DISEASES IN BEER. 257 



Hansen's epoch-making researches on disease yeasts proved 

 that amongst the wild yeasts there are groups which bring 

 about detrimental changes in beer, whilst others proved to 

 be harmless. Amongst the former there are some which 

 impart a bitter taste and disagreeable odour to beer (Sacch. 

 Pastorianus I.), usually without producing turbidity, whilst 

 others (Sacch. Pastorianus III. and Sacch. ellipsoideus II.) 

 only fully develop their activity at a late stage of the secondary 

 fermentation, and then make the beer turbid. This effect is 

 due to the abundant yeast deposit formed a comparatively 

 short time after the finished beer has been drawn off, which 

 rises at the slightest movement of the liquid. These disease 

 yeasts cannot produce turbidity if they only come in contact 

 with beer at the end of the principal fermentation. It is 

 possible, however, if the beer comes in contact with the two 

 species after storing, that an infection with young cells of S. 

 ellips. II. may produce turbidity. The disease yeasts which 

 influence the odour and flavour of beer are only of importance 

 when they occur at the beginning of the principal fermentation. 

 The chief danger lies in the pitching yeast. Weakly fermented 

 beer is much more liable to attack than other beer. Hansen's 

 observations on the disease yeasts have been confirmed and 

 extended by Gronlund, Will, and others. Becker made the 

 interesting observation that certain wild yeasts, which impart 

 .a bitter taste to beer, are capable of influencing the attenuation 

 when mixed with culture yeasts. The fermentation is increased 

 and in certain cases extends over a longer period than with 

 the pure culture yeast. Wild yeasts can also bring about 

 disturbing effects in top-fermentation breweries. For instance, 

 according to de Bavay, the " summer-cloud " of Australian 

 beer is caused by a Saccharomycetes, which causes turbidity, 

 and imparts a bitter and slightly acid taste. In English high- 

 fermentation beers the author found yeasts of the Saccharo- 

 myces anomalus type which produced turbidity ; in weakly- 

 fermented, Danish high-fermentation beers, Torula species 

 having similar properties occur. Similarly van Hest found 

 species of Torula producing turbidity in top-fermentation 

 Dutch beers. Chapman found that Sacch. Past. I. occurs in 

 English beer, and gives the well-known bitter taste known as 



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