HANSEN'S. METHOD OF SINGLE-CELL CULTURE. 223 



cultures to fresh medium at frequent intervals. If this be neg- 

 lected, and the cells are left for a long time in the spent liquid, 

 they will be injured by the products of metabolism and fermenta- 

 tion. The length of the intervals within which this re-inoculation 

 must be performed in order to prevent the danger aforesaid, 

 depends on the species of yeast as well as on the constitution of 

 the nutrient medium. Thus, H. MULLER-THURGAU (III.) states 

 that he was able to leave certain wine yeasts in their respective 

 wines for ten months without appreciable injury. On the other 

 hand, the author found that a spirit yeast, Race II., suffered a tran- 

 sitory, but decided, weakening (especially in reproductive power) 

 by remaining only three months in the fermented wort. The live 

 herbarium in the laboratory of a fermentation physiologist, that 

 is to say, the collection of cultures of micro-organisms, usually 

 contains a very large number of yeasts. These include certain 

 kinds especially yeasts for the production of wines and berry 

 wines, then the so-called wild yeast, and others for whose re- 

 inoculation there is little or no outside demand. Nevertheless 

 the re-inoculations take a good deal of time and labour when 

 as is desirable per se the collection is a large one numerically. 

 In order to reduce the work to a minimum the yeast should be 

 transferred to the medium in which it is known to keep longest 

 without suffering a change of character. Beer wort is not suitable 

 for the prolonged storage of brewery yeasts ; but, on the other 

 hand, the aqueous 10 per cent, solution of saccharose tried and 

 recommended by E. C. HANSEN (XXXV.) 

 usually behaves well. It is generally kept 

 in Freudenreich flasks or in the Hansen 

 modification shown in Fig. 162, these 

 being filled about half full. To minimise 

 the inevitable evaporation, J. C. HOLM 

 (III.) employs a modified form of cap or 

 hood. The amount of sediment of the 

 culture to be preserved must not be more 

 than imparts a slight turbidity to the 

 liquid, the reproduction of the sowing and 

 the chemical alteration of the liquid being 

 then very slight. The object in view is. 

 not the reproduction, but the preservation 

 of the cells, and it is therefore necessary 

 to prevent as far as possible the distribu- 

 tion of the store of nitrogenous food 

 introduced by the cells and consumed for 

 their own maintenance during storage, or 



a weakened progeny will certainly result. Moreover, the re- 

 inoculations should be kept in the dark and at a temperature not 

 far exceeding 1 5 C. Under these conditions nearly all the species 

 (about 50 in number) examined in this connection by HAXSEN 

 VOL. ii : PT. 2 ? 



FIG. 162. 



Freudenreich-Hanseu 



Flask. About half natural 



size. (After Hansen.) 



