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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. From the fact that any condition which restrains or inhibits the growth of micro-organisms has a tendency to improve the keeping quality of butter, it would appear that the detrimental changes in the quality of butter are due to biological causes. The most common defect known is that usually referred to as rancidity. There are, however, different types of changes that are probably included under this head and it is very probable that different causes are operative in their production. True rancidity is probably due to biological causes; the so-called tallowy change, in which the butter acquires the odor of tallow is probably due to the combined action of light and air on the fat. =Bacterial defects in butter.= There are a number of defects in butter that are positively known to be due to the growth of bacteria i
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