PURE YEAST AND BREWING OPERATIONS. 299 



feeble coefficient of reproduction. This, however, requires 

 further confirmation, for according to Jorgensen, 1 Van 

 Laer's composite yeasts do not maintain the constancy of 

 composition claimed for them. In connection with this it 

 is necessary to prove not merely that the different varieties 

 of primary yeasts maintain their relative proportions during 

 a series of fermentations but that the traces of secondary 

 yeasts which Van Laer maintains to be necessary likewise 

 remain constant. 



It would appear, for instance, highly probable that the 

 latter types being present only in very minute quantity 

 might readily become crowded out during the vigorous 

 primary fermentation in which they are acknowledged to 

 play but an insignificant part, and that the wild yeasts pre- 

 sent in the atmosphere will quickly take their place. If this 

 should prove to be the case, it is evident that the character 

 of the secondary fermentation will be largely a matter of 

 chance and will depend upon the number and nature of the 

 wild yeasts present in the atmosphere. Until Van Laer is 

 able to bring forward some experimental evidence that this 

 is not the case, his statement that his composite yeasts 

 maintain their uniformity in composition will certainly be 

 regarded with scepticism. 



Finally, a question of considerable importance has 

 reference to "the immunity as regards wild yeasts," which 

 Van Laer claims for " beers fermented with certain sym- 

 biotic mixtures of Saccharomyces ". He describes a com- 

 parative experiment in which wort was fermented in one 

 case with an ordinary composite yeast, and in the other 

 with a single race yeast, whilst the surrounding atmosphere 

 contained " legions of wild ferments ". The beer obtained 

 with the ordinary yeast behaved in a normal manner, whilst 

 that fermented with the single race yeast remained fiat for 

 about a fortnight and then "commenced to re-ferment with 

 intense energy". In this experiment, the single race yeast 

 employed was a variety which confessedly gave no after- 

 fermentation and in consequence left the beer flat and in a 



1 Trans. Inst. Braving, vol. vii., p. 95. 



