442 THE CHEMISTBY OF ESSENTIAL OILS
moisture or drought, too much sun, mist, or early autumnal rains will
sometimes change the whole aspect of the crop in a few days.
Bergamot oil is one which is adulterated with considerable skill, the
use of artificial esters being fairly common for the purpose. A compre-
hensive examination is therefore necessary, as the physical characters
can be, and frequently are, standardised to those of a pure oil.
A pure bergamot oil has the following characters :—
Specific gravity
Optical rotation
Refractive index
Acid value
Esters as linalyl acetate.
0-881 to 0-886 (rarely 0-880 to 0-887)
+ 8° „ + 22° (in some seasons to + 25°)
1*4640 to 1-4675
1 „ 4
34 per cent, to 42 per cent, (rarely
to 45 per cent.)
On evaporation of the oil on a water-bath it leaves a non-volatile
residue of 4*5 to 6*5 per cent, (rarely a little outside these limits). This
non-volatile residue should have an acid value of 18 to 30, and an ester
value of 115 to 180.
The above characters may be somewhat altered if fallen, unripe fruits
be used in the preparation of the oil. Oil from such fruits may have a
specific gravity as low as 0-879, and an ester content of 26 to 33 per cent.
Under the name " Nero de Bergamotto " is understood a bastard berga-
mot (Bergamottella), which, if mixed with the ordinary fruits when
pressed, will raise the specific gravity and lower the ester value. The
oil from these bastard fruits has a specific gravity 0*890 to 0*898, and an
ester content of 20 to 35 per cent.
Distilled bergamot oil, obtained by distilling the residues from the
expression process, is of very low odour value, having a specific gravity
as low as 0-865 and an ester content which may fall as low as 5 per cent.
The principal adulterants used to-day consist of artificial esters, with
or without a neutral oil such as lemon terpenes. Esters of the non-
volatile acids, or which are themselves of low volatility will be found in
the evaporation residue of the oil, which if over 6 per cent, will at once
arouse suspicion. The saponification value of this residue will therefore
be increased if such esters are present. If the saponification value is
over 185, it is almost certain, and if it is over 195, quite certain, that such
an adulterant is present. Ethyl citrate is the principal ester of this type
used as an adulterant. One per cent, of triethyl citrate added to the oil
will raise the saponification value enormously—for example, from 175 to
220.
In determining the ester value of the oil, it should be carefully
watched for the first five or ten minutes. The clear liquid will very soon
become turbid, on account of the separation of potassium citrate (which
is not very soluble in alcohol), if citric acid esters are present, and
eventually become clear again. Deniges l has devised a test by which
the presence of citric acid can be positively decided.
> Deniges' test consists in vigorously shaking 10 c.c. of the solution
with 1 to 1*5 grams peroxide of lead, adding 2 c.c. solution of mercuric
sulphate, and filtering. Of the filtrate, 5 c.c. is heated to boiling-point,
after which a 2 per cent, solution of permanganate of potassium is added
by drops, with stirring, until the latter ceases to become decolorised
immediately. If citric acid is present a flocculent, white or pale yellow
precipitate shows itself after the first drop.
1
Bull. Soc. Ph., Bordeaux (1898), 33.