decomposed, with the liberation of water
and production of compounds of higher carbon content. When heated
in contact with water, it undergoes hydration changes; gelatinous-like
products are formed, which are finally converted into a soluble
condition. In cooking cereals, the hydration of the starch is one of the
main physical and chemical changes that takes place, and it simply
results in converting the material into such a form that other chemical
changes may more readily occur. Before starch becomes dextrose,
hydration is necessary. If this is accomplished by cooking, it saves the
body just so much energy in digestion. Many foods owe their value
largely to the starch. In cereals it is found to the extent of 72 to 76 per
cent; in rice and potatoes in still larger amounts; and it is the chief
constituent of many vegetables. When starch is digested, it is first
changed to a soluble form and then gradually undergoes oxidation,
resulting in the production of heat and energy, the same
products--carbon dioxide and water--being formed as when starch is
burned. Starch is a valuable heat-producing nutrient; a pound yields
1860 calories. See Chapter XV.

10. Sugar.--Sugars are widely distributed in nature, being found
principally in the juices of the sugar cane, sugar beet, and sugar maple.
They are divided into two large classes: the sucrose group and the
dextrose group, the latter being produced from sucrose, starch, and
other carbohydrates by inversion and allied chemical changes.
Because of the importance of sugar in the dietary, Chapter V is devoted
to the subject.
11. Pectose Substances are jelly-like bodies found in fruits and
vegetables. They are closely related in chemical composition to the
carbohydrates, into which form they are changed during digestion; and
in nutrition they serve practically the same function. In the early stages
of growth the pectin bodies are combined with organic acids, forming
insoluble compounds, as the pectin in green apples. During the
ripening of fruit and the cooking of vegetables, the pectin is changed to
a more soluble and digestible condition. In food analysis, the pectin is
usually included with the carbohydrates.
12. Nitrogen-free-extract.--In discussing the composition of foods, the
carbohydrates other then cellulose, as starch, sugar, and pectin, are
grouped under the name of nitrogen-free-extract. Methods of chemical