CHAPTER III. 



TYPES OF MILK BACTERIA. 



IN classifying the bacteria of milk for our purpose it is 

 best to divide them into groups according to their action 

 upon milk. Although this is not the best scientific method 

 of classifying bacteria, it is the most useful for the purpose 

 of understanding their relations to milk. 



Practically all of the common changes which occur in milk, 

 subsequent to the milking, are due to the action of micro- 

 organisms. Milk, kept free from bacteria, may be preserved 

 for an almost indefinite, period with very little change. A 

 few years ago it .was supposed that this was absolutely true 

 and that milk, if protected from bacterial contamination, 

 would remain indefinitely without any chemical change. 

 Within the last few years it has been found that, even in 

 fresh milk, there is present a chemical ferment or enzyme 

 which has been called galactase. This is a normal ingredi- 

 ent in milk, derived from the cow, and it is capable of pro- 

 ducing a slow chemical change. Hence, even if uncontami- 

 nated by bacteria, milk will in time change its chemical 

 nature quite materially. This galactase probably plays a 

 part in the ripening of cheese, but its action is comparatively 

 slow and it has very little to do with those changes in milk 

 which take place rapidly. 



Odors and Flavors of Fresh Milk. Pure milk has very lit- 

 tle flavor or smell, but it frequently happens that our market 

 milk has strong tastes and odors. In some cases these fla- 

 vors are strongly developed in the milk at the moment it is 

 drawn. If, for example, a cow is fed upon garlic or tur- 

 nips, their odor shows itself in the milk, and in such cases 



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