Cerevisiae Fermentum.—Beer Yeast.
Preparation: Yeast Poultice
The ferment obtained in the brewing of beer.
SYNONYMS: Brewer's yeast, Barm.
Preparation.—When an infusion of malt (barley steeped in water, left to incipient fermentation, and dried in a kiln), technically called Wort, is subjected to the process of fermentation, a dirty, grayish-brown substance gradually separates, forming in part a frothy scum (top or upper yeast), and partly a sediment (bottom or lower yeast); this is yeast, or barm.
Description and History.—Yeast is a thick, glutinous, foamy-like fluid of a wine-acid odor, and a rather unpleasant taste; it is a very complex mixture, containing water, alcohol, carbonic, acetic, and malic acids, potassium and calcium salts, saccharo-mucilaginous extract, and a cryptogamous growth consisting of nucleated cells, which was formerly named Torula cerevisiae, of Turpin, but now known as Saccharomyces cerevisiae. When exposed to moist air, or to a temperature above 15° C. (59° F.), yeast rapidly decomposes; but when subjected to a gentle heat, so as to dissipate its watery parts, it forms a fragile solid, which may be kept without change for a considerable time, but with a diminution of its fermentative properties.
Neither water nor alcohol dissolve yeast. Its most important property is, that when placed in contact with saccharine solutions at a temperature between 10° and 26.6° C. (50° and 80° F.), it excites vinous fermentation in them, converting their sugar into carbonic acid and alcohol. This was first pointed out by Thénard, in 1803, but the fungi producing this change were observed by Leeuwenhoek as early as 1680. Dr. Christison, by means of yeast, has been able to detect one part of sugar in 1000 parts of urine, of specific gravity 1.030. The cells appear as small, ovoidal, ellipsoidal, or somewhat pyriform, transparent, nucleated bodies, varying in size from 1/7500 to about 1/250 of an inch (P.).
The fermentative power of yeast is much impaired by drying it; a heat of 100° C. (212° F.) destroys yeast when moist, as does the addition of some of the concentrated acids, and of undiluted alcohol. It is also destroyed by boiling water, sulphurous acid, sulphate or acetate of copper, salt, nitrate of silver, black oxide of manganese, carbolic acid, creosote, quinine, strychnine, free alkalies, pyroligneous acid, salts of mercury, essential oils, etc. If, however, fermentation has commenced, the vegetable bases have no specific power of arresting the process. Arsenous acid, acetate of lead, and tartar emetic have no retarding effect upon the progress of fermentation. The keeping qualities and the taste of the products of fermentation depend largely on the nature of the yeast employed. The introduction of pure cultures of these micro-organisms by E. C. Hansen, of Copenhagen, has raised the arts involving fermentation to an almost exact science. For some interesting remarks on fermentation, by M. Pasteur, see Amer. Jour. Pharm., Vol. XXX, p. 328; also see under Alcohol.
Yeast may be exposed to a very low temperature (—60° C. [—76° F.]) without losing its vitality, though the plant ceases to form and bud at a temperature of less than 5° C. (41° F.). On the other hand, it may be heated, if no water be present, to 100° C. (212° F.) without destroying its activity, but if water be present, its destruction is accomplished at 75° C. (167° F.). Commercial dry yeast is prepared by pressing out most of the water from yeast. Vienna yeast is the dried froth from the wort from barley malt, maize, and rye. Patent yeast is said to be flour dough mixed with yeast, or hop infusion, with malt added.
Action, Medical Uses, and Dosage.—Stimulant, tonic, nutritious, antiseptic, and laxative. Formerly used in typhoid fevers by mouth and injection, and in tympanitis by enema. With or without the addition of olive oil, which renders it more laxative, it will be found highly beneficial in all malignant ulcerations of the throat and mouth, in diseases where there is a disposition to putridity, in scarlatina, and low stages of fever. Externally, in combination with elm bark and charcoal, it forms an excellent emollient and antiseptic poultice in sloughing ulcers, stimulating the vessels, removing the tendency to gangrene, and correcting the fetor. In the furunculoid epidemic, which existed in this country and Europe, given internally, in conjunction with quinine, yeast was found effectual in the treatment of boils, carbuncles, and felons. Yeast in doses of 1 fluid drachm, 3 or 4 times a day, taken immediately before meals, has been advised in diabetes mellitus. It has in some instances proved efficient, and is supposed to act by decomposing sugar or preventing its abnormal production in the stomach. The dose of yeast is from 1/2 to 1 fluid ounce, every 2 or 3 hours.
King's American Dispensatory, 1898, was written by Harvey Wickes Felter, M.D., and John Uri Lloyd, Phr. M., Ph. D.