~The Effect of Heat upon Proteins.~--The fact that certain proteins are most susceptible to heat has already been stated, but the application of this knowledge in the preparation of protein foods is important. In milk, for example, whole raw milk forms a large hard curd; whereas boiled milk curdles in a much finer and softer form. Pasteurized milk shows smaller curds than raw whole milk, but larger than the boiled whole milk.[8]
An egg cooked by the application of a long-continued high temperature (212° F.) has a tough white; whereas an egg cooked until hard at a temperature under the boiling point shows a tenderness in the white which renders it distinctly more palatable. Soft-cooked eggs leave the stomach in less time than is required for hard cooked ones; poached (cooked in water under the boiling point), shirred eggs (cooked in hot dish), and soft-cooked eggs are among the most readily digestible forms of eggs. Raw eggs are slightly less stimulating to acid secretion in the stomach and require a longer time to leave the stomach than boiled eggs. Thus it is seen that in many cases the difference in preparation of the protein foods may make a difference in the way in which the digestive tract handles them. Necessarily, this point is emphasized more in abnormal than in normal conditions; for example, albuminized orange juice gives rise to a distinct gastric secretion, and leaves the stomach rapidly--a great advantage in certain abnormal conditions, and especially in those requiring liquid diet of high nutriment value.
The knowledge of the coagulation of proteins by heat points out the advantage of using cold water over hot in the preliminary cleansing of utensils in which protein foods have been prepared. Certain members of this group are soluble in pure water, and will readily dissolve; whereas, if the water is heated, their coagulation would prevent this taking place so readily.
~Functions of Protein in the Body.~--The proteins serve two distinct uses in the body; first, that of building and repairing tissues and furnishing, in conjunction with other substances, material for growth; second, that of producing energy for the internal and external work of the body. For this latter function a large percentage of the proteins ingested is used; consequently, since the carbohydrates and fats are primarily the energy furnishing material most readily used by the organism, it is clearly demonstrated that the average individual takes more protein into the body than is necessary for its maintenance. Except during the period when an allowance for growth must be made, it is probable that a much smaller daily consumption of protein could be made without disadvantage to the organism, leaving the bulk of the work, in so far as the running of the engine is concerned, to the other organic foodstuffs.