FERMENTATION, an intestine commotion, to which certain substances of vegetable or animal origin are, more or less, liable, from the spontaneous reaction of their con stituent elements. The process embraces a series of changes of composition, and terminates in the formation of new products, which differ essentially from the original substance, as well as from one another. Fermentation is accordingly divided into ; and to these, epithets have been applied descriptive of the products to which it gives birth, namely, the vinous, the acetous, and the putre factive. ..After making some remarks upon the proceSs in general, we shall consider the subject under these three heads.
It appears that no species of fermentation can take place without some portion of moisture, and a certain elevation of temperature. The presence of moisture is necessary, because no chemical action can be displayed by solids, with out the intervention of water, to give mobility to their component particles, and allow them to exert their mutual attractions for each other; and hence, vegetable or animal substances which are well dried, and kept free from mois ture, may be preserved for many years without suffering any material change in their composition. The degrees of heat necessary for fermentation vary with the different kinds of it ; but below a certain temperature, the process does not commence under any form, or is effectually check ed if it has already begun. Boerhaave imagined that the three kinds of fermentation which we have enumerated, al ways succeeded each other in the same invariable order ; but though this is often the case, it by no means holds uni versally. Many substances undergo the acetous, without having previously passed through the vinous fermentation ; and a still greater number run into putrefaction that never suffer any change analogous to the vinous or acetous pro cesses.
The vinous fermentation has been examined by chemists , with a good deal of attention ; and a variety of useful facts connected with the process have been noticed, though the circumstances that may be deemed essential to it are still involved in some degree of uncertainty. It is well known that saccharine matter, in some form or other, passes most readily into the vinous state, and that the product of the fer mentation is strongest when the substance which is sub jected to the process contains the largest portion of sugar ; but it has not been decidedly ascertained whether sugar is the only substance capable of being converted into ardent spirit. When nutritive grains are employed to afford fer mented liquors, they" are previously exposed, at least in part, to the operation of malting, the object of which is to convert the farinaceous part into sugar by germination. This operation was long held indispensibly necessary to render the grain capable of the vinous fermentation ; but experience has proved this opinion to be, in some measure, a mistake. Spirit distillers have of late been in the prac
tice of malting only part of the grain, and adding the rest in a mashed or ground state ; and they have found it to an swer their purpose extremely well, when the latter is used in a greater proportion than the former. It is not a little singular, that when the farinaceous part of the mashed grain is mixed with water, it passes into the state of an acid, without acquiring any vinous quality ; but when mix ed with a quantity of saccharine matter, it undergoes the vinous fermentation, and yields a larger portion of spirit than the sweet matter alone would have afforded : a fact which seems to indicate, that the matter already in the state of sugar has the property of acting upon the farinace ous part of the grain, and converting it into a similar sub stance. Mr Irvine remarks, that cc were it not for this pro perty of the farina, great loss would frequently be sustained by the farmers in unfavourable seasons ; as grain that has once begun to grow, and whose vegetation has been stop ped, can never be made to grow again. Such grain never can undergo any farther malting : when grain has been made to grow in this manner, it can hardly be supposed that the change into saccharine matter is perfect or com plete. It therefore would be less proper for the vinous fermentation, and would furnish a smaller quantity of spirit than grain which had been perfectly malted, This grain, however, when mixed with a quantity of perfect malt, and fermented, furnishes as much spirit as if the whole had been in the state of malt. The persons in this trade even prefer it to an equal quantity of malt ; for in good seasons, when no such half-malted grain can be got, they take good grain, reduce it to meal, and mix it with their malt, and are satisfied that they obtain more spirits in this way, than from an equal quantity of good malt." Though sugar, in some modified form, appears to be the only substance capable of the vinous fermentation, certain other substances are necessary, both for the commence ment and continuation of the process. A suitable quantity of water must be added to the saccharine matter : if the quantity, however, be in excess, the liquor is apt to pass into the acetous fermentation ; and if it be too little, the process goes on difficultly and slowly. When the liquor to be fermented consists of a solution of pure sugar, a quan tity of yeast is also necessary to excite the fermentation, and make it pass into the vinous state. Nor is the influ ence of temperature less essential : below of Fahren heit's scale the vinous fermentation proceeds very slowly ; and at the freezing point it is completely checked. Above 70° the process advances too rapidly, and unless it be duly moderated, is apt to pass into the acetous stage.