Protein holds an important place among the food principles, for it is the only one which supplies the nitrogenous materials necessary for the building and repair of the tissues of the body. As the tissues of the body contain nitrogen, foods must be supplied which contain nitrogen in a form in which the body can use it. Protein forms the basis of all living cells, being an essential part of the protoplasm, a jellylike content of the cell. Sugars and starches contain no nitrogen and for this reason obviously cannot serve the important purpose which protein does. Without protein foods the body will weaken and undergo nitrogen starvation, or a breaking down of the tissues.

The protein molecules are very complex, containing probably many atoms each of the various elements of which they are composed. While something is known of the arrangement of these atoms, much is still to be learned and their chemical formulae have not been determined. Besides their important functions as tissue builders, the proteins serve also as heat-giving foods, as they contain carbon and hydrogen. Some of the substances into which they are changed during digestion are capable of forming fat in the body. To depend chiefly on protein, however, for fuel and for body fat is undesirable for many reasons, one of which is that foods rich in protein are almost always high priced. Fortunately, carbohydrates and fats seem to act to some extent as protein sparers, and when they are used in suffi-cient quantity, they prevent undue destruction of protein for the production of heat.

Identification Test For Protein

Put raw white of egg in a saucer. Cut with a pair of scissors until you can take up a spoonful. Put 1/2 ts. of it in a test tube with a little water, add a few drops of nitric acid (HNO3). A white precipitate forms which when boiled turns yellow. This yellow color is characteristic of protein. When the mixture is cool, add a few drops of ammonia (NH4OH). An orange-brown color will result.

Experiment 38

Test various foods to detect the presence of protein. Tabulate results.

Experiment 39

Part 1. Prepare raw white of egg by cutting with scissors, and add to a small part of it ten volumes of water; filter through filter paper. Test the filtrate for protein. Is protein soluble in cold water ?

Part 2. Repeat, using boiling water with white of egg. Is albumen soluble in boiling water? In cooking meats in water, when should they be placed in cold water ? in boiling water ?

Experiment 40

Remove the residue of egg white left in the filter paper of Part 1 of the previous experiment, and put it in a test tube. Add a weak salt solution; mix well; filter. Test filtrate for protein.

Is globulin soluble in weak salt solution? When should you add salt to water in cooking meats?

Experiment 41

Effect of heat upon albumen and globulin.

Put half the white of an egg in a test tube. Put a thermometer in the tube and place it in a pan of cold water, having water deep enough to surround the egg; heat the water gradually. Note temperature when the first signs of coagulation are seen in the egg. At 180° F. remove some of the albumen; continue heating to 212° F. Compare the consistency of albumen cooked at 180° F. and 212° F. What is the correct temperature for "soft-cooked" eggs?

Experiment 42

(a) Boil 1 pt. water, place two eggs in it, remove from fire, keeping kettle closely covered to retain heat. At end of six minutes, remove one egg; remove the other at the end of ten minutes.

(6) Put an egg in boiling water and let boil three and one-half minutes.

Compare the consistency of the three eggs. Is there any difference ? What is the best method for cooking eggs soft ?

Experiment 43

(a) Cook an egg in boiling water for seven or eight minutes.

(b) Cook an egg in water just below the boiling point for twenty minutes or longer.

Compare (a) and (b). Note difference in consistency of yolks and whites. What is the best method of cooking eggs hard? What effect does the temperature of 212° F. have upon pro-tein ?