BIOLOGICAL RELATIONSHIPS OF YEAST. 277 



brings about a fermentation. The alcohol so formed sup- 

 presses most of the moulds. The true wine yeasts now 

 gradually begin to develop, and simultaneously the develop- 

 ment of wild yeasts, of bacteria, and of the Dematium species, 

 ceases. As soon as the alcohol content rises above 4 per cent., 

 as a result of the activity of the true yeasts, S. apiculatus is 

 suppressed, and the wine yeasts immediately take command of 

 the field to such an extent that, in an ordinary microscopical 

 examination, nothing but their cells can be observed. The 

 most powerful alcohol - formers amongst the yeasts again 

 gradually supersede the weaker species. 



Temperature plays a great part in the life of yeast cells, 

 and Hansen has made use of this relationship as one of the 

 most important means for characterising the species. 



In 1883 he proved that both the spores and vegetative cells 

 of different species possess different powers of resistance to 

 heating in water. In this respect the spores are more resistant 

 than the vegetative cells. 



In such determinations the condition of the cells has a 

 marked influence, and the result depends largely upon their 

 age. Thus the two-day-old cells of S. ellipsoideus II. grown 

 in wort at 27 C. were killed on warming to 56 C. for five 

 minutes in sterilised distilled water, whilst cells similarly 

 prepared, but two and a half months old, were heated to 60 

 C. for five minutes without being destroyed. 



Ripe spores of this species, developed at 17 -18 C., 

 and partially dried for eight days at the same tempera- 

 ture, withstood heating for five minutes at 62 C., but not 

 at 66 C. 



The vegetative cells of S. cerevisice I. were killed by five 

 minutes' heating at 54 C., and the spores at 62 C. 



An interesting classification of Hansen's six species in 

 relation to any given temperature, is obtained by cultivating 

 them in wort under conditions favouring the formation of 

 films. Thus, if the development is carried out at 36-38 

 C., the three Pastorianus species are killed in eleven days, 

 whilst S. cerevisice I. and the two ellipsoid species remain alive. 

 From this and similar experiments, it may be argued that the 

 rule formerly accepted that top-fermentation yeasts can 



