Acetic acid in a dilute state, combined with mucilage, and sometimes accompanied with flavouring ingredients. Though frequently resulting from spontaneous fermentation, this useful acid is usually obtained by the manufacturing processes of brewing and fermentation. There are four principal kinds; namely, wine vinegar, malt vinegar, sugar vinegar, and wood vinegar. The process of preparing the last-mentioned, has been already described under the article Acid, in the first volume; our attention is therefore here restricted to the three former.
Wine Vinegar. In Paris, the wine destined for making vinegar, is usually mixed in a large tun, with a quantity of wine lees; the whole is then transferred into cloth sacks, placed within a large vat, and the liquid portion of the matter is extruded through the sacks by supeincumbent pressure. What passes through is run into large casks, set upright, having a small hole in their tops. In these vessels, it is exposed to the heat of the sun in summer, or to that of a stove in winter. Fermentation takes place in a few days. If the heat should then rise too high, it is lowered by cool air, and the addition of fresh wine. In the skilful regulation of the fermentative temperature, consists the art of making good wine vinegar. In summer, the fermentative process is usually completed in a fortnight; in winter, about double the time is requisite; after which, it is run off into casks, containing some chips of birch-wood, where it is allowed to remain, until it has become clear and bright, which usually takes a fortnight more.
The vinegar is then put into close casks, and is ready for the market.
At Orleans, the manufacturers prefer wine of a year old, for making vinegar; but. if the wine has lost its extractive matter, by age or otherwise, it does not so readily undergo acetification, which is, however, brought about by the addition of bunches of grapes, slips of vines, or green woods, abounding with extractive matter. Almost all the vinegar of the north of France being prepared at Orleans, the manufacture of that place has acquired such celebrity, as to render the process employed there worthy of particular attention.
The Orleans casks contain nearly 400 pints of wine. Those which have already been used, are preferred. They are placed in three rows, one above another, and in their tops, have an aperture of two inches diameter, kept always open. The wine for acetification is kept in adjoining casks, containing beech shavings, to which the lees adhere. The wine thus clarified, is drawn off to make vinegar. One hundred pints of good vinegar, boiling hot, are first poured into each cask, and there left for eight days. Ten pints of wine are mixed in, every eight days, until the casks are full. The vinegar is allowed to remain in this state fifteen days, before it is exposed for sale. The used casks, called mothers, are never emptied more than half, but are successively filled again, to acetify new portions of wine. In order to judge if the mother works, the vinegar-makers plunge a spatula into the liquid; and, according to the quantity of froth which the spatula shows, they add more or less wine. In summer, the atmospheric heat is sufficient.
In winter, stoves heated to about 76° Fahr. maintain the requisite temperature in the manufactory.
In some country districts, the people keep, in a place where the temperature is mild and equable, a vinegar cask, into which they pour such wine as they wish to acetify; and it is always preserved full, by replacing the vinegar drawn off, by new wine. To establish this household manufacture, it is only necessary to buy at first a small cask of good wine. The following mode of preparing vinegar, which was described by Boerhaave, more than a century ago, is still in practice in various parts of France, and elsewhere:-
"Take two large casks or hogsheads, and in each of these, at the distance of a foot from the bottom, form a false-bottom of wicker-work; set the vessel upright, and on the grate place a moderately close layer of green twigs, or fresh cuttings of the vine. Then fill up the vessel with the foot-stalks of the grapes, commonly called the rape, to the top of the vessels, which must be left quite open. Having thus prepared the two vessels, pour into them the wine to be converted into vinegar, so as to fill one entirely, and the other only halfway up. Leave them thus for twenty-four hours, and then fill up the half-filled vessel with liquor, from that which is quite full, and which will now, in its turn, be left only half full. Twenty-four hours afterwards, repeat the same operation, and thus go on, keeping the vessels alternately full and half full, during twenty-four hours, till the vinegar be made. On the second or third day, there will arise in the half-filled vessel, a fermentative motion, accompanied with sensible heat, which will gradually increase from day to day.
On the contrary, the fermenting motion is almost imperceptible in the full vessel; and, as the two vessels are alternately full and half full, the fermentation is, by this means, in some measure, interrupted, and is only renewed every other day in each vessel. When this motion appears to have entirely ceased, even in the half-filled vessel, it is a sign that the fermentation is finished; and therefore, the vinegar is then to be put into casks, close-stopped, and kept in a cool place. A greater or less degree of warmth accelerates, or checks this, as well as the spirituous fermentation. In France, it is usually finished in fifteen days, during the summer; but if the heat of the air be very great, and exceed 25° of Reaumur's thermometer, (88 1/4o Fahr.) the half-filled vessel must be filled up every twelve hours; because, if the fermentation be not so checked in that time, it will become violent, and the liquor will be so heated, that many of the spirituous parts, on which the strength of the vinegar depends, will be dissipated, so that nothing will remain after the fermentation, but a vapid liquor, sour indeed, but effete.
The better to prevent the dissipationof the spirituous parts, it is a proper and usual precaution to close the mouth of the half-filled vessel, in which the liquor ferments, with a cover made of oak wood. As to the full vessel, it is always left open, that the air may act freely on the liquor it contains; for it is not liable to the same inconvenience, because it ferments but very slowly."
In observing the phenomena of this fermentation, M. Fourcroy remarks, (Elements of Natural History and Chemistry, London, 1790,) we perceive a great deal of boiling and hissing. The liquor becomes hot and turbid; a great many bubbles and filaments appear to run through it in all directions: there exhales from it a lively acid smell, which is no way dangerous: it absorbs a great deal of air. By degrees, these phenomena disappear; the heat falls, the emotion ceases, and the liquor becomes clear. It deposits a glareous sediment in reddish flakes, which stick to the sides of the casks. It appears, from a sufficient number of experiments, that the smaller the quantity of the wine, and the more it is exposed to the contact of air, so much the more readily does it pass into the state of vinegar. Care must be taken to draw off the vinegar clear, when it is thus prepared, in order to separate the lye, which, were this precaution neglected, would cause it to pass into the 3tate of putrid fermentation. Vinegar does not, like wine, deposit tartar by rest: that salt was dissolved, and combined with the alcohol and water, during the fermentation.
It is even probable, that the presence of the salt has a principal influence in calling forth the properties of vinegar from a latent state.
Malt Vinegar, which is chiefly used, and extensively manufactured in this country for foreign as well as home consumption, is made by macerating malt (in some instances mixed with a proportion of unmalted barley,) in hot water. From each boll of the grain is extracted one hundred gallons of wort, and when the temperature is reduced to about 75° of Fahr. four gallons of beer yeast are added to each hundred gallons. The liquor is next racked off into a series of upright vats, arranged in a stove-room, kept heated to a temperature of nearly 90° Fahr. The vats are provided with perforated false bottoms, on which is strewed a quantity of rape, the refuse from the makers of British wine, or some low priced raisins. Every twenty-four hours, or oftener, should the liquor grow too warm, the principal portion of the liquor of each alternate vat is pumped out, and discharged into the adjoining one, two vats being usually worked together, in the manner already described, until the active fermentation is completed.
After this, the liquor is drawn off clear into large casks or pipes, which are laid on their sides, exposed to the air, the bung-holes being only loosely covered, to exclude accidental impurities.