204 ORGANIC FOODSTUFFS. 



W + T = TOO 



T 1.509 ~ is ' 

 and, the sp. gr. of the sample being known, this gives the equation 



-**(.-$ 



whilst, the dry residue being known, the sp. gr. of the yeast 

 sample is found by the equation S = 296.5 : (296.5 - T). 



Contrary to the proposition advanced by Hayduck, however, 

 it is unfortunately impracticable to determine the quantitative 

 addition of starch in a sample of pressed yeast by this method, 

 the difference between the sp. gr. of anhydrous starch (mean 

 1.65) and that of the dry residue of the yeast (1.509) being too 

 small. This method is discarded with greater regret because the 

 existing chemical methods, based essentially on the hydrolysis of 

 the starch and the determination of the resulting sugar, are very 

 unreliable owing to the fact that this treatment saccharines the 

 glycogen of the yeast as well as the added starch, and that the 

 amount of the former (vol. ii. pp. 170, 171) is sometimes very 

 large, occasionally exceeding that of the starch itself. 



The ultimate composition of the organic matter in the dry 

 residue of yeast cells is influenced by the mode of nutrition as 

 well as by the kind and age of the cells, for which reason gene- 

 ralised values are unreliable. Moreover, the available analytical 

 data on this matter have not been obtained by working with actual 

 cells, but from the examination of pitching yeast or pressed yeast. 

 Now the invariable presence of admixtures in these samples, 

 already alluded to in 257 as preventing the acquisition of 

 reliable data on the ash constituents of the yeast cell, has a still 

 greater adverse influence when the determination of the amount 

 of carbon, hydrogen and oxygen in the cells is in question, many 

 of these admixtures being low in or free from ash, and consisting 

 solely of three or four of the elements just mentioned. Conse- 

 quently the results of the ultimate analysis may differ between 

 wide limits, according to the proportion of such impurities present. 

 As instances of this, and not merely to comply with an injudicious 

 demand for quantitative reports on the ultimate composition of 

 yeast, a few results obtained in this connection are reproduced 

 on p. 205. Although on the publication of the first analysis by 

 Marcet the useless character of such figures was pointed out by 

 QUEVENNE (I.) in 1838, similar results have been brought forward 

 from time to time since. In fact, some workers have gone so far 

 as to assume that an expression of the difference between top and 

 bottom yeasts can be found in the results furnished by ultimate 

 analysis a view that is, of course, untenable. 



Carbon compounds may be taken up by yeast for three pur- 

 poses : (i) for alcoholic fermentation and other enzyme actions; 



