Respiration, Aeration, and Fermentation 297 



172. Lactic fermentation. - - If no precautions are 

 taken to prevent the contamination of milk as it is drawn 

 from the udder, it normally undergoes lactic fermentation, 

 at the temperature of the dairy or living room. Among 

 the organisms usually finding access to the milk, are 

 bacteria (especially Bacterium lactis acidi) the action of 

 which produces a slight acidity or souring, and later a 

 marked effect exhibiting itself in the precipitation of the 

 casein (curdling). The acid developed is largely lactic, 

 and the course of the main changes referred to, involving 

 the milk sugar (lactose, which is first converted into 

 hexose sugars), is probably substantially thus : 



CisHooOn 4- H 2 = CH 13 O fi + C 6 Hi 2 6 = 4(C 3 H 6 O 3 ) 



lactose glucose galactose lactic acid 



One or more of a variety of organisms produce this end 

 result. In general, they utilize as food a portion of the 

 sugar, and they may produce, in small quantities, beside 

 lactic acid and carbon dioxid, the following : one or more 

 of several organic acids, also hydrogen, nitrogen, and traces 

 of methane, depending somewhat upon environmental 

 conditions. The production of lactic acid is so rapid in 

 milk that the medium is soon sterilized with respect to the 

 presence of other organisms ; but this is scarcely an ad- 

 vantage to the lactic forms, since this fermentation and the 

 activity of the lactic organism is usually brought to a 

 standstill when about .8 per cent of acid has been pro- 

 duced. According to certain investigators, a small amount 

 of lactic acid as an excrete product may be produced 

 during the anaerobic respiration of roots. 



173. Alcoholic fermentation. Alcoholic fermentation 



