A further difference consists in the fact that with certain yeasts the rate of fermentation of glucose is somewhat increased by monosodium phosphate whilst that of mannose is unaffected [Euler and Lundeqvist, [1911]].
Mixtures of glucose and fructose are fermented by yeast at the [p132] same rate as either the glucose or the fructose contained in the mixture would be alone. When, however, mannose and glucose are fermented simultaneously interference between the reactions takes place, and this is especially evident when the yeast has comparatively little action on mannose. The following are the results obtained by Slator:—
| Yeast. | Relative Rates. | ||
|---|---|---|---|
| 2·5 per cent. Glucose. | 2·5 per cent. Mannose. | 2·5 per cent. Glucose + 2·5 per cent. Mannose. | |
| S. Thermantitonum | 100 | 105 | 92 |
| Brewery yeast, 53 per cent. activity destroyed by heat | 100 | 21 | 33 |
| Brewery yeast, 60 per cent. activity destroyed by heat | 100 | 12 | 42 |
The case of galactose merits special attention. Previous investigations [see Lippmann, [1904], p. 734] have shown that the fermentation of galactose by yeast differs greatly from that of the other hexoses. The subject has been re-investigated by E. F. Armstrong [[1905]], and by Slator [[1908, 1]]. Armstrong carried out his experiments in the manner already described (p. [131]), and found that some yeasts had, and others had not, the power of fermenting galactose, although all were capable of fermenting glucose, fructose, and mannose.
Slator made quantitative experiments on the same subject. He was able to confirm the statement which had previously been made, that certain yeasts which have the property of fermenting galactose possess it only after the yeast has become acclimatised by culture in presence of the sugar. This was shown for brewery yeast and for the species mentioned below. This phenomenon is one of great interest and is strictly analogous to the adaptation of bacteria which has now been quite conclusively established [Neisser, [1906]].
| Yeast. | Mode of Culture. Grown in: | Relative Rates. | |
|---|---|---|---|
| Glucose. | Galactose. | ||
| S. Carlsbergensis | wort | 100 | <1 |
| " | hydrolysed lactose | 100 | 86, 83, 85, 25, 46, 51, 69, 54, 155 |
| S. Cerevisiæ | wort | 100 | <1 |
| " | hydrolysed lactose | 100 | 21, 26, 29 |
| S. Thermantitonum. | wort | 100 | <1 |
| " | hydrolysed lactose | 100 | 77, 53, 35 |
| S. Ludwigii | wort | 100 | <1 |
| " | hydrolysed lactose | 100 | <1 |
It will be seen that in one case the rate of fermentation of galactose was considerably greater than that of glucose. S. Ludwigii did not respond to the cultivation in hydrolysed lactose, but, as Slator points out, it is quite possible that repeated cultivation in this medium might effect the change, and this would be strictly analogous to the results obtained with bacteria. Slator's results have been confirmed by Harden and Norris, R. V. [[1910]], and by Euler and Johansson [[1912, 2]] who have made an exceedingly interesting study of the progress of the adaptation. As in the case of mannose the rates of fermentation of glucose and galactose are differently affected by agents such as heat and alcohol; moreover, the rate of fermentation of mixtures of dextrose and galactose is in no case either the sum or the mean of the rates obtained with the separate sugars. The temperature coefficient of the fermentation of galactose also differs slightly from that of the other hexoses.
| Yeast. | Relative Rates. | ||
|---|---|---|---|
| Glucose. | Galactose. | Glucose + Galactose. | |
| S. Cerevisiæ | 100 | 0 | 103 |
| " | 100 | 34 | 103 |
| S. Carlsbergensis | 100 | 155 | 119 |
| S. Thermantitonum | 100 | 76 | 124 |
Assuming that his conclusion that all yeasts which ferment glucose also ferment fructose and mannose is correct, Armstrong has drawn attention to the fact that these three hexoses are also related by the possession of a common enolic form (p. [97]) and has suggested that this enolic form is the substance actually fermented to carbon dioxide and alcohol [[1904]].