15. So much is involved in the cooking of foods containing protein that
the effect of heat on such foods should be thoroughly understood. The
cooking of any food, as is generally understood, tends to break up the
food and prepare it for digestion. However, foods have certain
characteristics, such as their structure and texture, that influence
their digestibility, and the method of cooking used or the degree to
which the cooking is carried so affects these characteristics as to
increase or decrease the digestibility of the food. In the case of foods
containing protein, unless the cooking is properly done, the application
of heat is liable to make the protein indigestible, for the heat first
coagulates this substance--that is, causes it to become thick--and
then, as the heat increases, shrinks and hardens it. This fact is
clearly demonstrated in the cooking of an egg, the white of which is the
type of protein called _albumin_. In a raw egg, the albumin is nearly
liquid, but as heat is applied, it gradually coagulates until it becomes
solid. If the egg is cooked too fast or too long, it toughens and
shrinks and becomes less palatable, less attractive, and less
digestible. However, if the egg is properly cooked after the heat has
coagulated the albumin, the white will remain tender and the yolk will
be fine and mealy in texture, thus rendering it digestible.
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