The Preparation of Yeast-Juice.
Buchner's process for the preparation of active yeast-juice is characterised by extreme simplicity. The yeast employed, which should be fresh brewery yeast, is washed two or three times by being suspended in a large amount of water and allowed to settle in deep vessels. It is then collected on a filter cloth, wrapped in a press cloth, and submitted to a pressure of about 50 kilos, per sq. cm. for five minutes. The resulting friable mass contains about 70 per cent. of water and is free from adhering wort. The washed yeast is then [p022] mixed with an equal weight of silver sand and 0·2 to 0·3 parts of kieselguhr, care being taken that this is free from acid. The correct amount of kieselguhr to be added can only be ascertained by experience, and varies with different samples of yeast. The dry powder thus obtained is brought in portions of 300 to 400 grams into a large porcelain mortar and ground by hand by means of a porcelain pestle fastened to a long iron rod which passes through a ring fixed in the wall (Fig. 1). The mortar used by Buchner has a diameter of 40 cm. and the pestle and rod together weigh 8 kilos.
Fig. 1.
As the grinding proceeds the light-coloured powder gradually darkens and becomes brown, and the mass becomes moist and adheres to the pestle, until finally, after two to three minutes' grinding, it takes the consistency of dough, at which stage the process is stopped. The mass is next enveloped in a press cloth and submitted to a pressure of 90 kilos, per sq. cm. in a hydraulic hand press, the pressure being very gradually raised in order to avoid rupture of the cloth. The cloth required for 1000 grams of yeast measures 60 by 75 cm. and is previously soaked in water and then submitted to a pressure of 50 kilos, per sq. cm., retaining about 35 to 40 c.c. of water.
The juice runs from the press on to a folded filter paper, to remove kieselguhr and yeast cells, and passes into a vessel standing in ice water.
The yield of juice obtained by Buchner in an operation of this kind from 1 kilo. of yeast amounts to 320 to 460 c.c. It may be increased by re-grinding the press cake and again submitting it to pressure, and then amounts on the average to 450 to 500 c.c.
Since the cell membranes constitute about 20 per cent. of the weight of the dry yeast, this yield corresponds to more than 60 per cent. of the total cell contents of the yeast. It has been computed by Will [quoted by Buchner, E. and H., and Hahn, [1903], p. 66] that [p023] only about 20 per cent. of the cells are left unaltered by one grinding and pressing, and only 4 per cent. after a repetition of the process, at least 57 per cent. of the cells being actually ruptured by the double process, and the remainder to some extent altered. It seems probable from these figures that a certain amount of the juice may be derived from the unbroken cells, and Will expressly states that many unbroken cells have lost their vacuoles.
Fig. 2.
If the yeast be submitted to a process of regeneration, which consists in exposure to a well-aerated solution of sugar and mineral salts until fermentation is complete, the juice subsequently obtained [p024] is more active than that yielded by the original yeast [Albert, [1899, 1]].
A modified method of grinding yeast was introduced by Macfadyen, Morris, and Rowland [[1900]], who placed a mixture of yeast and sand in a jacketed and cooled vessel, in which a spindle carrying brass flanges was rapidly rotated [Rowland, [1901]]. One kilo. of yeast ground in this way for 3·5 hours yielded 350 c.c. of juice.
This grinding process was at first adopted by Harden and Young in their experiments but was afterwards abandoned in favour of Buchner's hand-grinding process, as it was found liable to yield juices of low fermenting power, probably on account of inefficient cooling during the grinding process. A slight modification of Buchner's process has, however, been introduced, the hand-ground mass being mixed with a further quantity of kieselguhr until a nearly dry powder is formed, and the mass packed between two layers of chain cloth in steel filter plates and pressed out in a hydraulic press at about 2 tons to the square inch (300 kilos. per sq. cm.). The press and plates are shown in section in Fig. 2. It has also been found convenient to remove yeast cells and kieselguhr from the freshly pressed juice by centrifugalisation instead of by filtration through paper, and to wash the yeast before grinding by means of a filter-press.
Working with English top yeasts Harden and Young have found the yield of juice extremely variable, the general rule being that the amount of juice obtainable from freshly skimmed yeast is smaller than that yielded by the same yeast after standing for a day or two after being skimmed. The yield for 1000 grams of pressed brewer's yeast varies from 150 to 375 c.c., and is on the average about 250 c.c.
Very fresh yeast occasionally presents the peculiar phenomenon that scarcely any juice can be expressed from the ground mass, although the latter does not differ in appearance or consistency from a mass which gives a good yield.