911.] 



The Permeability of the Yeast-Cell. 



295 



cells and in the liquid surrounding the cells would be necessary. Attempts 

 were therefore made to obtain the yeast-cells minus the liquid which 

 normally fills the interstices in an ordinary cake of yeast. 



This was eventually accomplished by enclosing the moist yeast cake of the 

 brewery, or yeast obtained as residue after centrifuging, in chain cloth and 

 subjecting it to the pressure of a small hand press. A white friable cake 

 of yeast was obtained which appeared to be composed of dry cells. This 

 was proved to be the actual fact by the following series of experiments : — 

 (1) Total solid estimations in the same pressed cake gave uniform results, 

 showing the cake to be homogeneous. (2) Two pressings of the same yeast - 

 paste gave dry pressed cakes with the same total solid content. (3) Samples 

 of brewery yeast were pressed out and subsequently suspended in the 

 expressed wort, centrifuged and again pressed out, the total solids in the two 

 press cakes were exactly equal. (4) Direct estimations of a salt solution left 

 in the interstices of press cake showed that the greatest volume of liquid thus 

 held by 100 grm. of dry pressed yeast was - 5 c.c. 



Method of Experiment. 



The yeast was prepared by pressing out the cake of yeast as received 

 from the press of the brewery, washing being avoided in order to 

 prevent disturbance of the equilibrium of the cell contents. A considerable 

 amount of wort was thus removed. A known weight of this dry yeast was 

 then suspended in a certain volume of the liquid under experiment and 

 allowed to stand for about 20 hours in the cold, after which it was found that 

 osmotic equilibrium between the cells and the solution was attained. The 

 mixture was then centrifuged until the liquid portion was cleared from 

 suspended yeast-cells. The clear fluid was then poured off and the pasty 

 yeast residue was pressed out as described above. 



In order to ascertain the weights of yeast-cells and liquid after the 

 experiment, and the distribution of the solute under examination, the follow- 

 ing determinations were necessary : — Total solid estimations of the initial and 

 final liquid, and of the initial and final pressed yeast, together with estimations 

 of the dissolved substance in the initial and final liquid, and, in all eases where 

 this was possible, in the initial and final yeast. 



The assumption has been made that the total solid matter present in the 

 mixture remains constant during the experiment, an assumption only justified 

 when no loss of carbon dioxide owing to auto-fermentation of the yeast takes 

 place. 



VOL. LXXXIV. — 15. 



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