Of the higher sugars raffinose is fermented by juice from bottom yeast, but more slowly than cane sugar or maltose. No experiments seem to have been made with juice from top yeast. [p033]
As regards the fermentation of the higher carbohydrates, very little experimental work has been carried out. Buchner and Rapp found that the fermentation of starch paste was doubtful, but that soluble starch and commercial dextrin were fermented with some freedom. No special study has been made of the diastatic enzymes which bring about the hydrolysis of these substances.
The fermentation of glycogen by yeast-juice is of considerable interest, since it is known that the characteristic reserve carbohydrate of the yeast cell is glycogen [see Harden and Young, [1902], where the literature is cited], and moreover that in living yeast the intracellular fermentation of glycogen proceeds readily, whereas glycogen added to a solution in which yeast is suspended is not affected. Yeast-juice contains a diastatic enzyme which hydrolyses glycogen to a reducing and fermentable sugar, so that in a juice poor in zymase to which glycogen has been added, the amount of sugar is found to increase, the hydrolysis of the glycogen proceeding more quickly than the fermentation of the resulting sugar [Harden and Young, [1904]], but the course of this enzymic hydrolysis of glycogen by yeast-juice has not yet been studied. As a rule, it is found both with juices from top and bottom yeast that the evolution of carbon dioxide from glycogen proceeds less rapidly and reaches a lower total than from an equivalent amount of glucose.
Since nearly all samples of yeast contain glycogen, yeast-juice and also zymin usually contain this substance as well as the products of its hydrolysis. These provide a source of sugar which enters into alcoholic fermentation, so that a slow spontaneous production of carbon dioxide and alcohol proceeds when yeast-juice is preserved without any addition of sugar. The extent of this autofermentation varies considerably, as might be expected, with the nature of the yeast employed or the preparation of the material, but is generally confined within the limits of 0·06 to 0·5 gram of carbon dioxide for 25 c.c. of juice.
In juice from bottom yeast it amounts to about 5 to 10 per cent. of the total fermentation obtainable with glucose [Buchner, [1900, 2]], whereas in juice from top yeasts, which gives a smaller total fermentation with glucose, it may occasionally equal, or even exceed, the glucose fermentation, and frequently amounts to 30 to 50 per cent. of it. It is therefore generally advisable in studying the effect of yeast-juice on any particular substance to ascertain the extent of autofermentation by means of a parallel experiment.
The maceration extract of Lebedeff (p. [24]) is usually, but not invariably [Oppenheimer, [1914, 2]], free from glycogen, which is hydrolysed [p034] and fermented during the processes of drying and macerating, and therefore as a rule shows no appreciable autofermentation.
(e) Effect of Concentration of Sugar on the Total Amount of Fermentation.
The kinetics of fermentation by zymase will be considered later on (p. [120]), but the effect on the total fermentation of different concentrations of sugar, this substance being present throughout in considerable excess, may be advantageously discussed at this stage. The subject has been investigated by Buchner [Buchner, E. and H., and Hahn, [1903], pp. 150–8; Buchner and Rapp, [1897]] using cane sugar, and he has found both for yeast-juice and for dried yeast-juice dissolved in water that (a) the total amount of fermentation increases with the concentration of the sugar; (b) the initial rate of fermentation decreases with the concentration of the sugar. The following are the results of a typical experiment, 20 c.c. of yeast-juice being employed in presence of toluene at 22°:—
| Cane Sugar. | CO2 in grams after | |||
|---|---|---|---|---|
| Weight. | Per cent. | 6 hours. | 24 hours. | 96 hours. |
| 2·2 | 10 | 0·17 | 0·50 | 0·55 |
| 3·52 | 15 | 0·14 | 0·53 | 0·64 |
| 5 | 20 | 0·13 | 0·54 | 0·73 |
| 6·66 | 25 | 0·13 | 0·52 | 0·80 |
| 8·56 | 30 | 0·12 | 0·46 | 0·81 |
| 10·76 | 35 | 0·12 | 0·40 | 0·82 |
| 13·33 | 40 | 0·11 | 0·36 | 0·82 |
The results as to the total fermentations in experiments of this kind are liable to be vitiated by the circumstance that when a low initial concentration of sugar is employed, the supply of sugar may be so greatly exhausted before the close of the experiment as to cause a marked diminution in the rate of fermentation and hence an unduly low total. Even allowing, however, for any effect of this kind, the foregoing table clearly shows the increase in total fermentation and the decrease in initial rate accompanying the increase of sugar concentration from 10 to 40 per cent. Working with a greater range of concentrations (3·3–53·3 grm. per 100 c.c.) Lebedeff has obtained similar results with maceration extract [[1911, 4]], but has found that the total amount fermented diminishes after a certain optimum concentration (about 33·3 grm. per 100 c.c.) is reached.