194 MINERAL FOODSTUFFS. 



The foregoing objections apply equally to the percentage com- 

 position of the two kinds of ash ; and for this reason the per- 

 centage results of the series of ash analyses performed on wine 

 yeast by Braconnot must be omitted. Furthermore, the results 

 of analyses of commercial pressed yeast, by CHAMPION and PELLET, 

 and A. BELOHOUBEK (II.), in the table (vol. ii. p. 193) must not be 

 taken as unconditionally accurate, since this class of yeast contains 

 a considerable amount of (ash-bearing) starch, the proportions 

 being in these two instances 10 and 14 per cent, respectively. 



For the purpose of criticism we will take the first column of 

 figures. The 53.8 parts of phosphoric acid require 107 parts of 

 lime for complete saturation, whereas only about 40 parts of that 

 substance are present, i.e. only sufficient to neutralise 2 1.6 parts 

 of P 2 5 . r -^ ne remaining 28.4 parts cannot be completely combined 

 by the other bases, viz., i.o CaO and 6.0 MgO, so that 21.6- 

 (0.8 + 7.1)= 13.7 parts of P 2 5 are left in a free state. This 

 excess of free phosphoric acid accounts for the acid reaction of 

 yeast ash, or the solution thereof, which reaction was first 

 observed by QUEVENNE (I.). Being able to resist the action of 

 heat, this free phosphoric acid effects the expulsion of sulphuric 

 acid during incineration, and consequently the latter acid will not 

 be found in the ash unless measures have been taken to ensure its 

 combination and protection. Such measures were first adopted by 

 BECHAMP (VI.) in 1871, but were neglected by MITSCHER- 

 LICH (III.) in 1845, and even by J. LIEBIG (II.) in 1870, although 

 it was already known that yeast contains an appreciable quantity 

 of sulphur. In fact Liebig himself found 0.69 per cent, of sulphur 

 in the dry residue, whilst Mitscherlich, Reichenbach, and Demp- 

 wolff gave the figures 0.6, 0.57, and 0.39 per cent, respectively. 

 The same objection must be made with regard to the results of 

 analyses by H. Seyffert, given in subsequent paragraphs. 



In connection with the phosphoric acid, careful procedure is 

 also necessary to prevent its partial reduction to (volatile) phos- 

 phorus during the initial carbonising stage of the incineration 

 process. The possibility of this loss is apparently not precluded 

 even in the Mitscherlich method. With regard to the 14.4 per 

 cent, of silica mentioned in the fourth column, it is uncertain 

 whether this is entirely due to accidental contamination of the 

 sample with sand, &c. (vol. ii. p. 48). It is therefore evident 

 that it is by no means an easy task to secure a perfect analysis of 

 yeast ash ; and it is desirable that the hiatus, resulting from this 

 cause, in our knowledge of yeast and its vital activity, should be 

 filled up as early as possible. 



258. The Importance of Lime. 



We cannot expect to find any reliable particulars regarding 

 the ash requirements of yeast at a period when the authorities were 



