Euler and Bäckström [[1912]], however, have made the important observation that sodium hexosephosphate causes a considerable acceleration although it is itself neither fermented nor hydrolysed under these conditions. The extent of this is evident from the following numbers:—
| 20 c.c. of 20 per cent. glucose solution. 0·25 g. yeast [Yeast H of St. Erik's brewery]. | |||
|---|---|---|---|
| Without addition. | + 0·5 g. Na hexosephosphate. | ||
| Time. Min. | CO2. | Time. Min. | CO2. |
| 46 | 10·5 | 37 | 8 |
| 76 | 17·5 | 73 | 19 |
| 197 | 45 | 188 | 52·5 |
| 347 | 74·5 | 321 | 123 |
| 488 | 95 | 450 | 193·5 |
The observation has been confirmed with English top yeast (Harden and Young, unpublished experiments), but no explanation of the phenomenon is at present forthcoming.
Euler has also found [Euler and Cassel, [1913]; Euler and Berggren, [1912]] that yeast extract, sodium nucleinate and ammonium formate also increase the rate of fermentation of glucose by yeast, but these results have been criticised by Harden and Young [[1913]] on the ground that the possibility of growth of the yeast during the experiment has not been excluded.
Fermentation of Different Sugars by Yeast.
Many valuable ideas as to the nature of fermentation have been obtained by a consideration of the phenomena presented by the action of yeast on the different hexoses. Of these only glucose, fructose, mannose, and galactose are susceptible of alcoholic fermentation by yeast, the stereoisomeric hexoses prepared in the laboratory being unfermentable, as are also the pentoses, tetroses, and the alcohols corresponding to all the sugars. The yeast-cell is therefore much more limited in its power of producing fermentation than such an organism as, for example, Bacillus coli communis, which attacks substances as [p131] diverse as arabinose, glucose, glycerol and mannitol, and yields with all of them products of the same chemical character, although in varying proportions.
A careful examination of a number of different genera and species of the Saccharomycetaceæ and allied organisms by E. F. Armstrong [[1905]] has shown that all yeasts which ferment glucose also ferment fructose and mannose. Armstrong grew his yeasts in a nutrient solution containing the sugar to be investigated, and his experiments are open to the criticism that the organisms were hereby afforded an opportunity for becoming acclimatised to the sugar. His results, therefore, only demonstrate the fact that the organisms in question when cultivated in presence of the sugars examined brought about their fermentation, and do not exclude the possibility that the same organism when grown in presence of a different sugar might not be capable of fermenting the one to which it had in the other type of experiment become acclimatised.
This has actually been shown to be the case for galactose by Slator [[1908, 1]], and it is possible that this circumstance explains the negative results obtained by Lindner [[1905]] with S. exiguus and Schizosaccharomyces Pombe upon mannose, a sugar which, according to Armstrong, is fermented by both these organisms.
The same problem has been attacked quantitatively by Slator, who has shown that living yeast of various species and genera ferments glucose and fructose at approximately the same rate. Moreover, when the yeast is acted upon by various inhibiting agents, such as heat, iodine, alcohol, or alkalis, the crippled yeast also ferments glucose and fructose at the same rate.
With mannose the relations are somewhat different. The relative rate of fermentation of mannose and glucose by yeast is dependent on the variety of the yeast and the treatment which it has received. Fresh samples of yeast ferment mannose more quickly than glucose, but by older samples the glucose is the more rapidly decomposed. This is especially the case with yeast, the activity of which has been partly destroyed by heat, the relative fermenting power to mannose being sometimes reduced by this treatment from 120 per cent. of that of glucose to only 12 per cent. (Slator).