YEAST, or FERMENT, a substance which is deposited in an in soluble state during the fermentation of wine, beer, and vegetable juices. This substance, as is well is to produce fer mentation in saccharine solutions. According to Liebig, the insoluble part of yeast does not cause fermentation, for he states that if it be "carefully washed with water, care being taken that it is always covered with water, the residue does not produce fermentation." Neither, according to the same authority, does the soluble part of yeast excite fermentation until it has been allowed to cool in contact with the air, and to remain some time exposed to its action ; if in this state it be introduced into a solution of sugar, it produces brisk fermentation.
Yeast is a product of the decomposition of gluten, and when added to a solution of pure sugar, it gradually disappears; but when added to vegetable juices which contain gluten as well as sugar, it is repro duced by the decomposition of the gluten, in the same way as it w 49 originally formed. According to Professor Graham, the action of yeast and all other ferments is destroyed by the temperature at which water boils, by alcohol, by acids, salts of mercury, sulphurous acid, chlorine, iodine, bromine, by aromatic substances, volatile oils, and particularly empyreumatic oils, smoke, and a decoction of coffee; these bodies in some eases combining with the ferments or effecting their decom position.
Mr. Few= gives the following as one mode of producing yeast without the aid of a ferment. Wheaten flour and water are mixed to the consistence of a paste, and slightly covered up in a warm place; a sour odour is produced, and carbonic acid gas given off, about the third day; by about the sixth day the odour becomes vinous rather than sour; and then the substance has practically become yeast, or a sub stitute for it. It may be either used at once, or laid by for future use. In the latter case, it is made into small thin cakes, and dried in the air; when about to be used, the cakes are dissolved. This is nearly equivalent to the ancient mode of making leaven. Mr. Cooley describes a mode of making yeast with the aid of a ferment. About lb. bean flour is boiled for half an hour in 6 quarts of water. The solution is poured into a vessel; 31 lbs. wheat-flour is added and stirred in; when cooled down to about 5: °° Fehr., 2 quarts of beer-yeast are added ; and when the mixture has fermented for 24 hours, 7 lbs. of barley-flour or bean-flour is thrown in. The composition is kneaded into dough, made into cakes, and kept dry place till wanted for use.
This subject is further illustrated under BREAD; BREWING; FER MENT.