THE AUTOFERMENTATION OF YEAST. 543 



accurate investigation has yet been instituted on either of these 

 lines ; and up to the present nothing is known as to the practical 

 success of the Effront method. 



334. The Autofermentation of Yeast.* 



Except for an antecedent, though indefinite, announcement by 

 BERTHELOT (III.), Pasteur may be regarded as having been the 

 first to observe that, under certain conditions, the total carbon 

 dioxide and alcohol produced from a solution of sugar by the action 

 of yeast may exceed the theoretical yield calculated from the 

 sugar initially present in the nutrient solution. Thus, for in- 

 stance, o 424 grm. of sugar and 10 grms. of yeast (calculated to 

 dry substance), instead of furnishing about no c.c. of carbon 

 dioxide, in accordance with the equation of decomposition, actually 

 produced 300 c.c., together with 0.6 grm. of alcohol. Since only 

 a portion and not the whole of this yield is covered by the quantity 

 of sugar present in the nutrient solution, the remainder must have 

 originated in the constituents of the yeast cells, so that the latter 

 possess the faculty of fermenting, not merely the sugar in their 

 vicinity, but also the material of their own corpus. As a matter 

 of fact, under certain conditions (previous abundant nutrition, 

 restriction of the access of air, <fcc.), considerable quantities of 

 alcohol may be formed in a mass of yeast, without even a single 

 trace of sugar having been added. The term applied to this 

 phenomenon is "autofermentation." 



Pasteur's explanation of the phenomenon was that the yeast cells 

 contain a substance capable of being transformed into sugar and 

 afterwards fermented ; and, by boiling yeast with dilute sulphuric 

 acid, he succeeded in extracting no less than 20 per cent, of 

 fermentable sugar (calculated on the dry matter of the yeast). 

 His assumption, however, that the cell wall is the source of this 

 sugar was an error out of which his rival, LIEBIG (III.), made a 

 good deal of capital. This latter worker showed, in the case of 

 a number of samples of yeast, that the cellulose, forming the 

 residue of the method of extraction employed at that time, was in 

 considerably smaller amount than would be requisite to produce the 

 total alcohol resulting from autofermentation. Thus, to mention 

 only a single instance advanced by Liebig, 100.6 grms. of yeast, 

 found by a preliminary experiment to contain 13.9 per cent, of 

 so-called cellulose, furnished by autofermentation 13. 9 per cent, of 

 alcohol, whereas the theoretical yield from the amount of cellulose 

 in question was only 11.3 per cent, at the most. Liebig regarded 

 this observation as a proof of his own theory of fermentation ; 

 but it must be pointed out that, though the determination 



* This paragraph has been chiefly drafted on the reports of Professor 

 Lafar, to whom I am greatly indebted for his valuable assistance here, and 

 in other portions of chapters Ixiv. and Ixv. A. B. 



VOL. II : PT. II 2 M 



