BACTERIA IN BUTTER-MAKING. 243 



should commonly be a good one, and produce a desirable 

 type of ripening, when we remember that the milk obtained 

 in this way has opportunity for contamination with many 

 kinds of bacteria. A partial answer is, that the bacteria which 

 get into the milk, from a cleanly kept dairy and a cleanly kept 

 cow, are most likely to be species of a desirable character. 

 But the more important reason is that already referred to on 

 a. previous page (see page 1 90). It was there seen that the 

 common species of lactic bacteria are much better adapted to 

 the conditions occurring in milk, when it is kept at a tempera- 

 ture of about 65 to 70 F., than the great majority of milk 

 bacteria. When milk is maintained at this temperature these 

 lactic bacteria grow with excessive rapidity, while the other 

 species are evidently less favored, as indicated by their slow 

 growth. In milk maintained at such a temperature the lactic 

 bacteria present grow so much more rapidly than the other 

 species, that they soon outnumber them many fold, and by 

 the time the milk has soured they outnumber the other species, 

 ordinarily one hundred to one. This is very frequently true 

 even when the original milk or cream had very small numbers 

 of lactic bacteria. In some actual analyses of cream it has 

 been found that, although in fresh cream the miscellaneous bac- 

 teria outnumbered the lactic bacteria perhaps ten fold, in the 

 soured cream the lactic bacteria were considerably over ninety 

 per cent. From these facts it will follow that the milk of an 

 ordinary cleanly dairy, if left to sour at such a temperature, 

 will show, at the time of souring, a vast majority of the normal 

 lactic bacteria, even though it may have had at first a consid- 

 erable majority of miscellaneous species. Now, it is these lactic 

 bacteria which perform the chief part in the ripening of the 

 cream, although perhaps not the whole, and it consequently 

 follows that such a lot of soured milk will make a favorable 

 starter for cream ripening, simply because the lactic bacteria 

 which were present at the proper temperature, showed so much 



