STUDIES ON FERMENTATION. 225 



or years, will never contain in its deposit anything besides 

 the yeast that was used in its manufacture, provided that 

 that was pure to begin with. Now this can never be the 

 case in dealing with actual commercial beers, no matter what 

 they may be or in what brewery they may be produced. 

 In all beers, in the course of time, in addition to diseased 

 ferments, ferments essentially different from those used in their 

 manufacture wall appear, and notably sacckaromi/ces pastorianus ; 

 this result must be attributed to the general impurity of 

 commercial yeasts. 



In certain cases the intermixture of ferments is to be feared 

 almost as much as the presence of diseased ferments, when 

 these latter have not developed to any great extent. We have 

 often seen our fermentations invaded by ferments differing 

 absolutely from those w^hich we originally employed. The 

 repetition of growths, and more jDarticularly changes in the 

 composition of our fermentable media, purposely made with 

 the view of attaining certain results, often produce complica- 

 tions of this kind. For a long time we were unable to realize 

 the true significance of the results of some of our experiments, 

 in consequence of the facts which we have just explained, as 

 well as those detailed in the preceding paragraph, having 

 escaped our notice ; indeed, our ignorance of those facts added 

 greatly to the difficulty and length of our researches. Our 

 labours from the commencement of this work to the date of 

 its publication have extended over not less than five years, 

 and no one can know better than ourselves wdth what advan- 

 tage we might devote a still longer time to it ; but, as Lavoisier 

 says, one would never give anything to the world if he delayed 

 doing so until he fully attained unto his ideal aims, which 

 always seem more distant the more one increases one's efforts 

 in the attempt. 



Our preceding observations show how extremely important it is 

 to employ pure yeasts to obtain, on the one hand, well flavoured 

 beers, whilst adhering to the processes at present existing 

 in breweries, and on the other, beers of good keeping qualities, 



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