4o6 ACIDITY (IF WIXE. 



assume the presence of a particular enzyme, " j^lucacetase " in the 

 juice of the (crushed) pressed yeast, which decomposes the sugar 

 into acetic acid. (I would ix)int out here that i molecule of a 

 hexose could give 3 molecules of acetic acid. ) In view of the 

 above. Duclaux's explanation about the formation of volatile acid 

 by yeast will have to be modified in so far as to admit that, whilst 

 some volatile acid may be formed as he explained, a considerable 

 amount, if not most, is formed through the action of some enzyme, 

 such as the above supposed glucacetase, which is present in the 

 yeast cell. This will explain most olf the observations subse- 

 quently made, and which will be dealt with as we pnx^eed. 

 VVindisch (7), in referring to investigations made on this subject 

 by Bechamp, Thylman and Hilger, Kayser. Carles, Becker. Haas, 

 Rocques, and Reisch, states that the sum total of their conclu- 

 sions amounts to the following' : 



Different yeasts form, under otherwise identical conditions, different 

 amounts of volatile acid, the heer yeasts forming less than the wine or 

 elHplic and pastorianus yeasts. High temperatures, admission of air, and 

 a lengthy fermentation increase the volatile acids formed. The concentra- 

 tion of the fermenting liquid has nn influence. [Suhsequent work has 

 certainl}' disjjroved this! — A. I. P.I The liisrher the lixed aci(h'ty of 

 the must is. the lower is the volatile acid formed. 



He further states that Reisch (6) found that 

 the volatile acids rapidly form until nearly half the 

 -ugar is fermented out, when their formation soon 

 proceeds much more slowl\ , and stops altogether when half the 

 sugar has fermented out. Windisch then further states that, 

 when wine matures, the volatile acidity usually increases ; that 

 micro-organisms are no doubt mainly responsible for this, 

 although an oxidation of the alcohol, apart from an\' organisms, 

 iloes not seem out of the (|uestion ; further, that a wine's volatile 

 acidity sometimes decreases during maturation. 



R. von der Heide (9) in 1907 ])ublished the results of his in- 

 Aestigations on abnormal anKuints of volatile acid formed in 

 sweet musts by different yeasts. In his first .series of experi- 

 ments with one wine yeast (Steinberg. 1893) he used a must with 

 the same composition in every case excepting that the sugar rose 

 from 12 to 43 per cent, in the dififerent bottles. After two 

 tnotnhs the fermentation was over, and the yeast cells were nearly 

 all free from glycogen. The wines were now analyzed for 

 alcohol and volatile acid, and the following were the results ob- 

 tained : 



No. of Bottle 



I 

 2 



4 



