FERMENTATION 191 



stacked to undergo heating, and fermentation occurs, 

 possibly by the action of members of the proteus, subtilis, 

 and mycoides groups, or by one of the Bacilli tobacci, 

 but the main trend of opinion is to regard the process as 

 due to oxidising enzymes. 



Tanning. Hides m&y be freed from hair by treatment 

 with sulphites or milk of lime, or by allowing hides to 

 putrefy. This is followed by a ' pickle ' of bran and 

 canine or avian excreta. Lactic acid is developed, which 

 combines with the lime to form a soluble lactate. Gas- 

 forming bacteria (B. furfuri is the only one yet isolated) 

 grow and cause the hides to swell. The changes that take 

 place in the next stage, in the tan-pit ; are due to bacteria; 

 the process is essentially a souring process, in which 

 success lies in the proper regulation of the amount of 

 lactic acid produced. 



Retting of flax appears to be due to the growth of 

 bacteria, one of which has been isolated by Winogradsky, 

 and is used for the work as a pure culture. 



Fermentation processes are involved in the preparation 

 of tea, coffee, cocoa, and indigo. On the Continent low- 

 grade wines are improved by the use of pure cultures of 

 bacteria obtained from high-class vintages, whereby much 

 of the characteristic aroma and bouquet of the latter are 

 communicated to the wine. 



The Enzymes. 



The enzymes are unorganised ferments, often secreted 

 by bacteria and the cells of animals and plants. In many 

 cases (groups 1, 2, 3, 5 and 6) they act by hydrolysis, 

 or, more correctly, by zymolysis. The most important 

 groups are the following: 



1. Proteolytic, changing proteins into proteoses, pep- 

 tones, and further products of hydrolysis, as in the case 

 of pyocyanase, trypsin, and pepsin. 



2. Amylolytic, converting amyloses (starch, glycogen) 

 into sugars, such as diastase, ptyalin, and amylopsin. 



3. Inversive, which convert disaccharides (lactose, 

 maltose, cane-sugar) into glucose. Invertase, which 

 occurs in the cells of yeasts (p. 162) and in the intestinal 

 juice, is an example of this class. 



4. (Joagulative, converting soluble into insoluble proteins, 

 such as rennet (chymosin), the fibrin and myosin ferments. 



