60 Agricultural Bacteriology. 



contained in sour milk as & starter to prevent gassy 

 cheese. 



None of the ropy fermentations are harmful as far as 

 health is concerned, but they are often the cause of con- 

 siderable loss to the milk dealer. They do not appear to 

 injure the use of milk for butter. The source of the bac- 

 teria in some of the outbreaks studied has been found to 

 be the water to which the cows had access. 



Alcoholic fermentation of milk. As is well known in 

 order to produce any of the fermented drinks, such as 

 beer or whiskey, the wort or mash (the infusion of barley 

 or other grain used) is seeded with yeasts, which act on 

 the sugar formed from the starch of the grain, producing 

 alcohol and carbon dioxide. None of the bacteria pro- 

 duce alcohol in appreciable amounts, this property being 

 confined in the main to the yeasts. The ordinary type 

 of yeasts can not ferment milk sugar, but in milk, butter, 

 and cheese, yeasts are found that possess this property. 

 "When these are numerous in the milk they may injure 

 the butter and cheese, giving these products a yeasty 

 flavor and odor. Through the gas produced from the 

 sugar, holes may be formed in the cheese and not infre- 

 quently gas is formed in such abundance as to cause the 

 cheese to crack open. 



Yeasts grow best in an acid medium. This is shown 

 by the ease with which all fruit juices, as apple and grape 

 juice, undergo alcoholic fermentation, even when no yeast 

 is intentionally added. Wild yeasts occur in sufficient 

 numbers on the surface of the fruit to seed the juice. In 

 less acid substances the bacteria appear first. In %hey 

 vats that are not carefully cleaned, a favorable place 

 exists for yeast growth. The whey becomes acid due to 

 the growth of the acid-forming bacteria, and as it still 



