84 PRACTICAL DAIRY BACTERIOLOGY 



PROTECTIVE ACTION OF THE LACTIC ACID 

 BACTERIA 



The role which the lactic bacteria play in milk is one of con- 

 siderable importance, since they protect the milk from putre- 

 faction. 1 Most proteid materials readily putrefy from the action 

 of bacteria. Why does not milk undergo the same change ? 

 The other bacteria which are liable to be found here, especially 

 the liquefying type, if left to develop unhindered, would produce 

 a series of decomposition changes in the milk, as they do in 

 meat or eggs, resulting in a variety of bad tastes and odors, 

 ending in putrefaction, and in some cases would produce 

 poisons, which would be extremely injurious. But ordinarily 

 milk will not putrefy. The rapid development of the lactic 

 acid bacteria prevents the growth of these miscellaneous organ- 

 isms. As a result, although it contains a great quantity of easily 

 putrefiable proteid, it is prevented from undergoing this putre- 

 faction by the development of the lactic acid bacteria. One 

 practical side of this question relates to the pasteurizing of 

 milk. As we shall notice later, the ordinary pasteurizing of 

 milk destroys a large number and sometimes all of the lactic 

 bacteria, but leaves in an active condition many kinds of bac- 

 teria, especially those producing spores. It frequently happens, 

 therefore, that in pasteurized milk there are no lactic acid bac- 

 teria present whose growth can subsequently prevent the milk 

 from putrefaction by means of the other bacteria. Pasteurized 

 milk will ordinarily not sour and seems to keep good for a con- 

 siderable time; but it inevitably undergoes certain types of de- 

 composition, and the products which appear after the bacteria 

 have begun to grow are very much worse than those in sour 

 milk. Pasteurizing, therefore, deprives the milk of the pro- 

 tecting action of acid bacteria, so tkat after the other bacteria 

 have a chance to grow, as they are sure to do in a few days, 



4 Bouska. Land. Jahr. der Schweiz, 1903. 



