It is to these important substances that yeast owes its inverting and fermentative properties. They include sucrase (invertase), zymase, maltase (glucase), lactase, hexosephosphatase, reductase, carboxylase, melibiase, and endo-tryptase, as well as proteolytic enzymes, which have already been mentioned in the general description of the enzymes. But yeasts of different species do not all contain the same enzymes, and hence different yeasts behave differently towards the various sugars. A particular yeast may ferment one sugar but not another. So far as is known at present, only three of the aldo-hexose sugars, namely, dextrose, d-mannose, and d-galactose, are directly fermentable by yeast, and only one keto-hexose, viz., lævulos . According to E. F. Armstrong, all yeasts which ferment dextrose also attack mannose and lævulose. Most yeasts can invert, and then ferment, cane-sugar, because the enzyme sucrase (invertase) is of common occurrence in the yeasts. On the other hand, the enzyme lactase is absent from the majority of yeasts, and hence these are incapable of fermenting milk-sugar. S. fragilis and kephir ferment are exceptions to this: they contain lactase, and can bring about the fermentation of lactose.

The following synopsis shows at a glance how the more common sugars are affected by a number of yeast-species. Fermentation is indicated by the sign +, absence of it by the sign 0: -

A number of observations on the sensitiveness of the chief enzymes of ordinary yeast towards various reagents are recorded by T. Bokorny.1 Thus as regards the invertase, yeast placed in absolute alcohol for several days showed no loss of inverting power when removed. On the other hand, the invertase is destroyed in twenty-four hours by a 1 per cent, solution of caustic soda, though not by a 5 per cent, solution of formaldehyde. Towards acids also it is fairly stable. Fifty per cent, alcohol destroys the zymase activity within twenty-four hours, but 20 per cent, does not. Sulphuric or hydrochloric acid of 1 per cent, strength also destroys the activity within twenty-four hours, as do lactic, acetic, and butyric acids at 5 per cent, strength, though not at 2 per cent. Ammonia solution at a concentration of 0.05 per cent, destroys the zymase activity within forty-eight hours, and formaldehyde at 1 per cent, does so within twenty-four hours. Solutions of neutral salts at concentrations below 10 per cent, are harmless, and in some cases beneficial. As regards maltase activity, 1 per cent, caustic soda solution destroys this in a few hours, but 0 02 and 0.1 per cent, solutions have no appreciable effect in twenty-four hours. Within the same period, 10 per cent. alcohol, 1 per cent, acetic, lactic, or hydrochloric acid, or 0 1 per cent. formaldehyde, all act deleteriously.

"Permanent" yeast. - Zymin or permanent yeast ("acetone yeast".)is a dry preparation obtained by well mixing finely-divided pressed brewers' yeast (500 grams) with acetone (3 litres) for ten minutes to destroy thevitality of the yeast-cells, and then filtering the mass and draining it with the filter-pump. The yeast is then again mixed with acetone (1 litre) for two minutes, filtered, drained, roughly powdered, and well kneaded with ether (250 c.c.) for three minutes, after which it is once more filtered and drained, and then spread on filter paper or porous plates and allowed to dry in the air for an hour. Finally, it is dried for twenty-four hours at 45°.

The product is a nearly white powder in which the yeast-cells are dead, and which therefore cannot grow and reproduce itself. Its cell-walls, however, are intact, and its sugar-fermenting enzyme, zymase, is still active; so that even after keeping for years it will, when ground up and placed in a suitable sugar solution, induce a vigorous alcoholic fermentation.

T. Bokorny has recently described a method of preparing permanent yeast which appears to have some advantages. According to this writer, yerst-cells may be killed, without destroying the activity of the zymanre by treatment with dilute solutions (0.1 to 0.5 per stimulating effect if present in only minute proportion. Thus mercuric chloride at a concentration of more than 1 in 20,000 inhibits fermentative action, but at 1 in 300,000 or less it has a favourable influence. Salicylic acid in the proportion of more than Zeitsch. angew. Chem., 1916, 29, Ref. 319.

0 1 per cent, has a deterrent effect, whereas less than one-sixth of this amount acts as a stimulant. Phenol is inhibitive at a concentration of more than 0'5 per cent., and stimulative at less than 01 per cent. Minute amounts of other toxic substances, such as strychnine, nicotine, and carbon disulphide, have also been found to favour fermentation.

Enzymes Of Yeast

It is to these important substances that yeast owes its inverting and fermentative properties. They include sucrase (invertase), zymase, maltase (glucase), lactase, hexosephosphatase, reductase, carboxylase, melibiase, and endo-tryptase, as well as proteolytic enzymes, which have already been mentioned in the general description of the enzymes. But yeasts of different species do not all contain the same enzymes, and hence different yeasts behave differently towards the various sugars. A particular yeast may ferment one sugar but not another. So far as is known at present, only three of the aldo-hexose sugars, namely, dextrose, d-mannose, and d-galactose, are directly fermentable by yeast, and only one keto-hexose, viz., lsevulos . According to E. F. Armstrong, all yeasts which ferment dextrose also attack mannose and lævulose. Most yeasts can invert, and then ferment, cane-sugar, because the enzyme sucrase (invertase) is of common occurrence in the yeasts. On the other hand, the enzyme lactase is absent from the majority of yeasts, and hence these are incapable of fermenting milk-sugar. S. fragilis and kephir ferment are exceptions to this: they contain lactase, and can bring about the fermentation of lactose.