1. Materials

One liter of fresh whole milk, one liter of milk for the preparatory steps of the demonstration.

2. Preparation

(1) On the day before the demonstration fill a 500 c.c. open-mouthed cylinder with milk and put it in a cool place.

(2) Three days before the demonstration weigh out 10 grams to 50 grams of whole milk in a platinum dish, or in a thin porcelain dish. Place it in a drying oven at 90° C. to 95° C, and dry to constant weight. Record the dry weight.

(3) Before the hour of demonstration burn the residue by bringing the dish which contains the dry solids to a red glow in a Bunsen flame, allowing ample access of oxygen. After the dish and the white ashes have cooled in a desiccator, take the weight. All of these weights should, of course, be taken upon an analytical balance.

(4) Fill a dialyzer with diluted milk one day before the demonstration.

3. Experiments And Observations

(1) "What proportion of milk evaporates at the temperature above suggested? It may be taken for granted that this proportion represents practically the water of the milk.

(2) Of the solids of the milk, what proportion is organic and what proportion is inorganic?

(3) What bases predominate in the ashes?

(4) What is the character of the organic constituents of milk?

(a) Note that the milk that has been standing has separated into two layers, an upper yellowish layer and a lower bluish-white layer.

(b) Draw off with pipette a few cubic centimeters of the cream, and in a test-tube add an equal volume of osmic acid. To a few drops of olive oil in another tube add osmic acid. Shake both tubes vigorously. Osmic acid has the same effect upon cream as upon olive oil. The cream is, in fact, fat in physiological emulsion. Quantitative examination shows that about 4 per cent of milk, or 4/13 of the solids of milk, consists of fats in which olein predominates.

(5) Fill a siphon with water and introduce it through the cream to the bottom of the 500 c.c. cylinder; draw off 300 c.c. of the milk; add to it 4 volumes of water; slowly add 1 per cent acetic acid, while stirring with a rod until the casein separates as a flocculent, copious precipitate. After the casein has partially settled, decant off a few cubic centimeters of the supernatant liquid and subject it to the Fehling test. The abundant precipitate indicates the presence of a reducing sugar. It is milk-sugar, or lactose.

(6) Wash the casein by the repeated addition of water, followed by decantation; pour it into a linen sack or a towel and press out the water; further, extract the water with absolute alcohol; extract the remnant of fat with ether; dry in the air. The white granular material that remains is nearly pure casein, the most important protein of milk, and represents nearly 4 per cent of milk.

(7) Heat 100 e.c. of the fresh milk in a beaker. Before the boiling point is reached a membrane gathers upon the surface of the milk. This membrane represents the lactalbumin of the milk, which has been coagulated by the heat and has collected in the membranous coagulum at the surface. The lactalbumin represents only a small proportion of the milk protein. Subject the membrane to the xanthroproteic test to demonstrate that it is a protein.

(8) Dilute fresh milk to one fifth normal and subject it to the following tests, recording results:

(a) Trommer's test.

(6) The xanthoproteic test.

(c) The biuret test.

(9) Fill a dialyzer with the diluted milk. One day later examine the diffusate (a) For any of the inorganic constituents of milk.

(b) For the carbohydrate constituents of milk.

(c) For the protein constituents of milk.

(d) For the fatty constituents of milk.

(10) Formulate in a series of concise statements the facts demonstrated regarding milk:

(a) Its chemical constituents. (6) Its physical properties.