202 PHYSIOLOGY OF NUTRITION 



tion of the wasp the medium soon began to ferment. The yeast cells pass the 

 winter in the soil and find their way to the young fruits the following season. 



Fermentation results in an increase in the dry weight of the yeast. If a 

 fermentable liquid is inoculated with a slight amount of yeast, the cells rapidly 

 increase by budding, and if enough of the liquid has been used a considerable 

 amount of dry substance is finally obtained. Fermentation is thus a physio- 

 logical process connected with the growth and reproduction of the yeast cells. 



Glucose and other varieties of sugar are suitable material for fermentation. 

 Saccharomyces cerevisice 7, S. pastorianus 7, 77 and 777, and S. ellipsoideus I and 

 77, all contain the enzyme invertase, which hydrolyzes cane sugar to form fruc- 

 tose and glucose, the latter being subject to fermentation. Maltose is fer- 

 mented in the same way, but lactose is not affected. Saccharomyces marxi- 

 anus, S. ludwigii and S. exiguus attack only glucose and saccharose, without 

 affecting lactose or maltose; S. apiculatus ferments only glucose, but S. kiphyr 

 and S. lactis are able to hydrolyze lactose. 



Sugar solution alone fails to produce an abundant growth of yeast; nitrogen 

 and mineral substances are necessary for these cells just as in the case of other 

 plants. These other substances are plentiful in grape juice and beer-wort, but 

 must of course be included in artificial nutrient media if yeasts are to be 

 cultivated therein. Among the ash-constituents of yeast, phosphates play a 

 conspicuous role. 



The researches by Harden and Young 1 indicate that alcoholic fermentation 

 proceeds by two stages, as follows: 



Carbon 

 Glucose Phosphate dioxide Alcohol Water Hexose phosphate 



i. 2C 6 H 12 6 + 2M"HP0 4 = 2CO2 + 2C2H5OH + 2 H 2 + C6H 10 O 4 (M"PO4)2 



Hexose phosphate Water Glucose Phosphate 



2. C 6 H 10 O 6 (M"PO 4 ) 2 + 2H 2 = C 6 H 12 6 + 2 M"HP0 4 



Hexose-phosphate is thus formed and again decomposed during the process, and 

 it is for this reason that the addition of soluble phosphates accelerates fermenta- 

 tion. The phosphate may therefore be considered as a co-enzyme of zymase. 

 Harden and Young showed that after the filtration of yeast through a gelatine 

 filter neither the filtrate nor the precipitate is capable of producing alcoholic 

 fermentation, but fermentation does occur if the filtrate and precipitate are 

 again brought together. The necessary phosphates occur in the filtrate in this 

 experiment. 



Yeast cells may also develop in a medium without nutrient material, under 

 otherwise suitable conditions, and they still produce carbon dioxide and alcohol. 

 This is the so-called auto-fermentation of yeast, which results in a decrease 

 rather than in an increase of dry substance. Here the carbon dioxide and 

 alcohol are formed at the expense of the yeast material itself. A similar phe- 



1 Harden, Arthur, and Young, William J., The alcoholic ferment of yeast-juice. Proc. Roy. Soc. 



London 77: 405-420. 1906. Idem, same title. Ibid. 78: 360-375. 1906. Idem, same title. Ibid. 



80: 299-311. 1908. Idem, The function of phosphates in alcoholic fermentation. Centralbl. Bakt., 

 //, 26: 178-184. 1910. 



