20 TEXTBOOK OF BOTANY 



for use. Of the forms producing fermentation, the lactic 

 acid bacteria are of most interest. These grow and multiply 

 rapidljr in milk at ordinary temperatures, so that if only a 

 few get in they will cause the milk to sour in a short time. 

 However, lactic acid bacteria midtiply very slowly at lower 

 temperatures (4o°-5o° F.), and this is the reason why milk 

 remains sweet longer if it is kept cool. While lactic acid fer- 

 mentation spoils milk for most piirposes, it is necessary in 

 order to prepare the milk for butter-making. 



29. Sterilization and Pasteurization. — Milk as well as 

 other liquids may be sterilized by heating it three or four 

 times on successive days to the boiling point (212° F.). This 

 kills all bacteria whether in the active or the spore condition, 

 but it also produces changes in the chemical make-up of the 

 milk which render it less desirable for some purposes. By 

 heating the milk to about 155° F.'^ for twenty minutes and 

 then cooling it, the great majority of the bacteria in the active 

 condition (but not those in the spore condition) are killed 

 and little change is caused in the milk itself. This process 

 is called pasteurisation, and properly pasteurized milk, kept 

 in a cool place, should remain sweet for several days. In 

 time, however, some of the bacteria which were not killed 

 will grow, divide, and become numerous, and the milk will 

 sour or change in other ways, depending upon the kinds of 

 bacteria that it contains. 



30. Other Saprophytic Bacteria. — Among the many sap- 

 rophytic bacteria that occur in milk are those which are con- 

 cerned in the "ripening " of cheese. The characteristic flavors 

 of many different lands of cheese are due to substances produced 

 in the cheese by the action of particular species of bacteria. 

 To make one of these cheeses, therefore, it is necessary not 

 only to treat the materials in the proper way, but also to make 

 sure that the right kinds of bacteria are present. Another 



1 The temperature actually used in the pasteurization of milk varies considerably, 

 but is usually between 140° F. and 160° F. 



