The composition of normal butter does not favor the growth of the majority of kinds of bacteria that are contained in it. The washing process removes much of the material suitable as food for the bacteria, such as sugar and albumen. If considerable butter milk is left in the butter, the growth of bacteria will be quite rapid, at first, but does not continue for any considerable length of time. The addition of salt also tends to restrain the growth of most kinds of bacteria. Butter is at its best when it is perfectly fresh. Deterioration begins within a short time and the rapidity with which the changes go on is dependent on the temperature at which the butter is stored. The temperature of the butter rooms in the large cold storage plants is kept below 0° F. The butter in such rooms will deteriorate very slowly, but on removal from the cold rooms and in storage at ordinary temperatures deterioration goes on more rapidly than would have been the case when the butter was fresh. At the temperature of an ordinary refrigerator the changes go on much more rapidly. This fact has often been looked on as indicating that the factors causing the changes are biological ones. The influence of temperature in accelerating the changes would be the same if no biological factor were active. That biological factors are of importance is indicated by the fact that the keeping quality of the product is profoundly affected by the quality of the cream. Butter made from sweet, fresh cream, that has been thoroughly pasteurized, has the best keeping quality, while butter made from such cream, but not pasteurized, has the poorest keeping quality, especially when no salt is added. Every process by which the desirable lactic bacteria are increased in proportion to other kinds has a marked effect in enhancing the keeping quality of the butter. Thus, the use of pure cultures in raw cream, and pasteurization together with the pure cultures, have a marked beneficial effect. The addition of preservatives exerts an effect on keeping quality. Borax is the chemical most frequently employed for this purpose. Its use is allowed in Australia and New Zealand in butter that is shipped to England, but the use of all preservatives is forbidden in the United States. The size of the package also has an effect on the keeping quality; the smaller the package, the greater is the surface exposed to the air and the more rapidly the butter deteriorates. Butter used in the United States Navy is packed in hermetically sealed cans so as to exclude the air as far as possible.