BACTERIA IN BUTTER-MAKING 197 
cance to-day than in earlier years, because our butter-making is 
coming to be concentrated in large creameries. 
The Cause of Cream-ripening.—The ripening of cream is a 
phenomenon of bacterial 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 prodigiousnumbers. 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,- 
ooo perc.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 tempera- 
ture 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 num- 
