564 A MANUAL OF BACTERIOLOGY 



of beers and yeast i.e. abnormal fermentations giving rise to 

 inferior products are due to admixture of certain " wild yeasts," 

 as they are termed, with the brewer's yeast, chiefly the 8. ellip- 

 soideus and 8. pastorianus ; and, in order to detect these " dis- 

 ease " 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, fermen- 

 tation 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 fermentation process seems to be due to the attachment of 

 minute bubbles of carbonic acid gas to the cells, and it has not 

 yet been possible to convert the one form into the other. 



Characters of some of the more important yeasts. Hansen 

 divided the important yeasts into groups having the same general 

 characters, and distinguished the varieties in each by Eoman 

 numerals (I., II., etc.). 



CEREVISI.E GROUP. These are the yeasts producing the normal 

 fermentations resulting in beer, etc. They are round or slightly 

 ovoid cells, and four ascospores are produced. In old cultures 

 long sausage-shaped or even filamentous cells may be met with. 



