CHAPTER LII. 



THE EFFECT OF CERTAIN TECHNICALLY IMPORTANT 

 CHEMICAL INFLUENCES ON YEAST. 



267. Copper and its Salts. 



IN the previous three chapters we have set forth the formal con- 

 ditions relating to the nutrition and reproduction of yeast ; and 

 we have now to deal with certain important adverse influences to 

 which this organism is exposed either in nature or in fermenta- 

 tion on a practical scale. The effect of physical agencies may be 

 disregarded at present, since they are treated of elsewhere, and 

 we confine ourselves for the moment to chemical influences, 

 beginning with those of copper as the first to which the yeast 

 is generally exposed during reproduction or fermentation. 



The behaviour of yeast cells toward copper and copper salts is 

 of particular interest to the vintager. As the reader is aware, 

 the ravages of Peronospora viticola are combated by sprinkling 

 the vines with " bouillie Bordelaise" (introduced by A. Millardet 

 of Bordeaux), an approximately 3 per cent, solution of copper 

 sulphate in which as has been shown by BERLESE and SOSTEGNI 

 (I.) the copper is converted into hydroxide (or basic double salt) 

 by the addition of an equivalent amount of calcium hydroxide. 

 Experience has proved that this remedy produces the desired 

 effect, the conidia of the fungus being destroyed, and the leaves 

 and fruit of the vine preserved ; but at the same time the copper 

 exerts a toxic influence on the natural yeasts present on the 

 grapes, and since the various races of yeast probably differ in 

 their susceptibility to this action of copper, it may be supposed 

 that the treatment of the vines will result in an alteration in the 

 flora of the grapes, and that its influence will extend as far as the 

 must vat more especially when sprinkling has been performed 

 late in the season. Thus, A. ROMMIEII (I.) has observed that 

 must from late-sprinkled grapes gave no sign of fermentation, 

 even under favourable conditions of temperature ; whilst in other 

 cases the only living cells detected were those of Saccharomyces 

 apiculatus (probably less sensitive to copper), so that the fer- 

 mentation remained incomplete. This observation led Rommier 

 to investigate the influence of copper on yeast cells, with the 

 result that even an addition of 25 mgrms. of Cu ( = 98 mgrms. 



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