BACTERIA IN MILK AND FOOD 255 



the use of rennet to form a curd, rennet curd cheese, to which 

 all the important varieties belong. Milk for cheese should 

 be free from Bacillus coli or other deleterious bacteria. The 

 milk for cheese cannot be pasteurized as for butter. 



Testing Milk for Bacillus Coli. A sample of milk is incu- 

 bated at 35 C. for a few hours, noting the curd, whether 

 firm or soft and gassy. 



Wisconsin Test. Milk curdled by rennet; curd cut and 

 drained and jars kept at 30 C. to 40 C. The curd should 

 have clean acid odor and taste. 



After the curd has been formed, the cheese is allowed to 

 ripen, and this is due to acid-forming bacteria, which permit 

 the pepsin in the rennet to act. Various molds, notably 

 Penicillium and Oi'dium lactis, are used to give certain 

 foreign cheeses their characteristic flavor. 



Condensed milk has few bacteria in it. The sugar and 

 condensation heat tend to prevent further growth of micro- 

 organisms. 



Concentrated unsweetened milk is a form of pasteurized 

 milk which is reduced in volume one-fourth. It is not always 

 sterile, and bacteria may develop in it if exposed to warmth 

 and air. 



Buttermilk and similar fermented drinks depend on 

 Bacillus lactis acidi and added yeasts. Bacillus bulgaricus 

 gives more acid and allows partial sterilization. 



Foods as a Source of Infection. Foods eaten after little 

 or no cooking, such as fruits, salads, and the like, and also 

 oysters, are possible sources of bacterial diseases, and the 

 so-called ptomain poisoning observed after the consumption 

 of ice-cream, sausage, canned meats, etc., is the result of the 

 action of bacteria or their products. 



Oysters and fish from sewage-polluted waters have pro- 

 duced typhoid. Vegetables grown in manured ground or 

 sprinkled with polluted water may be a possible source of 

 disease. The practice of exposing meats and other food to 

 street dust and flies is no doubt responsible for some disease. 



Alcohol and Vinegar Fermentations. On grapes are to 



