STUDIES ON FERMENTATION. 233 



this flask, after fermentation, in an oven at a temperature of 

 20° or 25° C. (68° to 77° F.). If the beer, in the course of a 

 few weeks, does not thicken, or become covered with efflo- 

 rescence, if its deposit is microscopically pure, if, in short, it 

 only tastes flat, we may have every confidence in the purity 

 of the yeast which produced it. After we have purified a yeast 

 we are, unfortunately, never sure that it has not undergone 

 some change in the course of the manipulations to which it 

 has been subjected in purification. It is indispensable, there- 

 fore, that we should test it, and see if the flavour of the beer 

 produced by it is really the one that we want — viz., that of the 

 beer from which we took the yeast that we submitted to puri- 

 fication. 



In the course of a series of practical experiments that 

 we were carrying out in the large brewery of Tourtel, at 

 Tantonville, in 1875, in connection with the new process of 

 brewing, which will be explained in Chap. YII., the following 

 circumstance occurred. We had purified some of the yeast 

 of the brewery, by means of a succession of growths and 

 adding a few drops of phenol, and had obtained a yeast of irre- 

 proachable purity. It happened that this yeast, which was 

 repeatedly cultivated in the brewery during the summer of 1875, 

 from six to ten hectolitres (130 to 220 gallons) of wort being 

 used on each occasion, always produced a beer that had a yeast- 

 bitten flavour and defective clarifying powers, notwithstanding 

 that it possessed remarkable keeping properties, which it owed 

 to the pureness of the ferment employed. As a matter of fact, 

 the beer sufiered no injury from journeys of more than 300 

 miles, by slow trains, in ordinary casks, containing from 50 to 

 100 litres (10 to 20 gallons), during the great heats of June 

 and July, or from being subsequently stored for two months in 

 a cellar, the temperature of which rose daring that time from 

 12° to 18° C. (54° to 65° F.) The temperature of fermentation 

 had been 13° C. (55° F.). Beer from the same brewery, made 

 with the same wort by the ordinary process, did not remain 

 sound for three weeks in this same cellar. 



