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 the vitality 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, yeastt-cells may be killed, without destroying the activity of the zymas by treatment with dilute solutions (0.1 to 0.5 per cent.) of sulphuric acid, or of various other agents, including formaldehyde, sodium or ammonium fluoride, ferrous sulphate ammonium oxalate, potassium chlorate, ether, and chloroform.

Zeitsch. angew. Chem., 1916, 29, Ref. 319.

Continued action of the toxic solution, however, ultimately affects the zymase also. By washing out the solutions with water, and drying the yeast, permanent yeast preparations may be made, similar to the " acetone yeast." The process is much cheaper than the acetone and the ether-alcohol methods. With the fluorides the results are not satisfactory. Rapid treatment is not necessary. For instance, using sulphuric acid, it is advisable to leave the yeast in contact with a 01 per cent, solution for several hours, in order to make sure of killing all the cells.1

Zymin can be deprived of its fermenting power by washing with water, but addition of boiled yeast juice restores this power. Harden2 has shown that a similar restoration of fermenting activity is brought about by adding potassium or ammonium phosphate, together with either potassium pyruvate or acetaldehyde. By itself, the phosphate has no such effect. Thes experiments suggest that possibly acetaldehyde may be the "co-enzyme" of yeast juice. They also indicate that potassium and ammonium ions have some specific function in fermentation which is not possessed by the sodium ion, since sodium phosphate gave negative results with acetaldehyde.