In determining the acidity of cream, a definite volume is taken, and the acidity determined by titration, expressing the results as such a per cent of lactic acid. Manifestly, the amount of fat in the cream influences the apparent per cent of acidity. The acidity will not usually exceed 0.5 to 0.7 per cent, but in reality the serum will contain more than this, as the acid is formed in the serum, the butter fat having no role whatever. In a very rich cream, 40 to 50 per cent fat, it is impossible to develop more than 0.4 to 0.5 per cent of acidity, and the flavor of the butter will be low, because of the relation between the amount of acid and fat, while in a thin cream having the same acidity, the ratio between the amounts of fat and acid will be very different. For example, in one hundred pounds of 50 per cent cream of 0.5 per cent acidity there will be one-half pound of acid and fifty pounds of fat; in the same quantity of cream containing 20 per cent of fat and having an acidity of 0.5 per cent there will be one-half pound of acid to twenty pounds of fat. The flavor of the butter from the rich cream will be quite different in intensity from that made from the thinner cream. The acidity of cream cannot be determined with any degree of accuracy by the taste or odor. Every butter maker should have some method of determining the degree of acidity in his cream, so that he may better control the flavor of his product. Several methods have been devised for this purpose and the necessary apparatus is sold by all dairy supply houses. The effect of the ripening of the cream is shown not only in the flavor of the product, but in a number of other ways. Sour cream churns more easily, and more exhaustively than does sweet cream. It is supposed that the fat globules are surrounded by a film of albuminous material which prevents their coalescing readily. During the ripening process, the action of the acid apparently dissolves this enveloping substance, and the globules cohere more easily in the churning process. When raw cream is used the ripened-cream butter keeps better than that made from sweet cream. In sweet cream there are few lactic bacteria, the majority of the bacteria present being of various kinds, many of which may be injurious, so far as the keeping quality is concerned. In sour-cream butter the lactic bacteria make up over 99 per cent of the bacteria present, and their presence tends to prevent the development of undesirable non-acid forms.