THE RELATION OF MICROORGANISMS TO MILK 



397 



bers of this group quite clearly from those of the true lactic group which 

 grow chiefly below the surface of the medium. The members of this 

 group do not form spores, but certain varieties are quite resistant to 

 heat and will oft times survive pasteurizing temperatures which com- 

 pletely destroy the Bact. lactis acidi group. They grow most rapidly 

 at high temperatures, between 20° and 37°. 



Sweet Curdling Fermentation. — ^This phenomenon is caused by 

 a variety of organisms which cause the milk to coagulate without the 

 production of acid. The coagulation is brought about by a rennet-like 

 enzyme produced by this type of bacteria. The resulting milk is 

 either neutral or alkaline in reaction. Usually the coagulation of the 

 milk is followed by the digestion of the casein as a result of another 

 enzyme which is also produced by these bacteria. The coagulation 

 caused by these organisms is slower than in the case of the acid 

 formers and the curd is usually soft and mushy as compared with the 

 curd formed in the normal acid fermentation. The members of this 

 group get into the milk from and along with dust 

 and dirt associated with unsanitary conditions. 

 Some of the species produce spores and are not 

 kiUed by the ordinary methods of pasteurization. 

 This fact accoimts for the occurrence of sweet 

 curdling of pasteurized milk. This group of or- 

 ganisms is unable to develop rapidly in the pres- 

 ence 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 evi- 

 dence of insanitary 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 or 

 sUmy fermentation of milk. This is sometimes spoken of as stringy 

 milk (Fig. 131). Several species of organisms are capable of pro- 

 ducing this condition. 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 under- 

 lying layers of milk. B. lactis viscosus is perhaps the most common 

 species in this group. The slimy condition in the milk is supposed 



Fig. 131. — Ropy- 

 cream lifted with a fork. 

 {After Ward.) 



