THE RELATION OF MICROORGANISMS TO MILK. 



3 2 5 



unsanitary conditions. Some of the species produce spores and are not 

 killed by the ordinary methods of pasteurization. This fact accounts 

 for the occurrence of sweet curdling of pasteurized milk. This group 

 of organisms is unable to develop rapidly in the presence of the lactic 

 bacteria and for this reason we do not commonly get the sweet curdling 

 of raw milk. The presence of these organisms is evidence of unsanitary 

 conditions. Frequently they develop very disagreeable flavors in the 

 milk. 



ROPY OR SLIMY FERMENTATION. One of the most common milk 

 infections causing trouble to the milk dealer is that which causes a ropy 



%"'"" ',*' 

 OV^ 



-.^i..k*f.-;i-7, 



^ 



FIG 79. Bacillus lactis viscosus from a milk culture. 

 (After Ward ) 



or slimy fermentation of milk. This is sometimes spoken of as stringy milk 

 (Fig. 78). Several species of organisms are capable of producing this con- 

 dition. These organisms grow most freely in the presence of an abundant 

 supply of oxygen and for this reason the cream usually becomes slimy 

 before any changes are apparent in the underlying layers of milk. B. 

 lactis viscosus is perhaps the most common species in this group. The 

 slimy condition in the milk is supposed to be the result of a very viscid 

 capsule surrounding these organisms (Fig. 79). Representatives of this 

 group are quite resistant to heat and frequently pass uninjured through 

 the methods of cleansing and scalding used under ordinary dairy condi- 



