Experiment 108.- Coagulation of Egg Albumin.

Materials:

Fresh white of egg.

Thermometer.

Fill a test tube to the depth of 1 to 2 inches with fresh white of egg. Put a thermometer into the liquid and place the tube in a beaker of cold water over a low flame. Note the temperature at which the albumin coagulates (whitens). Express this temperature in Fahrenheit and in Centigrade degrees.

It is because of the coagulating effect of heat on albumins and on some other classes of proteins that in washing dishes which have held uncooked foods, such as milk or eggs, it is better to rinse with cold water before applying hot water.

The globulins are insoluble in water but soluble in dilute solutions of the neutral salts of strong acids with strong bases. They are, however, insoluble in concentrated solutions of these same salts. (The albumins are soluble both in dilute and in saturated sodium choride and magnesium sulphate.)

Among the globulins are the myosin of meat, the fibrinogen of blood (the change of which into fibrin is the cause of blood clotting), serum globulin, which remains in solution in the serum of blood, and ovoglobulin, a constituent of egg-white. Edestin is a globulin found in many plant seeds, including the cereals, flaxseed, and hempseed. Legumin is a globulin found in peas and beans.

Experiment 109.

Materials:

Hempseed, crushed.

5 per cent solution of common salt.

Cover a handful of crushed hempseed with 5 per cent sodium chloride and heat to 60° C. for about half an hour. Moisten a filter with hot 5 per cent sodium chloride solution and filter the hot liquid through it. Allow the filtrate to cool. Part of the edestin crystallizes out on cooling. Filter off a little of this crystallized edestin and wash it in test tubes with water. Boil a

little of it with Millon's reagent. Does it behave like a protein? Treat three equal portions of the edestin with, respectively, (a) distilled water, (b) a 5 per cent sodium chloride solution, (c) a saturated sodium chloride solution, warming each to about 60° C. Which of these three liquids is the best solvent for edestin?

The most important proteins of the interior of the wheat grain (and therefore of white flour) are those which are contained in the gluten. Gluten, which—as is evident from the method of its preparation from flour is insoluble in water, contains as its chief constituents two proteins called glutenin and gliadin. Both of these constituents are insoluble in water. Glutenin, however, is soluble in very dilute acids and alkalies and is representative of a class of proteins known as glutelins, which behave similarly. Gliadin is soluble in a mixture of alcohol and water containing 60 to 70 per cent of the former. It is, however, insoluble in absolute alcohol. Experiment 110. - Preparation of Wheat Gluten from Flour. Materials:

Wheat flour, strong and not over nine months old.

Cheesecloth.

Mix 30 grams flour with 5 cc. water to form a stiff dough. Knead in the hand in a stream of running water until the water runs through clear. (What constituent of the flour is carried out by the water, rendering it turbid?) Examine the residue of gluten left in the hand. Note its color and elasticity. The quality of flour depends not only on the quantity, but also on the quality of the gluten contained in it. The gluten of good flour is yellow, tough, and elastic. That of aged flour is grayish and short or friable.

Put the gluten from Experiment 110 in a mortar, cover it with a mixture of 5 cc. water and 15 cc. alcohol and rub well with the pestle. Filter. To one portion of the filtrate add water, to another alcohol. What effect does each have? Explain.

The albuminoids or scleroproteins (from the Greek, skleros, hard) are a class of simple proteins characterized by great insolubility. They include collagen, the protein of cartilage, skin, and bone; and keratin, the protein of hair, horns, hoofs, nails, etc. Collagen is converted by hydrolysis into gelatin which is soluble in hot water, forming a solution which sets to a jelly on cooling. Commercial gelatin is usually made from bones by treatment with hydrochloric acid, followed by treatment with boiling water or steam. Glue is a crude form of gelatin made from hoofs and hide clippings. Isinglass is a natural gelatin found in the swimming bladders of certain fishes.

The keratins contain a high proportion of sulphur. Hair and wool are therefore characterized by decided tests for sulphur a fact which is used to distinguish these fibers from silk, which is also essentially protein but contains no sulphur. (See Chapter XL.)

Experiment 112.

Materials:
Sulphur.

Wool, undyed.

Egg albumin, raw, coagulated (hard boiled), or dried.
Egg yolk, raw or coagulated.

Silk.

(a) Prepare sodium plumbite solution by adding sufficient sodium hydroxide solution (5 to 10 cc.) to 1 cc. lead acetate solution to redissolve (on warming) the precipitate which forms at first.

(b) Boil a little sulphur with sodium hydroxide solution. Add a little of the sodium plumbite. The black precipitate which forms is lead sulphide, PbS.

(c) Boil a small quantity of the wool with sodium hydroxide. Add to sodium plumbite solution and boil. Add sufficient water to enable you to see through the liquid. Has a black precipitate (lead sulphide) formed?

(d) Repeat (c) using egg albumin instead of wool.

(e) Repeat (c) using yolk of egg.

(f) Repeat (c) using silk.

CHAPTER XXXVI

THE FUNCTIONS OF FOOD

THE functions of food are:

I. To supply building material for the growth and repair of body tissue.

2. To furnish energy for the internal and external work of the body and heat to keep the body warm.

3. To regulate the physiological processes, i.e. the chemical and physical changes occurring in the body.

I. Food as Building Material

The human body is composed of the same classes of substances as foods. It is made up approximately as follows:

The amount of carbohydrates in the body is not only very small, but also exceedingly variable. The amount of fat is also widely variable and may fall very low without interfering with the normal physiological processes.

Proteins are essential constituents of all living cells, both vegetable and animal. There can be no life without them. Plants manufacture their own proteins from inorganic materials, making use of the carbon dioxide which they obtain from the air and of the water and nitrogen compounds which

Distinguished for his researches on the chemistry of nutrition.

Professor of Chemistry in Wesleyan University, Middletown, Connecticut, from 1873, the first director of an American agricultural experiment station, and the first Director of the Office of Experiment Stations of the United States Department of Agriculture, Atwater devoted special attention to agricultural chemistry and to problems of human nutrition. Under his direction numerous analyses and determinations of digestibility of American food materials were made. The respiration calorimeter described in the text was devised for the purposes of these nutrition investigations.