BACTERIA IN BUTTER -MAKING. 183 



The Cause of Cream-ripening. The ripening of cream is a 

 phenomenon of bacteria growth. The many bacteria in the cream 

 find it an excellent medium for food, and if kept at a fairly 

 warm temperature during the ripening period, their development 

 is rapid. For the twelve to twenty-four hours of ripening, the 

 bacteria multiply, and, by the time the cream is ripened and ready 

 to be churned, they are present in prodigious numbers. Analyses 

 of ripened cream have disclosed the fact that, whereas in the sweet 

 cream bacteria may be from 2,000,000 to 3,000,000 per c.c., in 

 the same cream when ready to churn there may be about 500,000,000 

 per c.c. The numbers at the time of ripening, however, vary widely, 

 being sometimes as low as 200,000,000, or even lower, and sometimes 

 as high as 2,000,000,000 per c.c. 



The growth of bacteria in the cream produces chemical changes 

 which considerably modify its nature. The lactic acid bacteria 

 always develop lactic acid, and the cream becomes sour; but there 

 are other changes as well. We do not yet know what all these 

 changes are or to what extent they contribute to the ripening 

 phenomenon. That the other changes have something to do with 

 the production of the flavor in butter is evident from the fact that 

 a butter flavor cannot be produced in the cream by adding lactic 

 acid to it, and if the ripening were wholly the result of souring, 

 the addition of lactic acid should produce the same results as normal 

 ripening. 



Growth of Bacteria During the Ripening. At the outset 

 cream contains many kinds of bacteria, and the composite cream 

 of a creamery has more kinds than that of a private dairy. The cream 

 is commonly kept between 60 and 70, at which temperature 

 many bacteria develop rapidly, but not all kinds with equal vigor. 

 During the first few hours there is a general increase in the number 

 of nearly all the kinds of bacteria originally present in the cream, 

 so that, after six or eight hours, there are higher numbers of all 

 species of bacteria than were found at first. During this time, 

 however, the lactic acid bacteria, especially of the Bact. acidi lactici 

 type, increase more rapidly than the others. In the very fresh 

 cream this species may have been comparatively small in numbers, 



