Fermentations of Milk. 55 



and in the dust of the barn. They are also found on the 

 milk utensils. Another type is found in great numbers 

 in the manure. The first is a desirable form and is nec- 

 essary in the making of butter and cheese, the latter is 

 very undesirable in every way. 



The number of lactic acid bacteria in freshly drawn 

 milk is always very small, often aggregating not more 

 than one in one thousand of the total number of bacteria 

 present. In sour milk, 99 per cent, of the bacteria are 

 lactic organisms. This great change in the flora is due 

 to the fact that the lactic bacteria find more favorable 

 conditions for growth in milk at ordinary temperatures 

 than any other kind that enters the milk. They grow 

 rapidly and the acid they produce is antagonistic to 

 many other species. Meat, eggs, and animal foods in 

 general putrefy. Milk does not under ordinary condi- 

 tions, because the acid formed by the lactic bacteria pre- 

 vents the growth of the putrefactive forms, just as vine- 

 gar keeps pickles from rotting. Meat can be preserved 

 by placing it in milk in a stoppered bottle. If it were 

 not for the lactic bacteria, milk instead of having a 

 pleasant taste and an agreeable smell, would putrefy and 

 give off as offensive odors as do meat and eggs. This is 

 shown when a little milk is spilled onto the stable floor 

 or into the water of a cooling vat. The lactic bacteria 

 are to be looked upon as the friends of man. They are 

 perfectly harmless and even beneficial to him. Butter- 

 milk, a form of sour milk, is enjoyed by most people as 

 a refreshing drink. It contains millions of lactic acid 

 bacteria in every drop. Clabber, curds, and cottage- 

 cheese are other forms of sour milk and are widely used 

 as human food. 



The curd which the desirable type of acid-forming bac- 



