VOLATILE ACIDS AND ALDEHYDES. 503 



The experimental basis on which this theory was constructed, so 

 far as yeast is concerned, was shown to be untenable by Nageli (II.). 

 The assumption that the plentiful admission of oxygen to cultures 

 of yeast causes this latter to develop like an aerobic thread fungus 

 and not set up alcoholic fermentation has not been proved, such 

 proof entailing the determination of the ratio between the carbon 

 dioxide liberated, the alcohol produced, and the resulting yeast 

 crop. The solution of this task was first undertaken by GILTAY 

 and ABERSON (II.) ; but their experiments are open to objection, 

 though not in the direction mentioned by DUCLAUX (XVIII.) in 

 a criticism refuted by GILTAY (I.). This worker arid Aberson 

 found that over 60 per cent, of the sugar consumed was converted 

 into alcohol and carbon dioxide, even in strongly aerated yeast 

 cultures in which energetic respiration occurred at the same time. 

 A similar result was obtained by BUCHNER and RAPP (V.) ; but 

 neither set of experiments was conclusive, the conclusions being 

 only a matter of probability, and therefore tending to refute the 

 accuracy of Pasteur's hypothesis. 



The chemical action of oxygen in alcoholic fermentation is a 

 far simpler question than this physiological influence. As already 

 stated 011 p. 501, vol. ii., alcohol readily undergoes oxidation to alde- 

 hyde and acetic acid. According to DURIN (II.), the carbon dioxide 

 liberated during fermentation constitutes a compact froth, and 

 the alcohol distributed all over the surface of the minute bubbles 

 of gas is readily transformed into aldehyde. ROSER (II.) also 

 found a larger quantity of aldehyde in fermentation conducted 

 with admission of air than without aeration. In any event, how- 

 ever, some action is exerted by yeast oxydases in connection with 

 the carrying of oxygen, especially when a film is produced by the 

 yeast. If a fermented liquid be left, together with the whole of 

 the veast, in contact with air for some time, KRUIS and RAY- 

 MAN (II.) state that nearly all the alcohol formed during fermenta- 

 tion is oxidised to carbon dioxide and water. The part played by 

 atmospheric oxygen during the storage of wines was referred to by 

 PASTEUR (XXXIII.), who found that young wine will retain its 

 original character for a long time when stored out of contact with 

 air. Which of the substances, however, combine with the oxygen, 

 and what products result, still remain undetermined. According 

 to WORTMANN (VII.) there is no doubt that considerable changes 

 are produced by atmospheric oxygen during the storage of wine 

 (see p. 509, vol. ii.) ; but the view that has prevailed since Pas- 

 teur's time, namely, that the matter is one of oxidation, is not 

 applicable in its entirety since physiological processes probably 

 form a contributory factor. The changes in question are chiefly 

 ascribed by Wort ma nn to the collaboration of organisms. Accord- 

 ing to RAPP (I.), the formation of esters is also increased by the 

 access of air a circumstance that can be easily explained, since 

 the acids resulting from the oxidation of alcohols combine more 



