There are two ferments or “starters” on the market which are commendable on account of their uniformity and purity. They are produced in laboratories, every care being taken to see that no foreign germs come in contact with them.
Preparatory Cultures.
—Into a commercial starter can put four gallons of sweet, skimmed milk, which tests two-tenths of one per cent acid, no more. Strain through a double thickness of perfectly clean cloth, heat the milk to 180° F., holding at this temperature for one-half hour. This “Pasteurizes” the milk by killing all life in it. It also destroys all flavor.
The milk should be cooled quickly to 90° F. As soon as this temperature has been reached pour in a four-ounce bottle of “starter” or culture, holding the bottle close to the milk so as to avoid contamination. The bottle should not be opened until ready for use. After adding the ferment cool down slowly to 70° F., stirring all the while so as to thoroughly mix the milk. The bacteria grow very rapidly at this temperature. In twelve hours, if kept evenly at this temperature in a tightly closed vessel, a sufficient quantity of the proper germs will have been developed to produce an acidity of five-tenths of one per cent. The milk will also have thickened somewhat. If the conditions are found at the termination of twelve hours, cool the milk quickly to 40° F. and hold at this temperature until ready to make a large “starter.”
If the milk has not grown sufficient germs to produce the above named acidity, that is, five-tenths of one per cent, reheat to 90° F. and hold at this temperature until it does. Do not stir it at this stage, as the “starter” should be kept with as little agitation as possible after the germs have been propagated.
The Regular “Starter”.
—When ready to make the factory “starter,” place twenty gallons of sweet, skimmed milk in a sterilizer, after having cleaned the vessel, and heat as before to 180° F.; hold at this temperature for one-half hour, then chill to 70° F. and add the four gallons of preparatory “starter” already described, stirring well while adding.
Let this stand twelve hours as before in a temperature of 70° F., when it should appear as the preparatory starter did, slightly thickened and showing an acidity of five-tenths of one per cent. When acidity test shows this to have been reached, chill to 40° F. (at this temperature the bacteria are dormant and will not develop), and hold until ready to ripen the day’s run of milk and cream for churning.
When the milk has been brought in the ripeners to the proper temperature, distribute the larger “starter” evenly through it. Three per cent is sufficient during the summer months, while even as high as ten per cent is necessary in cold weather.