Food Adulteration and Its Detection / With photomicrographic plates and a bibliographical appendix

by Jesse P. Battershall

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 Pharmacopœia, 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.