Vinegar is a dilute aqueous solution of acetic acid, containing inconsiderable proportions of alcohol, aldehyde, acetic ether, and extractive matters, which, to some extent, impart a characteristic flavour and aroma. The process most frequently involved in the preparation of vinegar is known as the acetous fermentation, and may be induced in various saccharine juices and infusions, such as those of apples, wine, malted grain, etc., when, in presence of a ferment, they are exposed to the action of the air, at a temperature between 24° and 32°. In the oxidation of alcohol, an intermediate compound (aldehyde) is at first formed, which, by the continued action of oxygen, is ultimately converted into acetic acid. A dilute solution of alcohol is, however, not oxidised to acetic acid by simple exposure to the air; it is usually necessary that a peculiar microscopic plant (mycoderma aceti) should be present. This fungus includes two varieties, viz., minute globules (micrococci) and rod-like forms (bacilli) varying in size; and is often developed in old casks that have been long employed for making vinegar. It constitutes a gelatinous mass (“mother of vinegar”) having the appearance of glue that has been soaked in cold water; the surface quickly becomes coated with a bluish mould (Penicillium glaucum).

Pasteur regards acetification as a product of the development of the mycoderma aceti, i.e., as a physiological fermentation—but it appears probable that the process is rather one of oxidation, and that the fungus accelerates the change by condensing the oxygen upon its surface and delivering it to the alcohol, possibly in the form of ozone. Indeed, the process of vinegar making may take place in the entire absence of the mycoderma, as when spongy platinum is brought into contact with alcoholic solutions; and Buchner has examined shavings which had been used in a vinegar factory for over twenty-five years, and found them to be absolutely free from the fungoid plant. In the United States, the best known and most esteemed kind of vinegar is that obtained by the acetification of apple cider; but by far the largest quantity is manufactured from alcohol and spirituous liquors. Cider vinegar is free from aldehyde but contains malic acid. The usual source of vinegar in Great Britain is a wort prepared from mixtures of malt with other grain; while, in Continental Europe, inferior sorts of new wine (especially white wine) are extensively employed for its production.

Malt vinegar possesses a brown colour and a density ranging from 1·006 to 1·019; that known as proof vinegar contains from 4·6 to 5 per cent. of acetic acid. In Great Britain the manufacturer is allowed by law to add 0·1 per cent. of sulphuric acid to vinegar, on account of its supposed preservative action, and, although the practice is now known to be unnecessary, it is still sometimes resorted to. The specific gravity of wine vinegar varies from 1·014 to 1·022. 100 c.c. should neutralise from 0·6 to 0·7 grains of sodium carbonate, and the solids obtained upon evaporation to dryness should approximate two per cent. According to the United States Pharmacopoeia, one fluid ounce of vinegar should require for saturation not less than 35 grains of potassium bicarbonate.

In 500 samples of imported wine and malt vinegar tested by the author, the minimum and maximum strength ranged from 3 to 10·6 per cent. of acetic acid, the specific gravity from 1·0074 to 1·0150, and the number of grains of potassium bicarbonate required to neutralise one troy ounce from 22 to 84. Of 273 samples of vinegar tested in 1884 by the Massachusetts State Board of Health, 52 were above the then legal standard of 5 per cent. of acetic acid, and 221 below; 109 of the latter contained more than 4 per cent.; the strongest sample showed 8·86 per cent., and the weakest contained but 0·66 per cent. of acetic acid. In the year 1885, 114 samples were examined, of which 45 were above and 69 below the standard of 4½ per cent. acetic acid.

In the State of New York, the legal standard for vinegar is 4·5 of absolute acetic acid, and, in the case of cider vinegar, the proportion of total solids must not fall below 2 per cent. In Massachusetts, also, the acidity must be equivalent to 4½ per cent. of acetic acid, and cider vinegar must contain, at least, 2 per cent. of solid matter. The English standard of strength is 3 per cent. of acetic acid.

Analysis.—For the requirements of the United States Customs Service, the only estimations ordinarily made are the specific gravity, and a determination of the acidity. The former is accomplished by means of the specific gravity bottle; the latter, by placing 10 c.c. of the sample in a beaker, adding about 30 c.c. of water, then a few drops of an alcoholic solution of phenol-phthaleÏn (to serve as the indicator), and titrating with a normal alkali-solution; the number of c.c. used divided by 10 and multiplied by 48, gives the amount, in grains, of potassium bicarbonate required to neutralise one troy ounce of the vinegar. In the presence of sulphuric acid, it is necessary to distil a measured quantity of the sample almost to dryness and titrate the distillate, it being assumed that 80 per cent. of the total acetic acid present passes over.

The determination of the extract or solid residue in vinegar is executed in the same manner as described under beer and wine. Several tests have been suggested for the detection of the presence of free sulphuric acid. The usual reagent—barium chloride—is not well adapted to the direct determination of this acid, since sulphates, which are as readily precipitated as the free acid, may also be present. The following methods may be employed:—

1. A piece of cane sugar is moistened with a small quantity of the sample and exposed to the heat of the water-bath for some time, when, in presence of free sulphuric acid, the residue will become more or less carbonised, according to the proportion of acid present.

2. Five centigrammes of pulverised starch are dissolved in a decilitre of the sample by boiling, and after the liquid has become completely cooled, a few drops of iodine solution are added. Dilute acetic acid does not affect starch, and if the sample is pure, a blue coloration will be produced; if, however, but a minute quantity of sulphuric or other mineral acid is present, the starch is converted into dextrine, and the addition of iodine fails to cause the blue coloration.

3. According to Hilger,[133] if two drops of a very dilute solution of methyl aniline violet (0·1 to 100) are added to about 25 c.c. of pure vinegar no change of colour takes place; whereas, in the presence of 0·2 per cent. of mineral acid, a bluish coloration is produced; in case the proportion of acid reaches 1 per cent. the liquid acquires a greenish tint.

4. A recent test for mineral acids has been suggested by Hager.[134] It consists in warming together two drops of East Indian copaiba balsam, and 30 drops of pure acetic acid, and subsequently adding to the mixture two or three drops of the vinegar under examination; if either sulphuric or hydrochloric acid be present, a blue-violet colour is produced.

The free mineral acids in vinegar may be quantitatively estimated by saturating a weighed quantity of the sample with quinine, evaporating the mixture to dryness over the water-bath, and dissolving the quinine salts formed in alcohol, which is then removed by distillation. The second residue is next dissolved in water, and the quinine precipitated by addition of ammonia, and separated by filtration.

The filtrate will contain the mineral acids present, and their amount is determined by the ordinary methods.

The free sulphuric acid in vinegar can also be quantitatively estimated, according to Kohnstein,[135] as follows: 100 c.c. of the sample are shaken with pure and freshly calcined magnesia until completely neutralised. The mixture is filtered, the filtrate evaporated to dryness in a platinum dish and the residue ignited at a moderate temperature. By this treatment magnesium acetate is converted into the corresponding carbonate, while any magnesium sulphate present will remain unaltered. The ignited residue is moistened and evaporated with a little carbonic acid water, then digested with hot water, and the solution filtered; the insoluble magnesium carbonate remains upon the filter, the sulphate going in solution; the precipitate is thoroughly washed. After removing the traces of lime possibly present, the amount of magnesia contained in the filtrate is determined as pyrophosphate, from the weight of which the proportion of free sulphuric acid originally contained is calculated. The presence of metallic impurities in vinegar is detected by means of the usual reagents, such as hydrosulphuric acid and ammonium sulphide. In addition to water and sulphuric acid, the most common adulterants of vinegar are capsicum, sulphurous acid and various colouring matters. The presence of capsicum and other acrid substances is usually revealed by the pungent odour produced upon burning the solid residue obtained by the evaporation of the sample to dryness, and by the peculiar taste of the residue. Sulphurous acid is sometimes detected by its characteristic odour; its determination is described on p. 177.

Caramel is recognised by extracting the solid residue with alcohol, and evaporating the solution to dryness; in its presence, the residue now obtained will possess a decidedly dark colour, and a bitter taste. Fuchsine, which is said to have been employed for colouring vinegar, is detected by the tests mentioned under Wine.

As already stated, a very large proportion of vinegar is made in the United States from spirituous liquors. It is probable that fully 90 per cent. of the total production is obtained by the acetification of whisky. Much of this product is mixed with cider vinegar, or simply coloured with caramel, and then put on the market as apple vinegar. It is certain that the manufacturers of whisky vinegar, who are permitted by law to make “low wines” on their premises, without being subjected to the usual Internal Revenue Tax, are frequently enabled to perpetrate a fraud on the Government by disposing of the spirits so produced to the whisky trade, instead of converting it wholly into vinegar. To so great an extent is this practice carried on, that many of the cider vinegar producers have found it impossible to successfully compete with the less scrupulous manufacturers. Whisky vinegar is nearly colourless, usually possesses a greater strength than cider vinegar, and is free from malic acid. Cider vinegar exhibits a light-brownish colour and a characteristic odour. Some of the differences between these two varieties are shown by the following results, obtained by the author by the examination of samples of pure apple and whisky vinegar, fresh from the factories:—

Naturally the addition of caramel or cider vinegar to whisky vinegar would greatly affect the above tests.

Attempts made to differentiate between the two samples by means of qualitative reactions for aldehyde and malic acid were not sufficiently distinctive in their results to be of much value.

It has been suggested that the presence of nitrates in vinegar would point to its origin from spirits. The apple vinegar manufacturer, however, frequently finds his product above the standard, in which case he reduces its strength by adding water, thus rendering this test of little or no avail.

Regarding the addition of mineral acids to vinegar in the United States, it is satisfactory to note that, of a large number of samples tested by the New York City Vinegar Inspector during the past year, not a single sample was found to contain these adulterants.

Fermented infusions of molasses, “black strap,” etc., are occasionally employed in the manufacture of vinegar. The product obtained from these sources has been found in some instances to contain acrid and probably noxious ingredients.