PURE YEAST AND BREWING OPERATIONS. 297 



least in the case of certain yeasts, only slowly attained. 

 Van Laer, however, regards as true secondary fermenta- 

 tions only such as are produced by means of composite 

 yeasts, and when an after-fermentation is obtained with the 

 help of a single race yeast he considers it to be "artificial" 

 and as the result of arresting the primary fermentation at a 

 certain point. He regards it in fact as the completion of 

 the primary fermentation. When, however, all legitimate 

 attempts to carry the primary fermentation to the limit- 

 attenuation fail, it is difficult to understand how the cask 

 fermentation, which must in consequence necessarily take 

 place, can be called artificial. In the case of the single race 

 yeast employed at Chester's brewery, the writer has recently 

 determined the limit-attenuation and found it to coincide 

 with the attenuation attained at the end of the secondary 

 fermentation and to be considerably lower than is reached at 

 the end of the primary fermentation, thus affording additional 

 proof that the after-fermentation is independent of diastatic 

 action or of accidental contamination by foreign yeasts. 



Van Laer describes a number of interesting experiments 

 illustrating the secondary fermentations obtained with mixed 

 yeasts of known composition. The following example 

 will suffice to show the effect produced by a trace of a 

 low type yeast when added to a primary yeast. Two 

 samples of the same wort were fermented, one with the 

 yeast E 2 , the other with the same yeast with the addition of 

 a trace of the yeast E 5 . After fermentation the resulting 

 beers were decanted into sterilised bottles, which were then 

 hermetically closed and exposed on the forcing tray to a 

 temperature of 30C. After eight days the rotatory power 

 of the beers was compared with that found at the time of 

 bottling, the difference indicating the extent of the secondary 

 fermentation. 



