THE VOLATILE ACIDLrY OF WINE: PARTICULARLY 

 THAT PR(,)DUCED BY PURE CULTURES OF YEAST. 



By Prof. Ai5RAH.\M Izak Pekold, B.A., Ph.D. 



Introduction. 



Whilst I do not for a moment deny that the bulk of the 

 volatile acid.s found in wines containing a high volatile acidity 

 must be ascribed to the action of bacteria, I hojje to show in the 

 course of this paper that appreciable, and even very considerable, 

 amounts of volatile acids are and can he (formed by pure cultures 

 of yeast. I shall first give a brief review of such literature on 

 this subject as was accessible to me. after which I shall g'wQ a 

 resume of some investigations on this subject made in the 

 Qinological Institute at Elsenburg. My assistant, .Mr. Franeois 

 Fevrier, B.A., carried out the practical part of these investiga- 

 tions under my su})ervision, for which I wish here to tender him 

 my heartiest thanks. These investigations were undertaken 



partly on account of their purely scientific interest, and partly on 

 account of the light they might shed on the question of the vola- 

 tile acidity m our wines looked at innw a commercial standpoint 

 I shall revert to this as]>ect of my suliject at a later stage. 



Historical Review. 



Pasteur (1)* and Bechamp (2j tirst show^ed that acetic acid 

 is formed during the alcoholic femientation of grape-juice or 

 must when working with pure cultures of elliptic (wi'ue) yeast. 

 Duclaux (4), in coni])aring this formation of acetic acid 

 with that of alcohol during the same fermentation, points out 

 that, whilst they both have the sugar as their source, the sugaf 

 need merely enter the yeast cell to meet the zymase which decom- 

 poses it into alcohol and carbon dioxide, whereas the volatile acids 

 are the products of secretion or excretion of protoplasmic activ- 

 ity. He continues : " lis sont le produit d'une action vitale, 

 aumeme titre que I'alcool est le produit d'une action diastasique." 

 He showed that, even when no sugar is present, yeast left by 

 itself will slowly produce some volatile acid at the expense of it«! 

 cell contents. He further showed that more volatile acid is. 

 formed in the presence of large numbers of yeast cells after the 

 sugar has been fermented out than before this happens. He also 

 states that more volatile acid is formed under conditions that are 

 unfavourable. 



Biiclmer and Meisenheimer (5) showed that acetic acid, 

 together with verv small (pumtities of higliL-r fatty acids, was 

 always formed during a fermentation without yeast cells. This 

 proves that acetic acid is a true ])roduct of fermentati(jn. They 



* This number — and subsequent similar numbers further on — appended 

 to the name of the authority quoted, refers ic tlic l>iliIio<rraphy at tlie end 

 of this paper. 



