68 MICRO-ORGANISMS AND FERMENTATION. 



varieties, than in the presence of 0'78 per cent. Those yeasts, 

 which are active in presence of 1 per cent, of acid, showed, 

 under these circumstances, a decrease in work done as com- 

 pared with the results in presence of 0'88 per cent. 



2. LACTIC ACID BACTERIA. 



When milk is exposed at a temperature of 35 to 42 C. 

 it soon turns sour, a considerable portion of the acid produced 

 being lactic acid, which is formed by the agency of various 

 species of bacteria. When a certain quantity of lactic acid 

 has been formed, the fermentation ceases. It will recom- 

 mence, however, if the liquid is neutralised with carbonate 

 of lime, or if a small quantity of pepsine or pancreatine 

 is added, causing the caseine of the milk to be dissolved. 



A method often employed for inducing a spontaneous lactic 

 acid fermentation is the following : To a litre of water are 

 added 100 grams of sugar, 10 grams of caseine or old cheese, 

 and an abundant quantity of powdered carbonate of lime. 

 This mixture is exposed in an open vessel to a temperature 

 of 35-40 C. The liquid is occasionally agitated, or a 

 current of air is passed through it. After completion of the 

 fermentation the liquid is evaporated, when calcium lactate 

 crystallises out, and from this the lactic acid is liberated 

 by addition of the calculated quantity of sulphuric acid. 



In addition to milk-sugar, lactic acid bacteria are also 

 capable of fermenting cane-sugar, glucose, maltose, and various 

 other substances. According to BOURQUELOT'S investigations, 

 a species of lactic acid bacterium, which makes its appearance 

 in the spontaneous souring of milk, is capable of fermenting 

 cane-sugar without previously inverting it. 



In nature, these bacteria seem to occur comparatively 

 rarely. On the other hand, they are of frequent occurrence in 

 the various fermentation rooms. 



In breweries, lactic acid fermentation takes place in malt, 

 especially during mashing; lactic acid is also produced during 

 fermentation. In the Belgian beers, obtained by spontaneous 

 fermentation, lactic acid is formed in large quantity, imparting 

 a sharp taste to the beer ; the so-called " white beers " owe 



