By DR. O. BACH.
There is no department in analytical chemistry in which so little
success has been attained as in the testing of commercial fats and oils.
All methods that have been proposed for distinguishing and recognizing
the separate oils, alone or mixed, bear upon them the stamp of
uncertainty.
The facts observed by J. Koenig, and described by him in his excellent
book entitled "_Die Menschlichen Nahrungs und Genussmittel_" (p. 248),
excited great expectations; viz., that the quantity of glycerine in
vegetable fats was much less than the amount required to combine with
all the fatty acids, and that the quantity of oleic acid in the oils
that he examined exhibited essential differences. Koenig himself asserts
that the fats have hitherto been too little investigated to found upon
it a method for distinguishing them, but that nevertheless it may
possibly do good service in some cases.
My own estimation of the amount of glycerine in different olive oils, by
Koenig's method, has shown, unfortunately, that the percentage may vary
from 1.6 to 4.68, according to the origin and quality of the oil. In
like manner the estimation of the oleic acid, which was conducted
essentially in the manner proposed by Koenig, showed that the amount of
oleic acid in different olive oils varied from 45 to 54 per cent. But
since cotton seed oil, for example, which is most frequently used to
adulterate olive oil, contains 5 per cent. of glycerine, and 59.5 per
cent. of oleic acid, it is easy to see an admixture of cotton seed oil
cannot be detected by this method, which appeared to be so exact.
The method of analysis that I am about to describe is based chiefly upon
the determination of the melting point of the fatty acids contained in
the oils, and upon their solubility in a mixture of alcohol and acetic
acid.
The oils employed in adulterating olive oil, and to which regard must be
had in testing it, are the following: Cotton seed oil, sesame, peanut,
sun flower, rape, and castor oils. The tests for the two last named
have hitherto never presented any difficulty, as rape seed is easily
detected, owing to the sulphur in it, by saponifying it in a silver
dish, and castor oil by its solubility in alcohol. But in recent times
another product has come into the market called sulphur oil or pulpa
oil, obtained by extracting the pressed olive cake with sulphide of
carbon. This also gives a sulphur reaction when saponified, while it
resembles castor oil by its solubility in alcohol. When this oil is
mixed with ordinary olive oil, it can easily deceive any one who uses
the ordinary tests.
My method of testing olive oil is as follows:
First, the so-called elaidine test is made, and then the test with
nitric acid. About 5 c. c. (a teaspoonful) of the oil is mixed in a test
tube with its own volume of nitric acid, spec. gr. 1.30, and shaken
violently for one minute. At the expiration of this time the oils will
have acquired the following colors: Olive oil, pale green; cotton seed
oil, yellowish brown; sesame, white; sun flower, dirty white; peanut,
rape, and castor oils, pale pink or rose.
As soon as the color has been observed, the test glass is put in a water
bath at the full boiling temperature and left there five minutes. It was
found that the action of nitric acid upon cotton seed and sesame oil was
the most violent, sometimes so violent as to throw the oil out of the
glass. At the end of another five minutes after the test tube is taken
out of the water bath, the following colors are seen: olive and rape
oils are red; castor oil is golden yellow; sun flower oil, reddish
yellow; sesame and peanut, brownish yellow; cotton seed, reddish brown.
After standing 12 to 18 hours at about 60° Fahr. the olive, rape, and
peanut oils will have solidified; sun flower, castor, and cotton seed
will be like salve (sticky), while sesame will remain perfectly liquid.
Mixtures of olive oil with small quantities of cotton seed or sesame are
distinguished by this characteristic--that, although the whole mass,
which is darker in color than olive oil, solidifies at first, at the end
of 24 or 36 hours a brown oil will be found floating upon the surface of
the solid mass, while the lower strata exhibit the yellow color of pure
olive oil. Oil of rosemary has no effect when shaken with cold nitric
acid, and imparts to it only a slightly darker color on heating. Oils
treated with lye act just like pure oils.
Far the purpose of determining the melting point of the fatty acids, 10
grammes of oil were saponified with 5 grammes of caustic potash on the
water bath; some water and alcohol being added. After all the alcohol
had been expelled the soap was dissolved in hot water, and the fatty
acids separated from the clear solution by adding hydrochloric acid.
After prolonged heating these acids will swim on the salt solution as
a perfectly clear oil, a portion of which is then put into a little,
narrow, thin walled tube and allowed to solidify. The point at with it
melts and solidifies is determined by putting this tube in a beaker
glass filled with water and warming with a small flame. A thermometer
is placed _in_ the fatty acids and moved gently about during the
observation, and the point accurately observed at which the whole mass
becomes perfectly clear, and also when the mercury bulb begins to be
clouded. It was found that the acids from pure olive oil melt between
26½ and 28½° C. (= 80° to 83° Fahr.) and solidify at a point not lower
than 22° C. (72° Fahr.). The melting point of the fatty acids in the
oils used to adulterate olive oil differs considerably from this. The
melting and solidifying points of the acids in cotton seed, sesame,
and peanut oils lie considerably higher, those of sunflower, rape, and
castor oils decidedly lower than those of olive oil.
The melting and solidifying points of these acids are as follows:
Cotton seed melts at 38.0°C. solidifies 35.0°C.
Sesame do. 35.0 do. do. 32.5 do.
Peanut do. 33.0 do. do. 31.0 do.
Sunflower do. 23.0 do. do. 17.0 do.
Rape do. 20.7 do. do. 15.0 do.
Castor oil do. 13.0 do. do. 2.0 do.
The above figures differ so much from those of olive oil, that
adulteratious carried to the extent that they are in trade can easily
be detected by the aid of an estimation of the melting point, for a
Gallipoli olive oil, mixed with 20 per cent. of sunflower oil, melted at
24° C. and solidified at 18° C. (of course, the fatty acids are meant).
A Nizza oil, mixed with 20 per cent. cotton seed oil, melted at 31½° C.
and solidified at 28° C. A Gallipoli oil with 33-1/3 per cent. of rape
oil melted at 23½° C. and solidified at 16½° C. When 0.50 per cent. of
rape is added, it melts as low as 20° and solidifies at 13½° C., etc.
In testing the solubility of the fatty acids in alcohol and acetic acid,
I employ the method proposed by David (in _Comptes Rendus_, 1878, p.
1416) for estimating stearic acid.
It depends upon the principle that when acetic acid is poured drop by
drop into an alcoholic solution of oleic acid, there comes a time when
all the oleic acid separates, but stearic acid, which is insoluble in
a mixture of alcohol and acetic acid, remains insoluble if the mixture
contains oleic acid.
The following manipulations are adopted in testing olive oil: Equal
parts of glacial acetic acid and water are mixed in a bottle. Then 1
c.c. of pure oleic acid, 3 c.c. of 95 per cent. alcohol, and 2 c.c.
of acetic acid are put in a small tube graduated in tenths of cubic
centimeters. The solution should remain clear; on adding another
one-tenth c.c. of acetic acid it becomes turbid, and when 1 c.c. of
oleic acid (or at first even more) floats on the mixture of acid and
alcohol, the liquid is ready for use. If this is not the case, the
proportions (of acetic acid and alcohol?) must be varied until the
addition of one-tenth c.c. of the former will cause all the oleic
acid to separate. The proportions having been ascertained from
these preliminary experiments, the alcohol and acid are then mixed
accordingly, e.g., 300 of alcohol to 225 of acid. One or two grammes
of stearic acid are added to the alcoholic acetic acid, and the clear
supernatant liquid used for the experiments.
One cubic centimeter of the oil (acids) to be tested is put in the tube,
and 15 c.c. of alcoholic acetic acid added, well shaken, and the whole
left to stand quietly at 15° C. (60° Fahr). If the olive oil is pure,
the acids dissolve to a clear solution that remains so. Cotton seed
oil is insoluble, and the solution obtained by heating the solution
solidifies at 60° Fahr. to a white jelly. Sesame and peanut oil react
in a similar manner. Sunflower oil dissolves, but at 60° a granular
precipitate falls. Rape oil is entirely insoluble and floats like oil on
the surface. Castor oil on the contrary dissolves completely, just like
olive oil, and hence cannot be detected therein by this method. To
detect this oil we must take the melting point of the acids along with
the solubility of the oil itself in alcohol.
Olive oil when mixed with 25 per cent. of cotton seed oil yields a
granular precipitate, and so does 25 per cent. of sesame. Smaller
quantities cannot be detected by these methods. For rape oil the limit
is 50 per cent., and in smaller quantities the oil does not collect on
the alcoholic solution. The decided lowering of the melting point of
the fatty acids in combination with the sulphur reaction, and the
insolubility of the oil in alcohol, also furnish a method of detecting
when present in smaller quantities in olive oil.
Although I am well aware that I am making public a research that is by
no means free from objections, I nevertheless believe that it may be of
use to those who have to undertake the ticklish and intricate analyses
of commercial fats.--_Translated from the Chemiker Zeitung_, p. 355.
Leipsic, Jan., 1883.
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