By the use of the pure-culture starters, the butter maker is able to add to his cream the same kind of bacteria from day to day, and the butter will be more uniform than when the less constant home-made starter is employed. In cream to which the starter is added, there are present a greater or less number of acid-forming bacteria, depending upon the age of the cream, and upon the condition under which it was produced. These will grow during the ripening process, and the flavor of the product will be the result of the mixture of the bacteria in the cream. The maker can not, therefore, be certain that the addition of a pure culture to raw cream will effectively control the type of fermentation. This can be secured only by first destroying the existing bacteria in the cream, before the selected culture is added. Heating the cream accomplishes this; and in cream thus freed from the various kinds of bacteria, the butter maker can insure the dominance of the desirable types, contained in the pure-culture starter. If the cream can be obtained in a sweet condition, the maker through this process of pasteurization, and the use of pure cultures, secures almost perfect control over the type of fermentation that occurs in the cream, and thus exercises control over the degree and kind of flavor of the product. This most scientific type of butter making is now used by the most progressive butter makers in the leading butter-producing regions of the world. Pasteurization of the cream also distinctly improves the keeping quality of butter, a condition doubtless due to the freedom of the same from organisms other than the lactic bacteria. This is a factor of as much importance as uniformity, because under modern business conditions, the surplus production must be kept in storage, and it is essential that the quality should not deteriorate materially during this time. =Process of pasteurization for butter making.= In the pasteurization of market milk, it is necessary to take into account the effect of heating on the physical and chemical properties of the milk, and the degree of heat that can be employed is limited. In pasteurizing cream for butter, there is no such limitation, and the cream may be heated to any temperature desired. In Denmark where the process of pasteurization has been used most extensively, temperatures ranging from 176° F. to 190° F. are used. The machines are of the "continuous flow" type, and the cream rather than the whole milk is treated. To prevent the spread of tuberculosis and other diseases, the Danish government requires that all cream and milk be heated to 176° F., before the skim milk or butter milk is returned to the farms. The heating of the butter fat to high temperatures has an injurious effect on the texture of the butter, unless the cream is cooled to 50° F., for a period of at least two hours previous to churning.