CH. XXV.] VARIETIES OP FERMENTS 407 



(6) Protcolytic those which change proteids into proteoses and 

 peptones. Examples: pepsin, trypsin. 



(c) Steatolytic those which split fats into fatty acids and 

 glycerin. An example, steapsin, is found in pancreatic juice. 



(d) Inversive those which convert saccharoses (cane sugar, 

 maltose, lactose) into glucose. Examples : invertin of intestinal 

 juice and of yeast cells. 



(e) Coagulative those which convert soluble into insoluble 

 proteids. Examples : rennet, fibrin ferment. 



Most ferment actions are hydrolytic i.e., water is added to the 

 material acted on, which then splits into new materials. This is 

 seen by the following examples: 



1. Conversion of cellulose into carbonic acid and marsh gas 

 (methane) by putrefactive organisms 



(C 6 H 10 O 5 ) + H Q O = 3CO 9 + 3rcCH 4 . 



[Cellulose.] [Water.] [Carbonic [Methane.] 



acid.] 



2. Inversion of cane sugar by the unorganised ferment invertin 

 C 12 H 22 U + H 2 = C 6 H 12 + C 6 H 12 O fl . 



[Cane sugar.] [Water.] [Dextrose.] [Levulose.j 



Some enzymes, called oxidases, are oxygen carriers, and produce 

 oxidation. They occur in living tissues. 



A remarkable fact concerning the ferments is that the substances 

 they produce in time put a stop to their activity ; thus, in the case of 

 the organised ferments, the alcohol produced by yeast, the phenol, 

 cresol, etc., produced by putrefactive organisms from proteids, first 

 stop the growth of and ultimately kill the organisms which produce 

 them. In the case of the enzymes also the products of their activity 

 hinder and finally stop their action, but on the removal of these pro- 

 ducts the ferments resume work. 



This fact suggested to Croft Hill the question whether ferments 

 will act in the reverse manner to their usual action ; and in the case 

 of one ferment, at any rate, he found this to be the case. Inverting 

 ferments, as we have just seen, usually convert a disaccharide into 

 monosaccharides. One of these inverting ferments, called maltase, 

 converts maltose into dextrose. If the ferment is allowed to act on 

 strong solutions of dextrose, it converts a small proportion of that 

 sugar back into maltose again. This discovery of Croft Hill's has 

 since been confirmed by others in relation to other enzymes. 



Ferments act best at a temperature of about 40 C. Their activity 

 is stopped, but the ferments are not destroyed, by cold ; it is stopped 

 and the ferments killed by too great heat. A certain amount of 

 moisture and oxygen is also necessary ; there are, however, certain 

 micro-organisms that act without free oxygen, and are called anae- 



