Cerevisiae Fermentum. Beer Yeast.
Beer yeast is the ferment obtained in brewing beer, and produced by Saccharomyces (Torula, Turpin) cerevisiae, Meyen (Order, Gymnoasceae). Yeast occurs as a viscid, frothy liquid, having a peculiar odour and bitter taste. Under the microscope it shows numerous isolated roundish or oval cells, or short branched filaments composed of united cells. The cells are transparent, with one or two vacuoles, and often contain a somewhat granular protoplasm. Compressed yeasts are distillers' yeasts and are obtained as bye-products in the manufacture of spirits from malt and raw grain. The skimmings from the fermentation vats are first mixed with water and then passed through a series of sieves. They are then washed by decantation two or three times and again sifted, this time through finer sieves, a process requiring care in order to avoid removal of cell contents. Finally, when the yeast has completely settled it is placed in filter-presses either alone or after admixture with starch. The best yeasts, however, are absolutely pure—i.e., free from starch. After compression the yeast is separated from the press cloths and made up into convenient form for distribution. Much of this yeast is imported into this country from Germany. German, or compressed, yeast, occurs as a pasty mass of putty-like consistence, and occasionally of a crumbly nature. It has an odour and taste like those of beer yeast, and can be dried by exposing it to a temperature of 30°. Dried yeast yields to alcohol 3 per cent. of extractive, consisting largely of unsaturated fatty acids, which may be fractionally separated by conversion into their calcium or lead salts.
Constituents.—An active unorganised ferment, named zymase, has been separated from yeast by crushing the cells by hydraulic pressure; it decomposes saccharine solutions with production of alcohol and carbon dioxide. Yeast also contains emulsin and nuclein or nucleol, a combination of nucleic acid with albuminates and carbohydrates.
Action and Uses.—The idea that the therapeutic action of yeast is due to its property of inciting fermentation has been proved to be fallacious, for it is still effective after the yeast has been heated to 130° for an hour. Its action is virtually that of nuclein; if injected it increases the proportion of leucocytes, after a transient leucopaenia, and it is stated to raise the opsonic index of the blood in respect to staphylococcus and the organism of tubercle. It is given internally to check the growth of boils, and has been added to poultices for application to unhealthy wounds. When fresh yeast is given by the mouth it grows actively in the stomach, and, besides being of value in some cases of furunculosis, it slightly diminishes the quantity of sugar passed by the urine in diabetes. It has also been given in phthisis, diabetes, and septic endocarditis, on the supposition that it has bactericidal or phagocytic properties. But living yeast cells do not live in the blood, as has been asserted, being disintegrated and absorbed, even when injected; the action of yeast is entirely due to its nucleo-albumin. Dried yeast has been given in doses of 5 decigrams (8 grains) for constipation; it occurs as a light grey powder, and is administered in capsules or tablets, keratinised or otherwise treated, to prevent solution in the stomach. Yeast soaps and combinations of yeast with ammonium ichthosulphonate, and salicylic acid, are used in acne and dermatitis. Yeast has also been used in the treatment of acute and chronic vaginitis, endocervicitis, etc. Thus, in vaginitis, a mixture of yeast and sugar solution is sometimes smeared over the vaginal walls, a douche being administered twelve hours later, and the treatment repeated every other day if necessary. Zymin is a variety of dried yeast, produced by dehydrating yeast by means of acetone. Similar preparations are sold under the trade-names Levurine, Levuretine, Faexin and Furonculine. The fatty acids of yeast extracted by alcohol are sold under the names Ceridin, Cerolin, Faexin Extract, etc., for use as mild laxatives, and for the other purposes to which yeast is applied, in doses of 1 to 2 decigrams (1 ½ to 3 grains). Yeast extracts, which closely resemble meat extract in colour, taste, and smell, are sold pure (Marmite), or mixed with meat extract; they have not the stimulating properties of meat extract, and may be distinguished chemically by their freedom from creatinine. Such extracts are not recommended in gout and allied conditions, on account of the large proportion of purin bases they contain, from which results an increase in the amount of exogenous uric acid excreted.
Dose.—Of liquid yeast, 15 to 30 mils (½ to 1 fluid ounce); of dried yeast, ½ to 2 grammes (8 to 30 grains).