288 MICRO-ORGANISMS AND FERMENTATION. 



It does not indicate what the true value of the yeast may be> 

 for this branch of the industry. Thus occasionally such a yeast 

 mass that has altered its character has produced excellent 

 results when applied in other places where the requirements 

 are different. A selection of a cell from the yeast mass that 

 has not degenerated has often proved the basis for regeneration, 

 in that the new culture possesses the properties of the original 

 stock. 



A very cautious treatment of a sample of purely culti- 

 vated yeast will throw some light upon this question. If 

 a number of cells are separated from a yeast mass derived 

 from a single cell, which has been in use in the industry for 

 some time, the pure cultures from these cells will show differ- 

 ences in a set of parallel fermentations, and sometimes im- 

 portant differences in respect to taste, smell, and other char- 

 acteristics of the fermented liquid ; also as regards the attenu- 

 ation, the character of the yeast layer, etc. Varieties may, 

 for instance, occur which produce a penetrating and unpleasant 

 bitter flavour, but in every other respect give a result in 

 agreement with the culture yeast. Thus it is interesting 

 to record a case where a selected variety gave considerably 

 more rapid clearing than the original race, whilst in every 

 other respect, practical and biological, it was identical. In 

 other cases the power of attenuation varied greatly. By 

 studying a number of selected growths a series of inter- 

 mediate forms could be detected, and by a proper selection 

 cultures were prepared which gave the normal attenuation, 

 in wort of the same character. 



A problem of great practical and theoretical importance is to 

 decide if such variations occurring in the yeast mass in practice 

 are constant or of a purely transitory nature. Hansen adopted 

 the view that, as a rule, " the races prepared from industrial 

 yeast cannot be maintained, but disappear," and that " so 

 long as the beer yeasts are kept under brewery conditions, 

 they only display slight alterations, which are of a transitory 

 character." This view, however, is in contradiction to the 

 results repeatedly obtained in the author's laboratory. 

 Strongly marked abnormalities may occur in practice in the 

 work of single cells, and certain of these variations prove to 



