212 LAURENCE F. FOSTER 



In a study of the acid fermentation of xylose by Fred, Peterson, 

 and Davenport (1919) the main products were found to be acetic 

 acid and lactic acid. The proportion of volatile acid to non- 

 volatile acid proved to be the same throughout the entire ten 

 to twelve days of fermentation; namely, 40 per cent of acetic 

 to 60 per cent of lactic acid. The two acids represented about 

 90 per cent of the sugar consumed. 



Speakman (1920) investigated the biochemistry of acetone 

 and butyl alcohol fermentation of starch and showed that acetic 

 and butyric acids are formed as intermediate products. A 

 reduction of these acids to the corresponding alcohol subse- 

 quently sets in. 



Methods 



Volatile acids. Volatile acids were determined by the steam 

 distillation method of Duclaux (1900) as modified by Dyer (1916). 

 This procedure is one in which the acid solution is distilled with 

 steam at a constant volume. The distillate is collected in 10 cc. 

 fractions until 100 cc. have passed over, after which the remainder 

 is taken off in 100 cc. portions. These fractions are then titrated 

 with N/50 alkali, using phenolphthalein as indicator, and the 

 percentage of acid is calculated. The amount of acid in a given 

 fraction, the "distilling constant" for the fraction, is plotted 

 against the corresponding portion of distillate on logarithmic 

 coordinate paper. Pure acids are graphically represented as 

 straight lines and arrange themselves consecutively from the 

 lower to the higher members of the series. With a mixture of 

 two volatile acids the first part of the curve occupies a position 

 intermediate to the lines representing the distilHng rates of the 

 higher and lower boiling acids. As the higher boiling acid is 

 removed, the curve gradually becomes parallel with the Hne 

 representing the lower boihng acid. 



It follows that the distillate from an unknown acid mixture 

 may be approximately determined by plotting a curve from the 

 distilling constants and comparing this with the curves estab- 

 lished for known acids. 



