8 



PRACTICAL DAIRY BACTERIOLOGY 



indeed, the heating of the milk to a temperature of 160, a 

 vast majority of the bacteria are killed; but "the few spores 

 that may chance to be in the milk are not thus killed, and 

 these will be able subsequently to develop. If milk contains 

 spore bearing bacteria, it cannot be sterilized by boiling, and 

 since it almost always does contain them, boiling is not suffi- 

 cient to sterilize milk. This phenomenon of the high-resisting 

 powers of spores must always be borne in mind in all problems 

 of sterilizing. 



YEASTS 



Among the micro-organisms that one is likely to find in milk 



are some that do not belong to 

 the bacteria proper, although 

 closely related to them and of 

 hardly less importance in dairy 

 phenomena. The first type is 

 YEAST. Yeasts are miscroscopic 

 cells of a round or oval shape, 

 and may usually be distin- 

 guished from any of the true 

 bacteria by their somewhat 



FIG. 6-RELATivE SIZE OF YEAST (&) lar g er size - The relative size 

 MOLD SPORES (c) AND BACTERIA (a) of yeast cells and bacteria 



cells may be seen from figure 



6. Occasionally we find small yeast cells and sometimes large 

 bacteria, so that the matter of size is not in all cases sufficient 

 to enable one to determine whether he is dealing with bacteria 

 or yeasts. The primary difference between the two is in their 

 method of multiplication. 



The multiplication- of yeasts is different from that of bac- 

 teria, and is known as the multiplication by budding. There 

 appears on the side of the yeast cell a minute bud which con- 

 tinues to increase in size (Figs. 7 and 8), until after a time it 

 becomes as large as the cell from which it was produced. The 



