VOLATILE ACIDS AND ALDEHYDES. 497 



tion that a very small quantity of the sugar had been converted 

 into lactic acid. EFFRONT (XII.) ascribes the formation of lactic 

 acid to a transformation of the proteid substances. Tn the fermen- 

 tation of sugar by expressed yeast juice, AHRENS (I.) observed a 

 non-volatile acid, which he stated to be lactic acid, but BUCHNER 

 and MEISENHEIMER (I Y.) were the first to prove in an indubitable 

 manner that this acid is a constant by-product of alcoholic fermen- 

 tation, and originates in the sugar (see p. 464, vol. ii.). 



324. Volatile Acids and Aldehydes as By-products 

 of Alcoholic Fermentation. Influence of Oxygen 

 on Fermentation. 



The acids in 323 are classified as non-volatile, or fixed acids, 

 and are contrasted, in zymotechnology, with the volatile acids, all 

 of which so far as concerns those present in fermentation pro- 

 ducts belong to the fatty series. (PRIOR VI.) fermented one and 

 the same beer wort with seventeen different pure yeasts, and deter- 

 mined the acidity in the resulting beers. The quantity of the 

 fixed organic acids formed per 100 c.c. of beer ranged from 

 the equivalent of 2.1 to 5.4 c.c. of decinormal caustic soda, and 

 that of the volatile acids between 2.1 and 5.8 c.c. of alkali. For 

 every TOO c.c. of alkali required to neutralise the fixed organic 

 acids, the quantity consumed in neutralising the volatile acids 

 formed by the various yeasts was 62.4-180.9 c.c. Consequently 

 the acidity due to the several races of yeasts fluctuates between 

 wide limits. According to ALFRED KAU (II.), the quantity of 

 volatile acids formed increases considerably at higher fermentation 

 temperatures (35 C.) ; and pressed yeast produces a larger amount 

 than beer yeast. STRAUB (I.) found that admission of air increases 

 the production of volatile acids ; and according to BIOURGE (I.), 

 the quantity formed is independent of the alcohol produced. The 

 concentration of the fermenting liquid is without any appreciable 

 influence, but, on the other hand, the yield is proportional to the 

 duration of fermentation, especially if the completely fermented 

 solution be stored for some time. AMTHOR (II.) found this to be 

 especially the case with Saccharomyces apiculutus (see p. 434, 

 vol. ii ). 



Formic acid is the first member of the volatile series. It was 

 observed here as long ago as 1891, by KRUIS and RAYMAN (II.), 

 in beers that had been left standing in contact with deposited 

 yeast for some years (see p. 126, vol. ii.). They also found it in 

 sterile beer worts after prolonged storage, and therefore attributed 

 its formation to a chemical reaction occurring in the wort itself, 

 and probably connected with the transformation of protein. 

 KHOUDABABACHIAN (I.) detected formic acid in fresh grape must, 

 the quantity increasing during fermentation when the surrounding 



