CHAPTER XXVII 



FERMENTED MILKS 



According to Metschnikoff, many of the degenera- 

 tive changes associated with old age are due to poisons 

 generated in the intestines by putrefactive bacteria. 

 Moreover, his investigations lead him to believe that 

 excessive intestinal putrefaction can be greatly lessened 

 by introducing bacilli which produce lactic acid in the 

 intestines. In fact, he ascribes the healthfulness and 

 longevity of certain people of eastern Europe to their 

 extensive use of sour milk as an article of diet. 



For many years the people of eastern Europe and 

 western Asia have looked upon sour milk as an essential 

 part of their daily diet. The term sour milk covers all 

 milks or parts of milk in which lactic acid fermentation 

 is pronounced. Ordinary buttermilk sours because of 

 the growth of lactic acid bacteria in the raw milk. 

 Sour milk from the dealers is more usually heated milk 

 to which some special culture of bacteria (''starter") 

 has been added. A starter, now extensively used, is one 

 supplied by Metschnikoff. 



Many different species of bacteria are able to provoke 

 the lactic acid fermentation, but ordinarily only a few 

 species are responsible for the natural souring of milk. 

 Chief among the latter are the common lactic acid bacilli 

 and an organism spoken of as the milk streptococcus. 

 Inoculation of sterilized milk with pure cultures of these 



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