ot coagulated by heat. They are produced from
other proteids by ferment action during the digestion of food and the
germination of seeds, and are often due to the changes resulting from
the action of the natural ferments or enzymes inherent in the food
materials. As previously stated, the insoluble proteids are present in
far the largest amount of any of the nitrogenous materials of foods.
Lean meat and the gluten of wheat and other grains are examples of the
insoluble proteids. The various insoluble proteids from different food
materials each has its own composition and distinctive chemical and
physical properties, and from each a different class and percentage
amount of derivative products are obtained.[1] While in general it is
held that the various proteins have practically the same nutritive
value, it is possible that because differences in structural composition
and the products formed during digestion there may exist notable
differences in nutritive value. During digestion the insoluble proteids
undergo an extended series of chemical changes. They are partially
oxidized, and the nitrogenous portion of the molecule is eliminated
mainly in the form of amids, as urea. The insoluble proteins constitute
the main source of the nitrogenous food supply of both humans and
animals.

20. Crude Protein.--In the analysis of foods, the term "crude protein"
is used to designate the total nitrogenous compounds considered
collectively; it is composed largely of protein, but also includes the
amids, alkaloids, and albuminoids. "Crude protein" and "total
nitrogenous compounds" are practically synonymous terms. The various
proteins all contain about 16 per cent of nitrogen; that is, one part of
nitrogen is equivalent to 6.25 parts of protein. In analyzing a food
material, the total organic nitrogen is determined and the amount
multiplied by 6.25 to obtain the crude protein. In some food materials,
as cereals, the crude protein is largely pure protein, while in others,
as potatoes, it is less than half pure protein, the larger portion being
amids and other compounds. In comparing the crude protein content of one
food with that of another, the nature of both proteids should be
considered and also the amounts of non-proteid constituents. The factor
6.25 for calculating the protein equivalent of foods is not strictly
applicable to all foods. For example, the proteids of wheat--gliadin and
glutenin--contain over 18 per cent of nitrogen,