132 BACTERIA AND THE CARBON DIOXIDE CYCLE 



(N. B. The yeast employed was a wine-yeast, but not a 'pure- culture' 

 according to modern ideas) (108): 



Grammes. 



Ethyl Alcohol 506-15 



Normal Propyl Alcohol ...... 0-02 



Iso-butyl Alcohol 0-015 



Amyl Alcohol 0-51 



Oenanthylic Ether 002 



Iso-butylene-glycol 1-58 



Glycerine 21-2 



Acetic Acid 2-05 



Succinic Acid 4-52 



Aldehyde traces 



In other words, 506 grm. alcohol and about 30 grm. of by-products, to 

 which is to be added about 450 grm. CO 2 (estimated). Some one per cent, 

 of the sugar was used by the yeast as food. It is noticeable that as much 

 as two per cent, glycerine is present. This is of some importance, for it has 

 more influence than might be supposed upon the taste of the wine. The 

 two next most important products are acetic and succinic acids. 



The example just given is of course only one of many different kinds 

 of fermentation ; it must not be taken as a standard. The by-products 

 particularly are very different both as regards quantity and composition in 

 the different technical fermentations. In distillation higher alcohols (e.g. 

 fusel oil) than ethyl alcohol are found. In wine the compound ethers 

 (esters, compounds of alcohols and organic acids) are of prime impor- 

 tance because, although present in homoeopathic quantities, they are the 

 substances which constitute the ' bouquet.' 



As soon as all the sugar in a fermenting fluid is used up the process 

 ceases, as it does also in any case when the alcohol has reached 12 or 14 per 

 cent. It is from this cause that many Spanish wines remain sweet, alcohol 

 being added to them in order to increase their permanence (?). 



Alcoholic fermentation (opt. 25-30 ; min. about o ; max. about 53) 

 can progress both aerobically and anaerobically. When air has free access 

 to the fermenting fluid the yeast cells multiply with great rapidity, but the 

 power of fermentation, that is to say, the amo^^nt of siigar broken up by 

 unit weight of yeast in unit time, is very small. Conversely, if oxygen be 

 absent or only very sparingly present, the fermentative power of the organisms 

 is great, but the rate of growth very slow. This is best illustrated by 

 a concrete example. To entirely ferment (i. e. to consume all the sugar in) 

 300 c.c. ' must,' twenty-three days were necessary. The time was the same 

 whether air was present or absent, but the number of cells in each case was 

 very different. Where complete aeration was carried out the fluid contained 

 at the end of fermentation 4,454,800 cells per cubic centimetre, whilst if air 

 was excluded only 50,160 per c c. were found (109). It is evident that the 



