ZYMASE 489 



the former of which grow on the surface of the wort, and are 

 generally employed for the brewing of the stronger varieties 

 of beer such as are produced in this country ; the bottom 

 fermentation yeasts, on the other hand, are used chiefly on 

 the Continent for brewing of Lager beer. 



Great care has to be taken to ensure the purity of the yeast 

 employed in fermentation, since contamination with wild 

 yeasts, which produce substances other than alcohol, may 

 result in the introduction of strange and unwanted flavours 

 in the resulting brew. 



MECHANISM OF FERMENTATION. 

 Gay Lussac proposed the following equation to represent 

 the action of the living yeast cell upon sugar : — ■ 

 (I) QHiPs = 2C,HeO + aCOj 



The mechanism of this reaction, involving the production of 

 carbon dioxide without the intervention of atmospheric oxygen, 

 has given rise to much speculation as to the association of 

 the phenomenon with the vital activities of the yeast cell, 

 especially from the point of view of its life without oxygen. 

 The significance to the yeast plant of its fermentative activity 

 and its relation to the whole chain of phenomena known by 

 the name of respiration will be dealt with elsewhere. 



Various theories have been put forward to explain the for- 

 mation of alcohol from glucose. Kostytschev * considers that 

 pyruvic acid and acetic aldehyde are intermediate compounds. 

 (a) CsHijOg = 2CH3CO . COOH + [4H] 



This acid is then decomposed by a carboxylase present 

 in the yeast to form acetic aldehyde — 



{b) 2CH3CO . COOH = 2CO2 + 2CH3CHO 



The aldehyde is then reduced to ethyl alcohol — 



(c) 2CH3CHO + [4H] = 2CH3 CH2OH 



A combination of these three equations gives equation (i) 

 above. 



It was first observed by Pasteur that in addition to carbon 

 dioxide and alcohol there are formed small quantities of 



* Kostytschev : " Zeit. physiol. Chem.," 1912, 79, 130, 359. 



