PURE YEAST AND BREWING OPERATIONS. 293 



secondary fermentation is caused by the pure yeast started 

 with and that the traces of wild yeast which cannot be ex- 

 cluded in practice play but an insignificant part. This is 

 illustrated in the following table, which represents one of a 

 considerable number of similar experiments. It will be 

 seen from the column headed "specific gravity" that the 

 secondary fermentation was really complete after nine days' 

 forcing, that the wild yeast, which could be detected by the 

 microscope, had produced no appreciable effect on the 

 gravity of the beer even after fifteen days' forcing, and that 

 it was only after twenty-five days' forcing that its effect in 

 lowering the gravity was noticeable. In these experiments 

 the proportion of wild yeast which gained admittance 

 through the unavoidable contact of the wort and beer with 



o 



the atmosphere was too small to produce any harmful effect 

 and the beer remained sound to the end. 



BEER PRODUCED WITH SINGLE RACE YEAST. 



It should be stated that no hops were added to the 

 samples of beer experimented with so that there is here no 

 question of diastatic action. 



An important paper on secondary fermentations has 

 been recently contributed by Professor Van Laer, 1 who^~— - 

 advocates the employment of "composite yeasts," that is to 



1 Trans, hist. Brewing, vol. vii., p. 55 









