SUPPLEMENT 67 



(DucLAUX, 1900). If the oxygen be permanently excluded, however, the yeast 

 in the long run dies, even though nutrients be still to hand (BEIJERINCK, 1894). 

 Alcoholic fermentation itself must thus also be furthered in the long run by 

 limitation of the amount of oxygen, but the statements in the literature on the 

 whole question are perhaps more conflicting than in any other department of 

 Physiology (CzAPEK, Biochem. I. 260). 



By way of summary, we must say that the yeasts are neither genuine 

 aerobes nor anaerobes. This statement, however, is, speaking generally, not 

 quite in accordance with the facts, because different functions of the yeast are 

 affected in quite different ways by oxygen. It is very desirable that the 

 minimum, optimum, and maximum of oxygen needed for the carrying out of 

 these functions in yeast should be determined as thoroughly as has been done 

 by WUND (1906) for many Bacteria. 



If the yeast forms alcohol under the conditions when normal respiration is 

 possible, this indicates a loss of energy, 



1. 50 P. 214, 1. 32, for This fact . . . higher alcohols read But this is 

 true only under the artificial conditions of a pure culture ; under natural 

 conditions, where there is always a mixture of organisms, apparently such 

 a suicidal action does not occur (BENECKE in LAFAR'S Mycologie, I. 330). 



We thus see that yeast obtains an advantage by this habit of always 

 forming alcohol, and we may suppose that other organisms that excite fermenta- 

 tion must behave in a similar way. By the term ' fermentation ' we understand, 

 therefore, a dissimilation process which does not lead to the formation of CO 2 

 and H 2 O, and in which the products which are formed have a biological signifi- 

 cance in relation to the organisms which produce them. 



Alcoholic fermentation does not lead merely to the formation of alcohol 

 and carbon-dioxide ; there are always by-products as well, and the occurrence 

 of lactic acid among these is quite intelligible since it is an intermediate stage 

 between sugar and alcohol ; but the case is otherwise with amyl-alcohol (fusel 

 oil), glycerine, and succinic acid. The source of amyl-alcohol has been most 

 satisfactorily explained by EHRLICH'S (1906) interesting demonstration of its 

 origin from leucin. It has nothing at all to do with the real alcoholic fermenta- 

 tion, but arises from a nitrogenous substance which occurs already in the 

 natural culture solution, or arises always as a down-grade product of proteids 

 in the yeast. Its amount may be greatly increased, just as readily by the 

 addition of more leucin to the culture as by a reduction in the concentration 

 of the sugar, and hence it may be clearly seen that there is no relation between 

 the formation of ethyl-alcohol and amyl-alcohol. The source of the glycerine 

 is as yet unknown ; it is very improbable that it arises from sugar, possibly the 

 fat or lecithin of the yeast may be its source. Its amount varies and depends, 

 in the first instance, on the species of yeast employed, and secondly, on the 

 nature of the nutriment provided, not only the fermentable carbohydrate but 

 also the source of nitrogen. LABORDE (1899) found that 2-5-7-75 g. of glycerine 

 were formed from 100 g. of fermented sugar as the result of the action of various 

 yeasts, and WORTMANN'S (1892) experiments with different wine-yeasts show 

 a very variable formation of glycerine. Succinic acid comes next to glycerine 

 in amount, and reaches about 0-5 g. per 100 g. of sugar, but the amount is liable 

 to considerable variation. Since both these secondary products are also formed 

 by yeast under conditions which exclude the formation of alcohol (UDRANSKI, 

 1889), we have obviously to deal with a special kind of metabolism whose 

 significance is unknown, but which has nothing to do with alcoholic fermenta- 

 tion (WORTMANN, 1898). Into the question of the occurrence of other by- 

 products such as aldehyde, volatile acids, &c., we will not enter ; these are, 

 doubtless, formed also in pure yeast fermentations, though not actually pro- 

 ducts of it. There are also other bodies present which give definite characters 



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