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

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

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

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:–

|Cider Vinegar.|Whisky Vinegar.
Specific gravity | 1·0168 | 1·0107
| |
Specific gravity of distillate}| 0·9985 | 0·9973
from neutralised sample }| |
Acetic acid | 4·66 p. c. | 7·36 p. c.
Total solids | 2·70 „ | 0·15 „
Mineral ash | 0·20 „ | 0·038 „
Potassa in ash |Considerable | None
Phosphoric acid in ash |Considerable | None
| |
Heated with Fehling’s solution}|Copious |} No reduction
}| reduction |}
| |
Treated with basic lead }|Flocculent |} No precipitate
acetate }| precipitate |}

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.