LACTIC FERMENTATIONS 781 



sugar originally present in the milk. This apparently spon- 

 taneous chemical change is the result of the vital activity of 

 bacteria which obtain access to the milk after it is drawn from 

 the cow, for if special precautions are taken to prevent their 

 admittance or to destroy them by heat and other means after they 

 have entered, the milk remains unchanged for an indefinite period. 



A large number of species of Micrococcus, Bacterium, and Strepto- 

 coccus have been isolated from milk and other substances which 

 are capable of inducing the above chemical change. Some of them 

 are comparatively rare. The organism most commonly and widely 

 distributed, and which is usually the cause of the souring of milk, 

 is Streptococcus lacticus K ruse, or S. Guntheri L. and N. — a short 

 somewhat anaerobic bacterium or oval coccus mostly occurring 

 united in pairs. It breaks down the milk-sugar into optically 

 inactive lactic acid, carbon dioxide gas, and small amounts of 

 acetic acid, and other substances. Unless precautions are taken 

 to neutralise the lactic acid with carbonate of lime or some 

 similar compound, only a small portion of the sugar is decom- 

 posed, the fermentative activity of the bacilli being checked 

 when the free acid formed reaches about -8 per cent. 



The amount of acid produced in the case of this organism 

 is sufficient to precipitate the casein of the milk in irregular 

 coagulated lumps, so that not only is the milk rendered sour, 

 but it is curdled as well. 



To the farmer whose object is the sale of fresh milk the lactic 

 bacteria are objectionable organisms. Their activity is greatest 

 at about 30° or 35° C, but below this temperature they can in a 

 few hours render milk unsaleable. To avoid their multiplication 

 and injurious effects milk should be cooled as soon as possible 

 after milking, and kept at a low temperature. Moreover, the 

 milk-cans and the vessels in which it is stored should be 

 thoroughly cleaned and scalded with boiling water before use 

 in order to destroy these and any other bacteria present. 



To the producer of butter the lactic bacteria are useful for they 



