ylose 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 deg. 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 suga