The starters may be propagated either in whole or skim milk; the former is preferable since, in most creameries, it can be more easily selected. The quality of the milk used has much to do with the quality of the starter; it should be as fresh and clean as it is possible to obtain. The clean bottle should be filled half to two-thirds full, covered and heated in some manner so that the milk shall be at a temperature close to the boiling point for fifteen to twenty minutes. The heating may be done by placing the bottles in water, which is heated on a stove or by steam, or the bottles may be subjected to streaming steam. The milk is cooled quickly and the contents of the package purchased added and well mixed with the milk. In the case of the dry starters, the mixing should be done with especial care. The bottle is kept in a warm place and in twenty-four to thirty-six hours, the milk should be curdled. A second bottle must be treated as before and inoculated from the first, and the process repeated daily since the bacteria must have fresh food, if they are to be maintained in good condition. In order to accomplish this, the maker must be able to maintain constant conditions from day to day, especially with reference to the amount of the ripened starter that is transferred to the fresh bottle of milk, and the temperature at which the bottles are kept. A spoon, arranged as shown in Fig. 31, enables one to carry a definite amount of the ripened starter to the bottle of milk to be inoculated and a constant temperature box (Fig. 32) permits of the maintenance of the same temperature from day to day. Through careful supervision of these points, and by taking care at every step to avoid the introduction of contaminating organisms, the purity of the culture can be maintained, and the bacteria kept in a healthy condition. The starter is used because of the acid-forming bacteria it contains; it is said to be ripe and in the best condition for use at the time it contains the greatest number of living bacteria. It has been found by experiment that this is at the time the milk curdles at ordinary temperature, or when the acidity is about 0.6-0.7 per cent. If the acidity is allowed to increase to 0.8 or 0.9 per cent, the number of bacteria will be less and a larger amount of the starter must be used in order to ripen a definite amount of cream in the desired time. The use of an overripe starter may also have an injurious effect on the flavor. [Illustration: Fig. 31.--Bottle for Mother Starters. A milk bottle with a tumbler for a cover and a spoon for inoculating the other bottles enables the butter maker to propagate the starters without contamination.]