SYSTEMATICALLY SELECTED YEASTS. 29 



a flask, we must in the first place secure the preservation of 

 this for future use. As a precaution, it is not sufficient to 

 keep merely a single sample, but a reserve flask should also 

 be kept, so that we may be certain that we always have a 

 living and a pure yeast. Most of the Saccharomycetes can 

 be preserved in flasks containing wort for a whole year, 

 or even longer, if kept at the ordinary room-temperature, and 

 protected from direct sunlight. In some cases, however, I 

 have found that they have perished in less than a year, and 

 when exposed even for a short time only to direct sunlight, 

 and therefore also to a high temperature, they perish much 

 sooner. It is necessary, therefore, to preserve these cultures 

 in a dark cupboard, or, at any rate, to protect them from the 

 direct rays of the sun ; this also applies when other liquids 

 have been employed for the cultivation. From some investi- 

 gations of Duclaux on the duration of the life of yeast cells 

 in beer wort, it might be assumed that this liquid leaves 

 nothing to be desired in this respect. As we have seen, 

 however, this is not fully supported by my experiments, 

 The best liquid for the preservation of yeast which I know 

 of, when it is a question of preserving it for several years, is 

 not wort, but an aqueous solution of cane sugar ; and for this 

 purpose I generally employ a 10 per cent, solution. I have 

 flasks containing this solution into which brewery yeasts and 

 wine yeasts were introduced fourteen years ago, and all the 

 species contained in them are still living. Many yeasts which 

 lived in the sugar solution for over ten years, were found to 

 perish in beer wort in from one to three years. 



The method which I have described for preserving yeast 

 in sterilised filter paper (see my treatise on ascospore-forma- 

 tion in ' Compte rendu des travaux du laborat. de Carlsberg,' 

 1883, p. 29) is very useful for sending small samples of yeast, 

 and is, in fact, generally made use of for sending samples from 

 breweries to the laboratories where the pure cultivation of 

 brewery yeasts is undertaken. As a result of my experiments 



