1. Maceration of Dried Yeast.
A valuable addition to the methods of obtaining an active solution of zymase was made in 1911 by Lebedeff [[1911, 2]; [1912, 2]; see also [1911, 3], [7], and [1912, 1]]. This investigator had been in the habit of grinding dried yeast with water for preparing samples of yeast-juice of uniform character and observed that when the dried yeast was digested with sugar solution and the mixture heated, coagulation [p025] took place throughout the whole liquid, the proteins of the yeast having passed out of the cells. Further examination revealed the interesting fact that dried yeast readily yielded an active extract when macerated in water for some time. The quality of the resulting "maceration extract" depends on a considerable number of factors, the chief of which are: (1) the temperature of drying of the yeast; (2) the temperature of maceration; (3) the duration of maceration; and (4) the nature of the yeast, as well as, of course, the amount of water added in maceration.
In general the yeast should be dried at 25°–30° and then macerated with 3 parts of water for 2 hours at 35°.
The temperature of maceration may as a rule be varied, without detriment to the product provided that the time of maceration is also suitably altered; thus with dried Munich yeast, maceration for 4·5 hours at 25° is about as effective as 2 hours at 35°, whereas treatment for a shorter time at 25° or a longer time at 35° produces in general a less efficacious extract. Yeast dried at a lower temperature than 25° tends to yield an extract poor in co-enzyme (p. [59]) and hence of low fermenting power, this being especially marked at air temperature.
The subsequent treatment of the yeast during maceration may, however, be of great influence in such cases. Thus a yeast dried at 15° gave by maceration at 25° for 4·5 hours a weak extract (yielding with excess of sugar 0·33g. CO2), whereas when macerated at 35° for 2 hours it yielded a normal extract (1·36g. CO2).
The nature of the yeast is of paramount importance. Thus while Munich (bottom) yeast usually gives a good result, a top yeast from a Paris brewery was found to yield extracts containing neither zymase nor its co-enzyme in whatever way the preparation was conducted. The existence of such yeasts is of great interest, and it was probably due to the unfortunate selection of such a yeast for his experiments that Pasteur was unable to prepare active fermenting extracts and therefore failed to anticipate Buchner by more than 30 years (see p. 15). The English top yeasts as a rule give poor results [see Dixon and Atkins, [1913]] and sometimes yield totally inactive maceration extract. It is not understood why the enzyme passes out of the cell during the process of maceration and the whole method gives rise to a number of extremely interesting problems.
Method.—A suitable yeast is washed by decantation, filtered through a cloth, lightly pressed by means of a hand press, and then passed through a sieve of 5mm. mesh, spread out in a layer 1–1·5cm. thick and left at 25°–35° for two days. Fifty grams of the dried yeast is [p026] thoroughly and carefully mixed with 150 c.c. of water in a basin by means of a spatula and the whole digested for two hours at 35°. The mass often froths considerably. It is then filtered through ordinary folded filter paper, preferably in two portions, and collected in a vessel cooled by ice. The separation may also be effected by centrifuging or pressing out the mass, and the maceration may be conveniently conducted in a flask immersed in the water of a thermostat. It is not advisable to macerate more than 50 grams in one operation. Under these conditions 25–30 c.c. of extract are obtained after 20 minutes' filtration, 70–80 c.c. in twelve hours. Dried Munich yeast can be bought from Messrs. Schroder of Munich and serves as a convenient source of the extract.[1]
[1] The material supplied is occasionally found to yield an inactive extract and every sample should be tested.
This extract closely resembles in properties the juice obtained by grinding the same yeast, but it is usually more active and contains more inorganic phosphate (see p. [46]).