LACTIC FERMENTATION 267 



sugar in the fermentation process should be replaced by 

 a definite amount of lactic acid, but the amount of acid 

 produced is always less than would be expected from the 

 above equation. Some of the sugar is used by the 

 bacteria for nutrition and respiration with the formation 

 of several compounds, such as acetic, formic, and succinic 

 acids, and the liberation of the gases, carbon dioxide, 

 hydrogen, nitrogen, and occasionally methane. The 

 quantity of lactic acid used, and the amounts and nature 

 of the by-products obtained, is dependent upon the 

 species of organisms present in the milk and on the 

 temperature and access of air. Under the most favour- 

 able conditions the yield of lactic acid is not more than 

 about 89 per. cent, of what would be expected from the 

 sugar which is lost, and is usually much less than this. 

 At a temperature of 10 C. (50 F.) the lactic organisms 

 grow very slowly and the amount of acid produced by 

 them is very small. The best temperature for their 

 physiological activity lies between 20 C. (70 F.) and 

 32 C. (90 F.). They are very sensitive to the action of 

 free acids, .15 per cent, of hydrochloric or .03 per cent, 

 of sulphuric acids completely check their work. More- 

 over, the fermentation of the sugar in milk generally 

 ceases when the lactic acid present is over .8 per cent, 

 before all the sugar is used up. The process can be 

 made to advance further by neutralizing the acid with 

 calcium carbonate as fast as it is produced. 



Ex. 134. Into one flask pour 500 c.c. of fresh milk and to 

 another similar flask add the same quantity of milk and boil it 

 for one minute : cool. 



Leave both in a warm place and test the acidity of samples 

 taken from each flask every twelve or twenty-four hours until 

 the milk is curdled ; shake it well before each test. 



