ENZYMES AND OTHER FERMENTS DIGESTION. 137 



Zymase is not a very stable ferment and in the solution 

 obtained is soon destroyed by other ferments present. The 

 yeast extract may, however, be concentrated at a low tempera- 

 ture and obtained in dry form which is more stable. Extracts 

 made from yeast by simple treatment with water may contain 

 invertase but no zymase. It seems probable that the ferment 

 is not confined to the yeast cell. It has long been known that 

 many overripe fruits produce a small amount of alcohol, even 

 when the possibility of the presence of yeast cells is entirely 

 absent. This formation of alcohol was finally ascribed to the 

 cell activity of the fruits themselves, but since the work of 

 Buchner it seems more rational to refer the appearance of alco- 

 hol to the presence of an enzyme in the. ripe fruit. 



It should also be said that sugar may be made to yield alco- 

 hol by a much simpler process. It has been found that a 

 sugar solution mixed with a little potassium hydroxide and 

 placed in bright sunlight yields some alcohol and carbon diox- 

 ide. This is of course a purely chemical decomposition, and 

 suggests the possibility of chemical reactions in other cases. 

 The old notion as to the necessity of the presence of living 

 cells to break down the sugar is thus completely disproved. 



ACETIC FERMENTATION. 



In this a true oxidation takes place, the oxygen of the air 

 being employed to convert weak alcohol into the acid accord- 

 ing to this reaction : 



C 2 H C + 2 = C 2 H 4 2 + H 2 0. 



The active agent concerned in the fermentation oxidation is 

 the cell found in " mother of vinegar." 



Mother of Vinegar is an old name given to the slimy scum 

 or sediment which forms in weak alcoholic liquids that turn 

 sour, in wine or cider, for example. Microscopic examina- 

 tion shows this substance to consist of minute cells which 

 have received the name of Micoderma aceti; more recently the 

 name Bacterium aceti has been given to the plant organism. 



