^ 



OccidL 



7 



CHAPTER XXXIX 



BENEFICIAL 



ACTIVITIES OF 



BACTERIA 



or- 



Curdling of milk. 



Souring of fruit juices. 



Why does milk turn sour? What makes sweet cider turn into 

 vinegar ? Why will the dead body of an animal putrefy ? What is 



the importance of the rotation of crops ? 







Bacteria which are of value to man may be grouped into 

 three classes : (1) those necessary for the preparation of certain 

 foods ; (2) those aiding industries ; and (3) those useful in 

 agriculture. 



Bacteria in food preparation. Certain bacteria in milk change 

 the milk sugar, lactose, to lactic acid. The production of this 

 acid coagulates or curdles the protein in milk. This process of 

 acid-formation and protein-coagulation is called souring. When 

 the acid reaches a certain concentration, the process ceases. The 

 presence of lactic acid in cream increases the yield of butter and 

 improves the flavor. Lactic-acid bacteria are necessary for the 

 production of sour-milk cheeses such as Swiss, Edam, and Ca- 

 membert. The flavor of these cheeses is partly due to the lactic- 

 acid fermentation and partly to mold activity. 



Acetic-acid bacteria convert alcohol into vinegar. Yeasts first 

 attack the sugar of fruits or grains and convert it into alcohol, then 

 the acetic-acid bacteria attack the alcohol and change it to 

 vinegar. This vinegar is useful in preserving foods because 

 bacteria of decay cannot work in a^trong acid medium. 



403 



