246 CHEMICAL INFLUENCES ON YEAST. 



classes the relative proportion changes in favour of the wild yeasts 

 when the culture is grown in a i o per cent, solution of saccharose 

 containing 4 per cent, of tartaric acid. The progressive diminu- 

 tion of the culture yeasts and increased percentage of wild yeasts 

 obtained by successive re-inoculations in such a liquid, forms an 

 excellent means of detecting the latter in critical cases. The Sake 

 yeast examined by K. YABE (III.) will not develop at all in this 

 solution. The influence of acetic acid was first investigated, in 

 the case of pure cultures, by LAFAR (II.), who found that the 

 fifteen species of wine yeast examined differed considerably in 

 point of sensitiveness. RODERICK MEISSNER (I.) afterwards found 

 the same result in the case of Saaz ; Frohberg and Logos yeasts, 

 which, however, cannot stand nearly so much of this acid as the 

 wine yeasts in question, the first two losing their fermentative 

 capacity almost completely in presence of 0.25 per cent., and the 

 third with 0.375 P er cent., whereas all the fifteen wine yeasts 

 continued to ferment in presence of 0.78 per cent, of acid, and 

 three of them even with i per cent. H. MULLER-THURGAU (XXI.) 

 found that the restrictive influence of this acid on development 

 can be ameliorated by aerating the nutrient medium (must). 

 According to DUCLAUX (I.), the presence of 0.4 grm. of formic 

 acid per litre will retard the development of cells of various kinds 

 of beer yeast sown in wort, reproduction ceasing entirely when 

 the addition is doubled. Oxalic acid is found by 0. LOEW (IX.) 

 to destroy the fermentative power of yeast in twenty-four hours, 

 when forming i per cent, of the solution ; and H. WILL (I.) states 

 that the same result ensues with 10 per cent, in five minutes. In 

 the case of succinic acid, M. HAYDUCK (VIII.) stated that even 

 0.59 per cent, does not hinder fermentation by yeast; and 

 E. KAYSER (X.) showed that this quantity is consumed by the 

 organism. The same worker (IX.) also found variable degrees of 

 sensitiveness to malic acid and citric acid on the part of different 

 yeasts. In the comparative tests made by J. BEHRENS (VIII.) 

 with Carlsburg bottom yeast No. i in unhopped beer wort, an 

 addition of 0.2-0.4 P er cent, of citric acid retarded the maximum 

 development of fermentative capacity to some extent, without 

 diminishing the total effect. 



The behaviour of yeasts toward hop resins a chemical biblio- 

 graphy of which has been compiled by Gr. BARTH (I.) has not 

 yet been sufficiently investigated. Three of these resins (see 

 vol. i. p. no) were isolated from hops by M. Hayduck (IX., X., 

 XII., I.), who showed that no appreciable antiseptic effect is pro- 

 duced by hop tannin or by the ethereal oil which imparts to hops 

 their characteristic aroma. On the other hand, according to this 

 worker, the two soft resins decidedly retard the progress of fer- 

 mentation; but L. AUBRY (III.) proved that the final attenuation 

 in hopped wort is higher than in unhopped wort, a result con- 

 firmed by J. BEHRENS (VIII.), though F. W. RICHARDSON (I.) 



