136 MORPHOLOGY AND LIFE-HISTOEY OF YEASTS 



i5°C and after forty hours at 25° C. Those of the second group 

 Avhilst remaining sporeless after forty hours at 2C^ C exhibit 

 .pores after seventy-two hours at 15" C. Finally, the members 



those last mentioned. On the other hand, the wild veasts 

 exhibit spores under both sets of conditions (15° C and 2V C ) 

 withm the time limits specified (seventy-two and forty hours 

 respectively), or even much earlier. Consequently the desired 



^T^T^^^^l^t^'"''''''^ ^'"" ""' ^^'^'^^^^ ^--"^^ 



According to Hansen's observations-which we shall have to 

 deal with in the paragraphs treating of mixed sowings— the wild 

 yeasts that have crept into the pitching yeast or the wort first 

 make their appearance in large quantities towards the end of 

 primary fermentation in the upper portions of the contents of 

 the fermenting vessel Consequently at this period a sample 

 should bedmwn from this part in a glass ; the suspended yeast cells 

 must be left to subside and then immediately transferred to the 

 prepared gypsum blocks, which are maintained at the tempera- 

 tures of 15 C. and 25° C. respectively. At the end of forty and 

 seventy-two hours respectively a sample is taken of each and 

 should spores be detected in either or both series, then the 

 presence of wild yeasts is demonstrated. When the brewerv is 

 unprovided with a suitable laboratory and an expert, a sample 

 ot the sedimental yeast must be sent to a laboratory, a drop of 

 the yeast being dried on blotting paper in the manner described 

 in a later paragraph dealing with this matter. Owing to the 

 abundant sporulation of the wild yeasts, this method is very 

 decisive, Holm and Poulsen having by this means succeeded in 

 detecting the presence of as little as about 1 per cent, of added 

 wild yeast (one or other of the above-named three pathogenic 

 yeasts) in twenty different species of l)ottom-fermentation^beer 

 yeasts For practical use this is sufficiently delicate. On the 

 other hand, to be of any value for testing yeast in a pure-culture 

 apparatus, a method must be absolutely reliable and capable of 

 detecting and isolating even the slightest trace of infection. 

 With this object the yeast from the apparatus is subiected to a 

 preliminary treatment, by means of which the amount of any 

 wild yeast present in the sample is increased. This matter will 

 be further described in a subsequent paragraph treating of the 

 influence of organic acids on yeasts. 



Differences also exist between the cultivated bottom-fer- 

 mentation beer yeasts and the wild yeasts, in connection with 

 the structure of the ascospores. Those of the latter are relatively 

 smaller, and their contents are homogeneous and of high lustre 

 whereas the ascospores of the culture yeasts mostly exhibit 

 vacuoles and granulation, and the membrane is clearly dis- 

 cernible. '' 



