282 MICRO-ORGANISMS AND FERMENTATION. 



under the usual conditions, and conversely, it was possible to 

 select a yeast colony derived from a cell containing spores y 

 and by further cultivating the colony to obtain a growth which 

 possessed the power of freely generating spores. By such 

 systematic choice the species was divided into three forms- 

 one distinguished by its vigorous spore-formation, another 

 by the fact that this power had almost disappeared, 

 and a third, which could not form spores. By frequent 

 infections in wort the third form reverted to the power of 

 forming spores. This took place slowly, but when Hansen 

 transferred it to a 10 per cent, dextrose solution with yeast 

 decoction this property was instantly restored. 



In other species, varieties which have lost their power of 

 spore-formation completely, or in part, may make their appear- 

 ance, without any known cause, both in liquid and on solid 

 nutrient media. In some cases (e.g., S. Ludivigii) that power 

 is restored if dextrose is added to the nutrient liquid. Similar 

 observations regarding asporogenesis have been recently made 

 by Beijerinck on 8. octosporus. 



If a pure culture of brewery yeast is developed in a wort 

 which has not been aerated after sterilisation, it generally 

 loses its normal " breaking " and clarifying properties, under 

 brewery conditions, and this to a degree dependent on the 

 species. These new variations must often be cultivated 

 through a great many generations in ordinary brewery wort 

 before regaining the original qualities of the species. As- 

 aeration brings about changes in the chemical composition 

 of the wort, it is evident that the effect on the protoplasm is- 

 due to such circumstances. 



The author of this book showed in 1890 that when a. 

 brewery top-fermentation yeast which has given a good clari- 

 fication in practice is kept for some time in wort-gelatine at 

 room temperatures, it tends to lose its clarifying properties for 

 a considerable time. At the same time, it brings about a con- 

 siderably stronger attenuation than in its original condition. 



As an additional instance of the effect of the chemical 

 composition of wort in producing new varieties, we may" 

 mention the observation, due to Hansen, that S. Pastor i- 

 anus /., which imparts an unpleasant taste and smell to beer- 



