Here, too, we have an explanation of the rapid change that occurs in brewers’ yeast when left to itself after fermentation. In such a mass of cells, kept apart from any food-supply, and only with 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 correspondingly 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 themselves, 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,[109] 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 reproducing; 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. On December 15th, 1872, we bought nine samples of beer in different large cafés in Paris, which had all come from the best breweries of Strasburg, Nancy, Vienna, and Burton. After leaving them for twenty-four hours, we decanted all our samples, and then sowed a drop of the deposit of each in flasks of pure wort. On January 2nd, 1873, we examined the ferments formed in these worts, which had been kept in an oven at a temperature of 20° C. (68° F.), and also tasted the beers produced; they all had an abominable taste, and each contained diseased ferments. At the same time, by way of comparison, we impregnated other flasks of wort with pure ferments. None of the beers of this series acquired a bad taste or produced foreign ferments; they only became flat.