BACTERIA IN FOODS 191 



bacilli do not as a rule liquefy gelatine, form spores, are 

 non-motile, and are easily killed by heat. 



When a certain quantity of lactic acid has been formed 

 the fermentation ceases. It will recommence if the liquid 

 be neutralised with carbonate of lime, or pepsine added. 

 Since Pasteur's discovery of a causal bacillus for this ferment- 

 ation, other investigators have added a number of bacteria 

 to the lactic acid family. Some of these in pure culture 

 have been used in dairy industry to add to the butter a pure 

 sour taste, a more or less aromatic odour, and a higher 

 degree of preservation. 



(2) Butyric Acid Fermentation. This form of ferment- 

 ation is also one which we have previously considered. 



Both in lactic and butyric fermentation we must recognise 

 that in the decomposition of milk-sugar there are almost 

 always a number of minor products occurring. Some of the 

 chief of these are gases. Hydrogen, carbonic acid, nitrogen, 

 and methane occur, and cause a characteristic effect which 

 is frequently deleterious to the flavour of the milk and its 

 products. Most of the gas-producing ferments are members 

 of the lactic acid group, and are sometimes classified in a 

 group by themselves. In cheese-making the gases create 

 the pin-holes and air-spaces occasionally seen. 



(3) Curdling Fermentations without Acid Production. Of 

 these there are several, caused by different bacteria. What 

 happens is that the milk coagulates, as we have described, 

 but no acid is produced, the whey being sweet to the taste 

 rather than otherwise. Digestion of casein may or may not 

 take place. 



We must now mention several fermentations about which 

 little is known. They are designated by terms denoting 

 the outward condition of the milk, without giving any in- 

 formation respecting the real physiological alteration which 

 has occurred. 



(4) Bitter Fermentation. Some bitter conditions of milk 



