YEAST, In the process of fermentation of saccharine fluids containing albuminous matter, as in brewing or wine-making, the originally clear fluid becomes turbid, car bonic acid is evolved, and the substance causing the turbidity gradually separates in a graying foaming mass of a bitter taste and an acid reaction. This is yeast; and on ex amining it under the microscope, it is found essentially to consist of aggregations of small oval cells of a vegetable nature, known as the yeast-cells, yeast-plant, or toruld cererisia (q.v.). Yeast, as is well known, has the property of setting up fermentation in saccha rine solutions; and, beer-yeast, the kind with which we are specially acquainted, pos sesses, according to prof. Miller, this power in the highest degree, as may be shown by dissolving 4 parts of pure cane-sugar in 20 parts of water, and adding 1 part of fresh yeast: if this mixture be exposed to a temperature of about SO', in less than an hour fer mentation will have commenced. The investigations of Mitseherlich have led chemists to distinguish two varieties of yeast—viz., the ober-liefe, or surface-yeast, and the miter heft, or sediment-yeast. the former collecting on the surface of the fermenting fluid, and the latter forming a sediment. Surface-yeast is propagated by buds (see TOR ULA CERE vi sliE) and sediment-yeast by spores; and each variety produces specific results upon the fermenting fluid. The fermentation induced by the surface-yeast is rapid and irregular; while that produced by the sediment-yeast is slow and quiet. The surface-yeast is formed when the saccharine fluid ferments at a temperature of from 65° to 77°; while the sediment-yeast is chiefly produced when fermentation takes place at the lower temperature of from 32° to 45°. In their chemical relations, the two varie ties present no apparent difference. On treating with a solution of potash, a ccl ulose-like substance remains, while an albumimiate is dissolved. The action of yeast is destroyed by exposing it to a temperature of 212°, by alcohol, by the strong mineral acids, chlorine, iodine, and bromine, oxide of manganese, creosote, etc.; on the other hand, it may be dried at a low temperature, or by pressure, and may be preserved in this state without losing its activity. The part which the globules of yeast play in ex citing the conversion of sugar into alcohol hod carbonic acid, is very obscure; but an experiment of Mitscherlich seems to show that the sugar ferments only in those points which are in actual contact with the globules. Pasteur's experiments render it probable that the process of fermentation is connected with the assimilation of the sugar by the yeast-plaid during the development of the yeast-globules, or, in words, that "the essential condition of fermentation is the conversion of albuminoid matter into organ ized globules."
According to 3litscherlich's analysis, the cells of ordinary washed yeast in a condi tion to excite fermentation contain (the ashes being deducted): carbon, 47.0; hydrogen, 6.6; nitrogen, 10.0; sulphur, 0.6; oxygen, 35.8; while spent yeast (after fermentation had ceased) contained only 5 of nitrogen. The inorganic matter amounted to 7.3 per cent of the dried yeast, and consisted entirely of phosphates.
The economic uses of yeast in bread-making, brewing, etc., are noticed in other ar ticles. Beer-yeast (cererisiee fermentum) is an article of the British Pharmacopoeia. It is employed as a stimulant in the advanced stages of low fevers,and is especially serviceable in cases where, in consequence of inflammatory symptoms, wine is inadmissible. Neli gan has found it of great service in intense tympanitis following parturition. The dose is two tablespoonfuls every three hours, and it may he given in camphor mixture or peppermint water. Yeast-poultice forms an excellent stimulating application to foul and irritable sores. It is composed as follows: Take of yeast, six fluid ounces; flour, fourteen ounces; water heated to 100% six fluid ounces. Mix the yeast with the water, and stir in the flour. Place the mass near the fire till it rises. This poultice should be renewed every six or eight hours. Its special efficacy depends upon the carbonic acid gas which it evolves.
If surface-yeast or under-yeast be collected and placed on a cloth to drain, and then pressed until nearly dry, it can be kept with care for several months, and in that state is whatss called GERMAN YEAST, for which a large trade has sprung up within the last few years; the imports to Britain from the continent having amounted, in the year end. ing bee. 31, 1875, to 163,521 cwts., or about 8,176 tons. It is chiefly imported from Hol land and Hamburg, and is obtained mostly from the great continental distilleries. Nearly the whole of this large quantity is consumed by the bakers. PATENT YEAST iS exactly similar, but is raised from a wort made purposely from malt and hops. ARTI FICIAL YEAST is a dough of wheat or other flour, mixed with a small quantity of cora• mon yeast, and made into small cakes, which are dried. If kept free from damp, it long retains its fermentive power.