466 A MANUAL OF BACTERIOLOGY 



by cross-lines etched on the glass and numbered. The preparation 

 is carefully examined with a J or J inch objective, and the positions 

 of single isolated cells are noted. This is not a difficult matter on 

 account of the comparatively large size of the yeast-cells, and their 

 position is determined by the numbered squares on the cover-glass. 

 The preparations are kept in a moist chamber in a warm place, 

 and when visible colonies have developed, those which are derived 

 from a single cell can be inoculated into tubes or flasks of a suitable 

 culture medium. 



It is found that the various yeasts form spores in different periods 

 of time when grown under similar conditions, and on this fact is 

 based what is known as the analysis of yeast a most valuable 

 method, which we owe to Hansen. The chief " diseases " of beers 

 and yeast i.e. abnormal fermentations giving rise to inferior pro- 

 ducts are due to admixture of certain " wild yeasts," as they are 

 termed, with the brewer's yeast, chiefly the S. ellipsoideus and 

 S. pastor ianus ; and, in order to detect these " disease " species, 

 the analysis consists in determining at what time ascospores appear. 

 The mode of procedure is as follows : 



The yeast is sown in a flask of sterile wort, and incubated at 

 25 C. for twenty -four hours. The yeast revives, and from the 

 deposit of young cells two cultures are made on plaster-of-Paris 

 blocks. These cultures are kept, one at 25 C., the other at 15 C., 

 and are examined twice daily. In an uncontaminated brewing 

 yeast ascospores should not be detected in less than thirty hours 

 in the culture kept at 25 C., and seventy-two hours in that kept at 

 15 C. The plaster-of-Paris blocks are sterilised by careful flaming 

 in the Bunsen, and are then placed in sterile glass capsules with 

 lids, containing sufficient sterilised water thoroughly to moisten the 

 whole of the blocks ; unless this is done no growth occurs. By 

 this method of analysis as little " wild yeast " as one two-hundredth 

 of the whole can be detected. 



Besides the distinct species of yeasts, there are also a number of 

 varieties employed in brewing, etc., differing but slightly in 

 morphological and cultural characters, yet giving rise to varied 

 products. These varieties may be divided into two groups the 

 surface, high or top, and the sedimentary, low or bottom, fermenta- 

 tion forms. In this country beer is brewed by fermenting an 

 infusion of malt (" wort ") with yeast, which, during fermentation, 

 rises to the surface, and belongs to the first group ; while the German 

 beers are obtained by yeast, which sinks to the 'bottom, and belongs 

 to the second group. The floating of the yeast in the high fermenta- 

 tion process seems to be due to the attachment of minute bubbles of 



