CHAPTER L. 



ORGANIC FOODSTUFFS. THE REQUIREMENTS IN 

 RESPECT OF OXYGEN. 



261. Sources of Carbon. 



THE true water content, and therefore also the amount of dry 

 residue (dry matter) in the yeast cell itself, is not yet accurately 

 known. The figures cited in the literature are based on 

 experiments performed, not on the cells only, but on samples of 

 sedimental yeast from the brewery, or pressed yeast, neither of 

 which, as we have already seen (vol ii. pp. 119, 175, 176), is of 

 uniform nature, but includes a variety of organic and inorganic 

 admixture?. The amount of dry residue in pressed yeast is 

 determined, of course, by the amount of pressure employed. R. 

 KUSSEROW (III ), in testing eight different samples, free from 

 starch, found 22.1 per cent, as the minimum, 29.9 per cent, as the 

 maximum, and 25.6 per cent, as the average water content. In 

 practice, 26 per cent, is usually estimated. 



The sp. gr. of the cells of pressed yeast was determined as i.i 

 by P. GUICHARD (I.) in 1894, though the method, suspension of 

 the cells in a mixture of alcohol and chloroform, was not per- 

 fectly reliable, and probably gave results in excess of the truth. 

 On the other hand, the pycnometric method adopted by Kusserow 

 for determining the sp. gr. of his eight samples of pressed yeast 

 gives values that are probably too low. He weighs out exactly 

 10 grms. of the yeast, triturates them with a little distilled water 

 in a porcelain basin, and swills the mixture into a pycnometer, 

 which is then filled up to the mark with distilled water and 

 weighed. Taking as the weight of the whole and P the weight 

 of the pycnometer filled with water alone, the difference O-P 

 being equal to a, the sp. gr. of the sample of pressed yeast works 

 out to S = 10 : 10 - a. Kusserow determined the maximum value 

 as 1.1093 an( ^ * ne minimum as 1.0821. 



The sp. gr. of the dry residue of these samples ranged from 

 1.580 and 1.491, with 1.509 as the mean value. By assuming 

 (which is not strictly accurate) that the volume of the yeast 

 sample is equal to that of the percentage, by weight (T), of the 

 dry residue, plus that of the water content (W), and taking the 

 above mean into consideration, we obtain the equation 



203 



