BACTERIA m OUR DAIRY PRODUCTS. 769 



" ripen/' Tlie cream in a creamery is placed in large vats, and 

 tlien kept at a constant warm temperature for about twenty-four 

 hours. The cream is stirred frequently during this time, and at 

 the end of the ripening it is seen to have changed its character. 

 It is somewhat acid in taste, is slightly thickened, and has a pleas- 

 antly sour odor, though one quite different from that of sour 

 milk. The cream is now churned, and the butter is found to sepa- 

 rate readily, the quantity is at its maximum, and the butter ob- 

 tained has the proper butter aroma. 



Bacteriological study of the last few years has shown that this 

 "ripening" is nothing more than a breeding of bacteria on a 

 large scale. There were many bacteria in the cream at the begin- 

 ning, and the ripening has been conducted at just the tempera- 

 ture at which bacteria grow rapidly. The result is, that their 

 multiplication is marvelously rapid, and the number of bacteria 

 present in ripened cream is beyond comprehension and almost 

 beyond calculation. Five millions in a drop would not be too 

 high an estimate for some specimens. 



Now, what are the bacteria doing in the cream during their 

 twenty-four hours' growth ? They can not multiply so rapidly 

 without producing profound changes in the cream. So far as the 

 butter-maker is concerned their action is twofold : 1. There is pro- 

 duced in the cream a considerable amount of lactic acid, together 

 with small quantities of other acids. 2. Various decomposition 

 processes are going on which fill the cream with decomposition 

 products, and these give rise to the odor and taste of ripened 

 cream. 



To understand the effect that this ripening has upon the but- 

 ter-making, we must first ask what happens to the cream during 

 the churning. If we look at a drop of milk under the microscope, 

 we find that the butter-fat is in the form of the most minute 

 drops. So small are they that they can not be readily separated 

 from the liquid part of the milk. In cream we simply have the 

 great mass of these drops together, but still not at all fused, like 

 a lot of snow-balls floating in water. In the churn, however, the 

 cream is agitated until the drops are shaken together and made 

 to fuse with each other. They now form masses of fat large 

 enough to be removed from the liquid in which they float, and 

 these masses form the butter. But, looking at the cream more 

 closely, we find a mechanical difficulty in the way of their ready 

 fusion. The fat-drops are not free to move at will, for they are 

 bound together in groups by a sort of slimy substance. As we 

 watch the cream with our microscope we see the fat-globules are 

 not easily shaken together, for the slimy matter prevents their 

 direct contact. This slimy substance must be broken down and 

 the drops shaken into each other before the butter can form into 



VOL, XL. — 52 



