STUDIES ON FERMENTATION. 221 



occurs in brewers' yeast when left to itself after fermentation. 

 In such a mass of cells, kept apart from any food-supply, and 

 only wdth difficulty able to keep themselves in life by consuming 

 their own soluble contents, we have an excellent field for the 

 development of foreign germs. In this way we may have a 

 rapid putrefaction in yeast, to which there will be a corre- 

 spondingly rapid growth of organisms in the liquid, where they 

 find, as well as in the yeast-cells, appropriate nourishment. 

 Nothing could better confirm this view of the matter than the 

 array of facts, by way of antithesis, already described, in which 

 we have seen a pure yeast remain for an indefinite time in 

 contact with pure air, without undergoing any putrefaction, or 

 manifesting other changes than those which result from the 

 combustions peculiar to living cells when left to support them- 

 selves, in a moist state, in contact with oxygen. 



In the process of brewing, as soon as fermentation is finished^ 

 or rather, as soon as certain physical effects are produced, for 

 instance, when the beer falls bright, or, as the French say 

 technically, when the yeast breaks up,* the beer is racked ; 

 subsequently the yeast, which is left in a plastic layer at the 

 bottom of the vessels, is collected, washed, and kept under 

 water in a cool place, to be used again in the course of twenty- 

 four or forty-eight hours. Brewers never care to keep their 

 yeast for a longer time before using it, especially in summer. 

 We can understand how this practice prevents the foreign 

 germs which are mixed with the yeast from living and repro- 

 ducing ; but although the conditions of brewing, as far as the 

 treatment of the yeast is concerned, may, in a certain measure, 

 prevent the development of these germs of disease, nevertheless 

 they are there, and from their extreme minuteness, pass into 

 the beer in greater or less number, however bright it may have 

 been rendered by racking. There they only await conditions 

 favourable to their existence to enable them to develop, and to 

 affect more or less injuriously the qualities of that delicate 

 beverage. 



* " La cassure de la leviire." 



