This is the name applied to an enzyme, present in yeast, which has the property of fermenting pyruvic acid.3 The latter substance is an intermediate compound formed during the alcoholic fermentation of sugar.

The proteolytic enzymes of malt are of considerable

importance, inasmuch as they break down the complex proteins of the grain into simpler products, namely, albumoses, peptones, and amides, thus rendering them soluble and diffusible, and making them available as nutrients of the yeast during the subsequent fermentation process. There appear to be two of these enzymes in malt, one a peptase, which converts the albumins into albumoses; the other a tryptase, which effects a more complete transformation into amides and amino-bodies such as asparagine. The former acts rapidly, and has its optimum temperature at 51°; the tryptase action is slower, and the most favourable temperature is a little lower - namely, between 45° and 50°. Both act best in a slightly acid medium; alkalis retard their action.

1 "Alcoholic Fermentation," 1911, p. 49.

2 Harden and Norris, Biochem. J., 1914, 8, 100.

3 C. Neuberg, Biochem. Zeitsch., 1915, 71, l.

Three proteolytic enzymes have also been found in yeast.1 They are analogous to, but differ in some particulars from, proteolytic enzymes of the animal organism. They comprise (i), a yeast pepsin, which can degrade proteins to peptones, but not further; (ii), a yeast tryptase, which does not act on the proteins of yeast itself, but can degrade certain proteins, such as acid albumins, gelatin, and caseinogen, into peptides and amino-acids; and (iii), a yeast ereptase, which readily degrades peptones and polypeptides into amino-acids.

The only other sucroclastic enzyme that need be mentioned here is melibiase, which is contained in " bottom ' yeasts, but not in yeasts used for " top ' fermentation. It inverts the sugar melibiose, itself a product of the partial hydrolysis of raffinose, found in sugar-beets and also in barley. Hence the bottom yeasts can ferment raffinose completely, whereas the top fermentation yeasts cannot do so.

"Phosphatese." - Before leaving the subject of enzyme action we may note that in some cases this action has been shown to be reversible, effecting a synthesis or condensation instead of a hydrolytic decomposition. Thus by acting upon dextrose with maltase, Croft Hill2 obtained evidence of the production of maltose (or isomaltose; Emmerling3). Lipase, which decomposes fats (esters of glycerol), can synthesise the ester ethyl butyrate from ethyl alcohol and butyric acid; and other instances of reverse action are known.

Of these may be mentioned that of the enzyme "phosphatese," which can be separated from the zymase complex. It synthesises the hexosephosphate of yeast.4

1 K. G. Dernby, Biochem. Zeitsch., 1917, 81, 109.

2 Trans. Chem. Soc, 1898, 73, 634. 3 Ber., 1901, 34, 600.

4 Euler, Ohlsen, & Johanssohn, Biochem. Zeitsch., 1917, 84, 402.

Malt.

Malt is grain, usually barley, in which germination has been artificially induced, allowed to proceed as far as may be desired, and then stopped by a moderate drying of the grain at the ordinary temperature (" green malt "), followed in general by a slow application of heat (kilned malt). Maize, oats, rye, and wheat are also malted, but are generally used in conjunction with more or less barley malt.

The effect of malting is to develop the enzymes of the barley grain; to modify the texture of the grain by converting the hard, tough corn into a friable, mealy one; and in the case of kilned malt, to produce a pleasant aromatic or biscuity flavour together with a certain amount of amber or brown colour. To the brewer of beer the flavour and colour of the malt are of importance, but they are of no particular consequence to the distiller of alcohol.

When barley planted in the earth germinates naturally, the process is started by absorption of moisture from the soil. In a similar manner, the maltster sets up artificial germination of the grain by moistening it and exposing it to suitable conditions of temperature and aeration. We are here concerned chiefly with the biological changes which take place in the grain during this germination - that is, during the growth of the embryo or rudimentary plantlet contained in the barley corn. Before discussing these changes, however, it is desirable to give a short account of the manufacturing operations through which the barley passes in the course of being converted into malt.