When yeast acts upon the saccharified starch of the wort, it converts the maltose at first into dextrose: -

1 K. Antal, Zeitsch. Spiritusind., 1911, 34, 239, 252.

This conversion is brought about through the agency of the enzyme, maltase, contained in ordinary yeast.

The dextrose is then decomposed by the action of another yeast enzyme, namely, zymase, chiefly into alcohol and carbon dioxide. In the main, the change proceeds according to the following equation: -

This, however, is very far from representing all the products of the complex series of changes included under the name of alcoholic fermentation. Many by-products result, the nature and proportion of which depend upon the conditions - e.g., the character of the wort and yeast. Chief among these by-products as regards quantity are glycerol, fusel oil, and succinic acid; in smaller proportions are other fatty acids (formic, acetic, propionic, butyric, and lactic) with aldehydes and various esters (ethyl acetate, butyrate, caproate, etc.).

Pasteur first observed (1858) the constant occurrence of glycerol in the fermentation of sugar solutions; the production of succinic acid had been noted by Schmidt eleven years previously. The quantity of glycerol found by Pasteur was 3.16 per cent., and of succinic acid 0 67 per cent., of the weight of cane-sugar taken; but, as already indicated, these amounts vary somewhat with the particular conditions. Probably the glycerol is derived from the sugar. The fusel oils and the succinic acid, however, have a different origin. In the fermenting wort, and also in the yeast, there are proteid substances which become hydrolysed to amino-acids; these, in their turn, take up the elements of water, and are decomposed with the elimination of ammonia and carbon dioxide and the production of higher alcohols (Ehrlich): -

These higher alcohols constitute the "fusel oil" (q.v., Chap. X).

The succinic acid is formed in a somewhat similar manner from glutamic acid, but oxidation is also involved. The ammonia eliminated is apparently assimilated by the yeast.

Cane-sugar solutions are not directly fermentable by yeast. The sugar is first hydrolysed to "invert"-sugar, a mixture of dextrose and laevulose, by the enzyme invertase contained in brewers' yeast: -

Both the dextrose and the lævulose are then, like the dextrose obtained from maltose, converted into alcohol and carbon dioxide by the instrumentality of the zymase. The production of invert-sugar, however, is more rapid than its fermentation, so that the invert-sugar accumulates in the solution until the cane-sugar is all inverted.

When a soluble phosphate is added to a mixture containing yeast juice and undergoing alcoholic fermentation, the rate of fermentation is much increased, though the increase is not permanent. There is evidence to show that hexosephosphates, discussed. According to this, the sugar itself is converted into some intermediate compound with three carbon atoms, before being finally transformed into alcohol and carbon dioxide. Lactic acid, methyl glyoxal, polymers of formaldehyde, glyceraldehyde, and dihydroxyacetone are some of the compounds which have been suggested; but there is little positive evidence in favour of the actual occurrence of these compounds as stages of alcoholic fermentation. On the contrary, there is good evidence against the probability of some, at least, of these compounds being the necessary precursors of alcohol in the transposition of sugar. Thus lactic acid, C3H6O3, might conceivably be split up into alcohol and carbon dioxide, as shown in the simple equation: -

C6H10O4(PO4R2)2, are formed during the fermentation of sugars, and they play an important part as intermediate compounds in the production of alcohol.1

This increased rate of fermentation on addition of phosphates does not, however, occur with fermentation by living yeast-cells, as distinct from fermentation by yeast-juice. The difference is perhaps due to the fact that with living yeast the process takes place within the cell, which already contains a sufficient supply of phosphate. Moreover, the cell-membrane may not be sufficiently permeable to phosphate to allow of an additional quantity being utilised.

It seems probable that when the sugar diffuses into the cell a compound of sugar and enzyme is formed, which reacts with the phosphate present to form a hexosephosphate and, simultaneously, alcohol and carbon dioxide. Harden suggests that two molecules of sugar take part in the reaction, according to the following equation: -

2C6H12O6+2PO4HR2=2CO2+2C2H6O+2H1O+C6H10O4(PO4R2)2.

The hexosephosphate is then decomposed, and the liberated phosphate again enters into the reaction with more sugar.

The hexose present in the phosphate appears to be lævulose. Free hexosediphosphoric acid itself is dextrorotatory, ([a]D 15o = +3.55); but when the acid is hydrolysed the rotation changes, and lævulose is produced. According to C. Neuberg and his collaborators,2 neither the free acid nor its soluble or insoluble salts with alkalis or alkaline earths can be fermented by living yeast, even if co-ferment is added, or artificial activators.

Another view of the process of fermentation has been much

1 A. Harden, " Alcoholic Fermentation," Chap. III.

2 Biochem. Zeitsch. 1917, 83, 244.

C3H6O3 = C2H6O + CO2.

As an experimental fact, however, lactic acid is found not to be fermented by living yeast. Hence the lactic acid hypothesis is not accepted, although there is evidence (see below) that a small quantity of this acid is, in fact, produced during fermentation. There is somewhat better reason to think that dihydroxyacetone is actually an intermediate product in alcoholic fermentation; but with our present knowledge no final conclusion can be drawn as to the precise chemical changes which occur.

Fern bach, however, has shown that pyruvic acid is formed in relatively large proportion during alcoholic fermentation, and it is produced at the expense of the sugar. By fermenting glucose in the presence of calcium carbonate, the pyruvic acid was neutralised as it was formed; and the calcium salts separated, which in some experiments amounted to one-fourth the weight of sugar fermented, were mainly composed of calcium pyruvate.1 Some calcium lactate was also produced, the lactic acid being presumably formed by the reduction of pyruvic acid. The latter is readily fermentable by yeast, the products being carbon dioxide and acetaldehyde; and Fernbach suggests that the aldehyde is further converted into alcohol by the reducing action of the yeast. Also a possible source of the succinic acid produced in fermentations is suggested by the fact that pyruvic acid readily condenses to a lactone, which with elimination of CO2 is transformed into methyl succinic acid.

Some further evidence in support of the acetaldehyde-pyruvic acid theory of fermentation has recently been obtained by Neuberg and Reinfurth.2 According to the theory as set forth by these authors, when 1 molecule of sugar is decomposed a part of the molecule forms 1 molecule of aldehyde, the other part acting as "acceptor" for hydrogen. By fermenting sugar in the presence of disodium sulphite to "fix" the aldehyde formed, it has been found possible to isolate aldehyde to the extent of about three-fourths of the amount required by the theory.

1 J. Inst. Brewing, 1016, 22, 354. 2 Biochem. Zeitsch., 1918, 89, 365.

E. F. Armstrong has suggested that the enolic form of the hexose-sugars is the substance actually entering into fermentation.1 This view is based upon the facts (i) that, according to this investigator, all yeasts which ferment dextrose also ferment lævulose and mannose; and (ii) that these three sugars have a common enolic form.