No. 6. DEPARTMENT OF AGRICULTURE. 667 



11. Sources of Contamiiiatiou besides Bacteria. 



While many of the undesirable flavors of milk are due directly to 

 the result of bacterial action, milk may acquire taints that are not 

 dependent on a bacterial origin. Milk, as it comes from a cow, 

 possesses a peculiar "cowy" odor, due to direct absorption of vola- 

 tile substances within the aoimal. This odor usually disappears 

 quickly, when milk is exposed to the air; but it becomes abnormal 

 in quality and persistence when a cow eats such things as leeks, 

 turni[)s, cabbage, rag-weed, etc., a few hours before milking. If 

 milk is not drawn for eight to twelve hours after such food is eaten, 

 such taints either disappear or are greatly diminished. The same 

 experience in milder form may come from the use of some green 

 fodders, such as rape and green rye, and from the feeding of exces- 

 sive quantities of swill, brewers' grains and distillery slops. 



If milk, after being drawn, is exposed to contact with strong odors, 

 such as those coming from manure or other decaying organic matter, 

 or even from ensilage, it readily absorbs them. Milk, whether warm 

 or cold, possesses a very striking power for absorbing and holding 

 volatile odorous substances. 



12. Kinds of Bacteria in Milk. 



Many different kinds of bacteria have been found in milk. Some 

 appear to be very generally present, others only rarely. Many of 

 them have little or no appreciable effect upon milk or upon human 

 beings; while some furnish products that are distinctly beneficial, 

 others produce only injurious effects in milk, imparting to it bad 

 flavors or unpleasant appearance, and, in some cases, even causing 

 the formatiors of violent poison. It is also possible that the bacteria 

 causing dread diseases may be found in milk in some instances. 

 We present below the chief classes or types of bacteria that give 

 rise to the fermentations most commonly occurring in milk. 



(1.) Lactic Acid Fermentation or Souring of Milk. — The souring 

 of milk is due to the formation of lactic acid, which is produced by 

 the action of lactic acid bacteria upon the sugar in the milk. In 

 this form of fermentation, one can begin to taste the acid when 

 it amounts to about 0.3 per cent, of the milk. As the amount of 

 acid increases to 0.4 per cent., the milk begins to curdle or thicken, 

 and, as the amount of acid increases above this, the curd becomes 

 more solid. These bacteria continue actively converting milk sugar 

 into lactic acid until the acid reaches 0.8 to 1 per cent, of the 

 milk, and then they cease their activity, because they cannot live 

 in a solution containing this amount of acid. Their activity is 

 stopped by the accumulation of the product of their own activity 



