VINEGAR
Vinegars may be adulterated by the addition of mineral acids as sulfuric
or hydrochloric. Caramel or the coal-tar dyes may be employed to improve
the color or to give color to an artificial product. Malic acid is
always present in cider vinegar. Potassium acid tartrate occurs in true
wine vinegar. Poisonous metals may be present in vinegars containing
free mineral acid. Entirely artificial cider vinegar is often found on
the market.
PREPARATION OF THE SAMPLE FOR TESTING
If the vinegar is turbid from any suspended matter, it should be
filtered. The samples should be analyzed at once, and in the laboratory
they should always be kept in glass-stoppered bottles.
_General Observations._--Ignite a little of the vinegar residue on a
clean platinum wire in a colorless Bunsen flame, and if it is pure cider
vinegar the flame will be colored the characteristic lilac color of
potassium. The sodium flame is absent or only a mere trace of it is
present. But in all artificially colored vinegars, spirit sugar and
glucose vinegars, the sodium flame predominates.
The residue of cider vinegar is thick, viscid, or mucilaginous, of a
light brown color, astringent acid taste though not unpleasant. The
solids of sugar-house vinegar, those from colored spirit and wood
vinegar, each have a bitter taste on account of the caramel used to
color them. The residue of the sugar-house vinegar has the odor of
molasses. Wood vinegar when present gives a residue with a tarry or
smoky taste and smell. Glucose vinegar gives the odor of scorched corn.
Solids of fruit vinegars are quite soluble in alcohol, except a granular
residue in grape vinegar, while the solids of malt and glucose vinegars
are almost insoluble.
The ash of fruit vinegars and malt vinegars has a distinct alkaline
reaction, while that of spirit and wood vinegars is very feebly
alkaline.
FREE MINERAL ACIDS
The ash of pure cider vinegar is always alkaline. If a vinegar should
show a neutral reaction this would certainly indicate the presence of a
free mineral acid. If the ash be alkaline, no acid except nitric could
have been present, and this is seldom, if ever, used as an adulterant of
vinegar.
WHEN THE ASH IS ALKALINE APPLY
_Ashby’s Test._--Extract 0.5 gram of logwood in 100 cc. of water and dry
a drop or two on a porcelain surface. Then add a drop of the vinegar and
dry again. If the residue is red, a mineral acid is present; if yellow,
mineral acids are absent. When only a very small amount of the acid is
present the red coloration will be destroyed on diluting with water, but
may be restored by concentrating the liquid.
SULFURIC ACID
Sulfuric acid, if present, will cause the vinegar to leave a charred
mass when evaporated over the water-bath.
_Frear’s Method._--Mix 5 cc. of the sample and 5 or 10 cc. of water, and
add a very little of a solution of methyl violet (made by dissolving one
part of methyl violet 2 B. in 100,000 parts of water). A blue or green
coloration shows the presence of mineral acids.
SULFURIC ACID AS DISTINGUISHED FROM SULFATES
_Allen’s Method._--Evaporate 100 cc. of the vinegar down to one tenth
its volume, and when cold add 50 cc. of alcohol. Sulfuric acid remains
in solution while the sulfates are precipitated. Dilute the solution and
precipitate the acid with barium chlorid.
HYDROCHLORIC ACID
FREE
Place a definite quantity of the vinegar in a distilling flask and
distil off half. Add a few drops of silver nitrate to the distillate. If
a precipitate forms, hydrochloric acid is present.
MALIC ACID
_Leach’s Method._--To 5 cc. of the sample, add a few drops of a solution
of calcium chlorid (1 : 10); make slightly alkaline with ammonia. Filter
off any precipitate that may form, add 20 to 30 cc. of 95 per cent
alcohol to the filtrate and heat to boiling. If malic acid is present, a
voluminous flocculent precipitate will form. A precipitate may form in
vinegars containing dextrine. Make a further test for malic acid by the
following: Filter and treat the precipitate with a little alcohol, and
when dry add concentrated nitric acid and evaporate to dryness on a
water-bath. Treat the residue with sodium carbonate, boil for a short
time, filter. Add acetic acid to the filtrate till slightly alkaline,
boil till carbon dioxid is expelled, and if on the addition of calcium
sulfate a precipitate forms, it indicates the presence of malic acid.
COLORING MATTER
CARAMEL
The residue of vinegar to which much caramel has been added has an
unusually dark color and bitter taste.
_Crampton and Simons’ Method._--Shake well together in a corked flask 50
cc. of the vinegar with about half as many grams of fullers’ earth;
after standing for half an hour filter. Vinegar containing no artificial
color will show scarcely any change in color when thus treated. A
caramel-colored vinegar will be decolorized in proportion to the amount
of caramel present.
COAL-TAR COLORS IN WINE VINEGAR
Test by the usual test for coal-tar dyes. See under canned vegetables.
METALLIC IMPURITIES
Vinegars containing free mineral acids are sometimes found to contain
poisonous metals.
Evaporate 200 to 400 cc. of the vinegar to dryness, add a little sodium
hydroxid to this residue and burn to an ash over a low flame. It may be
necessary to add a little potassium nitrate once or twice. Add a little
dilute hydrochloric acid and saturate with hydrogen sulfid and test for
lead, zinc, copper, and arsenic according to Allen’s method given under
canned meats.
SPICES TO INCREASE PUNGENCY
_Leach._--Neutralize a portion of the vinegar with sodium carbonate. The
presence of spices is easily detected by tasting this mixture.
_Another Test._--Exactly neutralize a little of the vinegar as above,
evaporate to smaller bulk and taste as before, then shake the
concentrated liquid with ether, separate the ethereal layer and
evaporate it, and taste the residue.
TARTAR IN WINE VINEGAR
The presence of tartar in vinegar proves it to be wine vinegar.
_Allen’s Method._--Evaporate a portion of the vinegar and treat the
residue with alcohol; a granular residue of tartar remains undissolved.
To prove that it is tartar, decant the alcohol and dissolve the residue
in a little hot water, cool, rub the inside of the vessel with a glass
rod, and if tartar was present acid potassium tartrate will be deposited
where the rod touched the vessel. The test will be more sensitive if an
equal volume of alcohol is added.
FREE TARTARIC ACID IN WINE VINEGAR
_Test as for Tartar._--Treat the alcoholic solution of the extract with
an alcoholic solution of potassium acetate. Rub the sides of the vessel
as before, and if tartaric acid is present the streaks and sometimes a
precipitate forms where the rod touches the vessel.
GLUCOSE
Glucose is present when both direct and invert readings are
dextro-rotatory.