L. A. ROGERS 399 



The rate of development of these flavors varies greatly in different lots of butter 

 but, other conditions being equal, is proportional to the temperature. While low 

 temperature greatly retards flavor changes, even the very low temperature (0° F.) 

 of commercial storage will not entirely prevent deterioration. 



The three factors which may be of importance in the deterioration of butter are 

 the enzymes secreted with the milk, or by bacteria in the milk or cream; bacteria, 

 either by direct action in the butter or by changes brought about in the milk or cream 

 from which the butter is made; and chemical changes which may take place spon- 

 taneously in such a complex and unstable product. 



The enzymes of the milk, especially the lipase, probably play some part in flavor 

 changes in butter made from unpasteurized cream, but they are nearly or quite in- 

 activated by the temperature of pasteurization. Bacterial enzymes may be a factor 

 under some circumstances. 



Butter made from ripened cream contains, in its fresh state, many millions of 

 bacteria per gram. In creamery butter made from pasteurized cream by approved 

 methods these bacteria are almost exclusively the lactic streptococci introduced as 

 starters. These bacteria decrease slowly as the butter stands in cold storage, and 

 more rapidly at higher temperatures. 



There is usually little or no multiplication of micro-organisms in salted butter. 

 This is easily understood in view of the fact that butter contains 25-3^ per cent of 

 salt dissolved in about 15 per cent of water. Sometimes torula yeasts grow for a time, 

 and there may be some multiplication of the salt-tolerant bacteria. Molds may grow 

 on the surface if the butter is not properly packed or is held at or above the freezing- 

 point of water. The molds cause rapid changes in the butter in the vicinity of the 

 colony but occur only under abnormal conditions. The yeasts and salt-tolerant 

 bacteria have never been shown to have any significant part in the changes causing 

 deterioration. 



The direct action of bacteria in the butter as a primary cause of undesirable fla- 

 vors is excluded by the fact that all of the typical storage flavors appear in the butter 

 held at temperatures at which bacteria cannot possibly grow. On the other hand, the 

 by-products of bacterial growth in the milk or cream may be the precursors of fla- 

 vor-producing substances which develop later in the butter. Acidity, which was for- 

 merly considered essential to good keeping quality, is now known to accelerate de- 

 terioration, and butter for storage is made from cream of low acid content or even 

 from pasteurized cream in which no acid is developed. 



About 10 per cent of the volume of butter is air, incorporated by the working 

 process, in the form of minute bubbles. In a short time the oxygen of this air disap- 

 pears, presumably through oxidation of some of the constituents of the butter. More- 

 over, an oxidation of fats may take place without free oxygen, probably through in- 

 termolecular rearrangement.' An acid reaction is essential to these changes, and they 

 are further accelerated by such catalysts as the salts of copper, iron, and nickel which 

 are dissolved by milk and cream from cans, pasteurizers, and other equipment. 



' Holm, G. E., Greenbank, G. R., and Deysher, E. F.: Indust. &• Engin. Chem., 19, 156. 1921. 



