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].