The search for methods of buttermaking that would yield a uniform product of good keeping quality led to the introduction of cream pasteurization and the ripening of the treated cream with selected cultures of bacteria. At about the time this procedure was developed, evidence began to accumulate which indicated that salted butter made from sweet cream deteriorated less on holding than salted butter made from ripened cream and, eventually, the influence of acid on certain types of chemical deterioration in butter was definitely established. The conflict between a high acidity in the cream and good keeping quality in the resulting butter has necessitated drastic changes in the methods of using butter culture in the manufacture of salted butter. Over the years, in an attempt to avoid certain types of deterioration in the butter, there has been a gradual lowering of the cream acidity at churning. Most butter plants in the United States now use a relatively low acidity in the cream. The general procedure is to hold the mixture of cream and butter culture at a low temperature in order to avoid significant increases in acidity.