Ch3ch2oh 02 Ch3cooh H20

CH3CH2OH + 02 CH3COOH + H20 Alcohol Oxygen Acetic Acid Water The interesting mechanism of this change has only been under stood of quite recent years. Just as the alcohol in wine or beer is produced by living agency (see FERMENTATION), so the sub sequent acetous fermentation is due to a micro-organism, the vinegar bacillus or Bacterium aceti. The curious and complicated recipes of mediaeval writers for the preparation of vinegar owed their success to the fact that they unwittingly involved infection with and the promotion of these organisms. Distinguished scien tists, such as Liebig and Dobereiner, had chemical theories for acetification but the truth was not discovered until Pasteur in 1864 confirmed experimentally the view put forward in 1837 that the living cells which formed a scum on the beer were really responsible for the changes observed. Hansen in 1878 described for the first time the three species of vinegar bacilli.

Vinegar brewing as a separate industry dates from about the 57th century when it was established in France. Prior to that it was just a by-product of the wine producer and the brewer. The connection between the brewing and vinegar industry in England is shown by the Revenue Acts which, in the reign of Charles II. charged different duties on beer and on vinegar-beer. There are two well-known methods of manufacture, the slow process and the quick one ; the latter has now almost superseded the former except for the preparation of pure wine vinegar as distinct from ordinary or malt vinegar. In the slow process the alcoholic wash, whether prepared from wine or not, must contain io% of alcohol; it is filled into casks holding 5o or ioo gallons, which are half-full of beech shavings and have previously been impregnated with vinegar. In these casks it is exposed to the atmosphere, by the simple expedient of leaving the bung-holes open until all the alcohol has been converted into acetic acid. The exposure was formerly in a field or yard but is now often in a special building, so that the casks may be kept slightly warm ; the acetified gyle is filtered and stored. This process is chiefly used in the Orleans district of France and has the merit of producing a vinegar of peculiarly fine aroma ; it is, however, slow and waste ful and liable to various disturbing factors, such as the develop ment of vinegar eels.

The Modern Process.

The more modern and efficient process is that of malt vinegar manufacture, the first stages of which are closely similar to those used in the preparation of beer. A quan tity of malt—about one quarter per 150 gallons—is crushed and extracted with hot water in the mash tun or tank at a temperature of about 68° C (155° F), the whole mash being well raked over so that all the soluble sugars may be extracted. After running off the infusion the grain is "sparged" by a stream of hot water from perforated revolving pipes arranged over the top of the mash tun. The liquid or gyle so prepared, which should have a specific gravity of about 1,060, is run over a refrigerator to reduce the temperature to about 21° C (70° F), then fermented by "pitching" with yeast. The yeast begins to ferment or attenuate the gyle and is allowed to operate for about 3 days, by which time all the sugar is converted into alcohol and the specific gravity is reduced to 1,005 or lower; the gyle now contains about 6% of alcohol. The yeast is skimmed off and the wash stored

until required for acetification ; prolonged storage is advantageous. The acetifier is essentially a large vat with a false bottom on which is packed beech twigs, lump pumice stone, corncobs, beech wood shavings, coke, rattan, excelsior, or basket work, previously well cleaned, and through which vinegar has been percolated. The acetic bacilli grow on the twigs and begin to operate when the gyle is sparged or sprinkled over them. The essential conditions are an adequate supply of air and a suitable temperature. The latter is maintained by the reaction itself, and is regulated by the air supply which is obtained by means of holes round the sides of the vat below the false bottom, and is adjusted so that the temperature rises to about 41° C (106° F) at which the acetifica tion is most efficiently effected. The gyle is sprinkled on to the twigs from the top, trickles through the mass and is pumped from the bottom back again, so that a continuous percolation is obtained. Acetification is complete in a fortnight or three weeks as compared with as many months in the slow process. In theory 6% alcohol should yield 7.5% of acetic acid but in practice there is always some loss so that the vinegar coming from the acetifier seldom contains more than about 6% of acid. It appears on the market at various strengths from 4% to 6% or more. Two common sources of disturbance in the acetifier are "mother" and eels. The former is a slimy film, sometimes called "tripe," which gradually forms on the twigs, due to a peculiar so called zoogloeal condition of the bacteria, which impairs the efficiency of the acetifier so that it ultimately has to be emptied and cleaned out. The vinegar eel is a curious creature resembling a thread worm, which may often be seen near the surface of vinegar which has been exposed to the air. Its presence much reduces the activity of the acetifiers but is not of any physio logical importance from the point of view of the consumer.

Special Vinegars.

In addition to wine vinegar and ordinary malt or table vinegar, there are various special kinds. Artificial or wood vinegar is an entirely factitious product, made by diluting acetic acid, manufactured by various synthetic processes, with water and colouring it with caramel; it has neither the food value nor the aroma of the genuine article. Cider vinegar is manu factured on a considerable scale in America and on a small scale in parts of England by processes analogous to that of malt vinegar. Wine vinegar is prepared mainly in France and varies in colour according as red or white wine has been used as the raw material. Spirit vinegar is a product manufactured from diluted alcohol which is acetified and coloured.

The question of a standard for vinegar is a vexed one ; in the United States and Australia there is a legal minimum of 4% acetic acid; in England a standard of 4% was suggested by the Local Government Board and is generally enforced.

See C. A. Mitchell, Vinegar: its Manufacture & Examination (1927); Brannt, Vinegar (1914). (H. E. C.)