Fermentation and Kindrid Phenomena. 97 



us to enquire why in grape juice alcoholic fermentation always 

 occurs, whilst in milk we seldom if ever find the yeast organism, 

 but always that of the lactic or butyric fermentation. Both 

 liquids contain sugar, yet each undergoes a different fermentative 

 change. 



A little reflection will, however, enable us to understand the 

 reason. We must remember that other things besides sugar are 

 necessary for the proper growth of yeast, the lactic and butyric 

 ferments. Thus we have seen that yeast requires certain mineral 

 matters, and also certain nitrogenous substances, for its develop- 

 ment, and it is the same with the lactic and butyric ferments. 



We know that certain plants thrive best in particular soils 

 and often become self-sown : thus the common sea pink flourishes 

 in the immediate neighbourhood of the sea, but is not found 

 inland. There is probably some peculiarity of the soil of seaside 

 localities which fit it especially for the nourishment of that plant. 

 In a precisely similar manner grape juice appears to be the liquid 

 most suitable for the yeast organism, whilst milk probably 

 contains those mineral and nitrogenous substances which are 

 essential for the nourishment of the lactic and butyric ferments. 



The Acetic Ferment. — I suppose that everyone knows that 

 beer and wine when exposed to the air become sour, in fact 

 produce vinegar, which is manufactured commercially by this 

 very process. In proportion as the spirituous liquid grows acid 

 it is found to lose spirit ; hence it is obvious that the acid is 

 produced from the spirit. The chemical name for the acid which 

 is formed is acetic acid (from the Latin acetum, vinegar), and 

 it is an elementary fact in chemistry that it can be produced by 

 the oxidation of spirit. Thus 



C 2 H e O 



+ 



o 2 = 



H 2 



+ 



C 2 H 4 2 



Spirit. 





Oxygen. 



Water. 





Acetic Acid. 



It was originally believed that vinegar was produced by the 

 simple chemical action of the oxygen of the air upon the spirit, 

 and it was considered that the action was especially induced by 

 porous bodies, which acted first as spongy platinum does, by 

 condensing the oxygen and thus bringing it into closer contact 



