294 



SCIENCE PROGRESS. 



say of mixtures of different varieties of yeast. Van Laer 

 was among the first to introduce the Hansen system into 

 high fermentation breweries in Belgium, and after meeting 

 with a varying degree of success he has given up single 

 race yeast in favour of his composite yeasts. He has not 

 yet published the method by which he prepares his pure 

 composite yeasts but he appears to be largely guided by the 

 composite nature of ordinary commercial yeasts. The 

 purity of his composite yeasts would seem to apply in the 

 main to their freedom from bacteria, although dangerous 

 wild yeasts are also presumably excluded. But many 

 varieties which are generally regarded as wild yeasts, and 

 which are acknowledged by Van Laer to be highly objec- 

 tionable when used alone, are regarded by him as necessary, 

 in that they " complete or perfect the effect of other yeasts " 

 {loc. cit., p. 64). 



In discussing secondary fermentations Van Laer dis- 

 tinguishes between (a) beers with dry hops added at rack- 

 ing and (6) beers to which no dry hops have been added. 

 In the former case the diastase, which Brown and Morris 

 have shown to be present in hops, exerts a hydrolytic 

 action on the carbohydrates present, producing maltose 

 which readily ferments, and the secondary fermentation is 

 thus at once explained. This is illustrated by the following 

 table taken from Van Laer's paper : — 



In these experiments equal quantities of the primary 

 yeast E x were added to five samples of the same sterilised 

 wort and to four of these samples (Nos. 2, 3, 4 and 5) were 

 further added traces of the yeasts E 4 , E 5 , E 7 and E s . The 

 worts were fermented at a temperature of 2O C. and the 



