Standards of Cream.—The composition of cream varies with almost every sample. The standards for cream vary in different states and cities. The national standard requires 18 percent of fat.
Skimmed Milk.—The residue which is left from the removal of cream is known as skimmed milk. Skimmed milk contains the principle part of the nitrogenous constituents of milk, the greater quantity of its sugar and a very large quantity of its mineral matter. It is still a very valuable food product, lacking only the element of fat. When eaten with nuts or other oily food skimmed milk would complete the ration and make a well balanced food. The chief prejudice against skimmed milk is that it has been so often sold for whole milk. When sold and consumed under its own name it is not a fraudulent body and is deserving of a higher place in the dietary than has been ascribed to it. In the large creameries of the country the skimmed milk is usually fed to animals. It is one of the most highly esteemed foods for pigs and poultry, and is largely used for those purposes.
Composition of Skimmed Milk.—Naturally the composition of skimmed milk would be that of milk corrected for the abstraction of fat. It contains some little fat when prepared by the gravity method and only a very small portion when separated mechanically. The abstraction of the fat increases the relative proportions of sugar and casein.
Curd Test for Purity of Milk.—The Wisconsin curd test is conducted as follows: 1. Sterilize milk containers so as to destroy all bacteria in vessels. This step is very important, and can be done by heating cans in boiling water or steam for not less than one-half hour.
2. Place about one pint of milk in covered jar and heat to about 98 degrees F. (Figs. 15 and 16).
3. Add ten drops of standard rennet extract and mix thoroughly with the milk to quickly coagulate.
4. After coagulation, cut curd fine with case knife to facilitate separation of whey; leave curd in whey one-half hour to an hour; then drain off whey at frequent intervals until curd is well matted.
5. Incubate curd mass at 98 to 102 degrees F. by immersing jar in warm water. Keep jars covered to retain odors.
6. After 6 to 9 hours incubation, open jar and observe odor; examine curds by cutting the same with sharp knife and observe texture as to presence of pin holes or gas holes. Observe odor.
7. Very bad milks will betray presence of gas-producing bacteria by the spongy texture of the curd and will have an off flavor.
8. If more than one sample is tested at the same time, dip knife and thermometer in hot water before each time used.
"Normal milk contains practically no organisms but the straight lactic acid bacteria. These germs produce no gas and no bad odors, but purely lactic acid and the curd formed therefrom is such as is represented in Fig. 17.
