Human Foods and Their Nutritive Value | Page 9

161° F. Whenever a food material is soaked
in water, the albumin is removed and can then be coagulated by the
action of heat, or of chemicals, as tannic acid, lead acetate, and salts of
mercury. The globulins are proteids extracted from food materials by

dilute salt solution after the removal of the albumins. Globulins also
are coagulated by heat and precipitated by chemicals. The amount of
globulins in vegetable foods is small. In animal foods myosin in meat
and vitellin, found in the yolk of the egg, and some of the proteids of the
blood, are examples of globulins. Albuminates are casein-like proteids
found in both animal and vegetable foods. They are supposed to be
proteins that are in feeble chemical combination with acid and alkaline
compounds, and they are sometimes called acid and alkali proteids.
Some are precipitated from their solutions by acids and others by
alkalies. Peas and beans contain quite large amounts of a casein-like
proteid called legumin. Proteoses and peptones are proteins soluble in
water, but not 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, making the nitrogen
factor about 5.68 instead of 6.25. If wheat contains 2 per cent of
nitrogen, it is equivalent to 12.5 per cent of crude protein, using the
factor 6.25; or to 11.4, using the factor 5.7. The nitrogen content of
foods is absolute; the protein content is only relative.[9]
21. Food Value of Protein.--Because of its complexity in composition,
protein is capable of being used by the body in a greater variety of
ways than starch, sugar, or fat. In addition to producing heat and
energy, protein serves the unique function of furnishing material for the
construction of new muscular tissue and the repair of that which is
worn out. It is distinctly a tissue-building nutrient. It also enters into
the composition of all the vital fluids of the body, as the blood, chyme,
chyle, and the various digestive fluids. Hence it is that protein is
required as a nutrient by the animal body, and it cannot be produced
from non-nitrogenous compounds. In vegetable bodies, the protein can
be produced synthetically from amids, which in turn are formed from
ammonium compounds. While protein is necessary in the ration, an
excessive amount should be avoided. When there is more than is
needed for functional purposes, it is used for heat and energy, and as
foods rich in protein are usually the most expensive, an excess adds
unnecessarily to the cost of the ration. Excess of protein in the ration
may also result in