[Sidenote: Four general classes of food]
The chemistry of carbon compounds and the general composition of
plant and of animal substances were discussed in Lesson III. We are
now prepared to take up the chemistry of food. The chemistry of
food substances will be considered under the common divisions of
carbohydrates, fats, proteids, and mineral salts. (See "Classification
of Organic Carbon Compounds," Lesson III, p. 89.)
[Sidenote: Classes vs. groups of related compounds]
In the food tables and analyses commonly published, the above terms are
used with very little explanation, and read by the average person with
meager comprehension. When one reads that a food is composed of glucose,
citric acid, or globulin, he is likely to become confused, not being
able to understand how a food at one time can be said to be composed of
carbohydrates, proteids, and fats, and at another time to be composed
of other substances. The explanation is that the first classification
does not refer to definite chemical substances, but to groups of related
compounds having properties in common.
[Sidenote: The different methods of analyzing food]
There is still another way of giving the chemical composition of a
food, namely, to specify the chemical elements that it contains. It
will be remembered that the relation between chemical elements and
chemical compounds was explained in the first lesson. As an example, I
will take the analysis of milk. We will first say that milk contains a
certain percentage of protein, carbohydrates, and fat. We might then
say that the proteid of milk is part casein and part albumin, and that
the albumin contains certain percentages of oxygen, sulfur, etc.; also
that the chief carbohydrate in milk is milk-sugar, which in turn is
composed of carbon, hydrogen, and oxygen. Or, we could consider the
milk as a whole, without dividing it into groups, and give the per cent
of each chemical element in the milk. Thus, the carbon of the proteid,
milk-sugar, and fat would be all considered together, and show a certain
per cent of carbon in the milk as a whole.