=277. Nomenclature.=—The terms fat and oil are often used
interchangeably and it is difficult in all cases to limit definitely
their application. The consistence of the substance at usual room
temperatures may be regarded as a point of demarcation. The term fat,
in this sense, is applied to glycerids which are solid or semi solid,
and oil to those which are quite or approximately liquid. A further
classification is found in the origin of the glycerids, and this
gives rise to the groups known as animal or vegetable fats and oils.
In this manual, in harmony with the practices mentioned above, the
term fat will be used to designate an animal or vegetable glycerid
which is solid, and the term oil one which is liquid at common room
temperature, _viz._, about 20°. There are few animal oils, and few
vegetable fats when judged by this standard, and it therefore happens
that the term oil is almost synonymous with vegetable glycerid and fat
with a glycerid of animal origin. Nearly related to the fats and oils
is the group of bodies known as resins and waxes. This group of bodies,
however, can be distinguished from the fats and oils by chemical
characteristics. The waxes are ethers formed by the union of fatty
acids and alcohols of the ethane, and perhaps also of the ethylene
series.[229] This chemical difference is not easily expressed and the
terms themselves often add confusion to the meaning, as for instance,
japan wax is composed mostly of fats, and sperm oil is essentially a
wax.
=278. Composition.=—Fats and oils are composed chiefly of salts
produced by the combination of the complex base glycerol with the fat
acids. Certain glycerids, as the lecithins, contain also phosphorus
in organic combinations, nitrogen, and possibly other inorganic
constituents in organic forms. By the action of alkalies the glycerids
are easily decomposed, the acid combining with the inorganic base and
the glycerol becoming free. The salts thus produced form the soaps
of commerce and the freed base, when collected and purified, is the
glycerol of the trade.
When waxes are decomposed by alkalies, fatty acids and alcohols of the
ethane series are produced.
The natural glycerids formed from glycerol, which is a trihydric
(triatomic) alcohol, are found in the neutral state composed of three
molecules of the acid, united with one of the base. If R represent the
radicle of the fat acid the general formula for the chemical process by
which the salt is produced is:
Glycerol. Acid. Salt. Water.
O.H O.R
C₃H₅O.H + 3R.OH = C₃H₅O.R + 3H₂O.
O.H O.R
The resulting salts are called triglycerids or neutral glycyl
ethers.[230] In natural animal and vegetable products, only the neutral
salts are found, the mono- and diglycerids resulting from artificial
synthesis. For this reason the prefix tri is not necessarily used in
designating the natural glycerids, stearin, for instance, meaning the
same as tristearin.
=279. Principal Glycerids.=—The most important glycerids which the
analyst will find are the following:
Olein, C₃H₅O(O.C₁₈H₃₃O)₃.
Stearin, C₃H₅O(O.C₁₈H₃₅)₃.
Palmitin, C₃H₅O(O.C₁₆H₃₁O)₃.
Linolein, C₃H₅O(O.C₁₈H₃₁O)₃.
Butyrin, C₃H₅O(O.C₄H₇O)₃.
Olein is the chief constituent of most oils; palmitin is found in palm
oil and many other natural glycerids; stearin is a leading constituent
of the fats of beeves and sheep, and butyrin is a characteristic
constituent of butter, which owes its flavor largely to this glycerid
and its nearly related concomitants.
=280. Extraction of Oils and Fats.=—Preparatory to a physical and
chemical study of the fats and oils is their separation from the
other organic matters with which they may be associated. In the case
of animal tissues this is usually accomplished by the application
of heat. The operation known as rendering may be conducted in many
different ways. For laboratory purposes, the animal tissues holding the
fat are placed in a convenient dish and a degree of heat applied which
will liquify all the fat particles and free them from their investing
membranes. The temperature employed should be as low as possible to
secure the desired effect, but fats can be subjected for some time to
a heat of a little more than 100°, without danger of decomposition.
The direct heat of a lamp, however, should not be applied, since it
is difficult to avoid too high a temperature at the point of contact
of the flame and dish. The dry heat of an air-bath or rendering in an
autoclave or by steam is preferable. The residual animal matter is
subjected to pressure and the combined liquid fat freed from foreign
matters by filtering through a jacket filter, which is kept at a
temperature above the solidifying point of the contents.
On a large scale, as in rendering lard, the fat is separated by steam
in closed vats which are strong enough to withstand the steam pressure
employed. For analytical purposes it is best to extract the fat from
animal tissues in the manner described, since the action of solvents
is slow on fat particles enveloped in their containing membranes, and
the fats, when extracted, are liable to be contaminated with extraneous
matters. In dried and ground flesh meal, however, the fat may be
extracted with the usual solvents. For the quantitive determination of
fat in bones or flesh, the sample, as finely divided as possible, is
thoroughly dried, and the fat separated from an aliquot finely powdered
portion by extraction with chloroform, ether, or petroleum. The action
of anhydrous ether on dried and powdered animal matters is apparently
a continuous one. Dormeyer has shown that even after an extraction of
several months additional matter goes into solution.[231] The fat in such
cases can be determined by saponification with alcoholic potash and the
estimation of the free fatty acids produced.
From vegetable substances, such as seeds, the fat is extracted either
by pressure or by the use of solvents. For quantitive purposes, only
solvents are employed. The dry, finely ground material is exhausted
with anhydrous ether or petroleum spirit, in one of the convenient
forms of apparatus already described (=33->43=). In very oily seeds
great difficulty is experienced in securing a fine state of subdivision
suited to complete extraction. In such cases it is advisable to conduct
the process in two stages. In the first stage the material, in coarse
powder, is exhausted as far as possible and the percentage of oil
determined. The residue is then easily reduced to a fine powder, in an
aliquot part of which the remaining oil is determined in the usual way.
[Illustration: FIG. 79.—OIL PRESS.]
In securing oils for physical and chemical examination both pressure
and solution may be employed. The purest oils are secured by pressure
at a low temperature. To obtain anything like a good extraction some
sort of hydraulic pressure must be used. In this laboratory a press
is employed in which the first pressure is secured by a screw and
this is supplemented by hydraulic pressure in which glycerol is the
transmitting liquid. The construction of the press is shown in the
accompanying figure.
The whole press is warmed to nearly 100°. The hot finely ground oily
material, enclosed in a cloth bag, is placed in the perforated cylinder
and compressed as firmly as possible by turning with the hands the
wheel shown at the top of the figure. The final pressure is secured by
the screw shown at the bottom of the figure whereby a piston is driven
into a cylinder containing glycerol. The degree of pressure obtained is
equal to 300 atmospheres.
Even with the best laboratory hydraulic pressure not more than
two-thirds of the total oil contents of oleaginous seeds can be secured
and the process is totally inapplicable to securing the oil from
tissues when it exists in quantities of less than ten per cent. To get
practically all of the oil the best method is to extract with carefully
distilled petroleum of low boiling point.
In the preparation of this reagent the petroleum ether of commerce,
containing bodies boiling at temperatures of from 35° to 80°, is
repeatedly fractioned by distillation until a product is obtained
which boils at from 45° to 60°. The distillation of this material
is conducted in a large flask heated with steam, furnished with a
column containing a number of separatory funnels and connected with an
appropriate condenser. The distillate is secured in a bottle packed
with broken ice, as shown in Fig. 80. A thermometer suspended in the
vapor of the petroleum serves to regulate the process. Too much care
to avoid accidents cannot be exercised in this operation. Not only
must steam be used in heating, but all flame and fire must be rigidly
excluded from the room in which the distillation takes place, and
the doors leading to other rooms where gas jets may be burning must
be kept closed. In the beginning of the process, as much as possible
of the petroleum boiling under 45° must be removed and rejected. The
distillation is then continued until the temperature rises above 60°.
The parts of the distillate saved between these temperatures are
redistilled under similar conditions. Other portions of the petroleum,
boiling at other temperatures, may be secured in the same way. The
products may be in a measure freed of unpleasant odors by redistilling
them from a mixture with lard. When used for quantitive purposes the
petroleum ether must leave no residue when evaporated at 100°.
[Illustration: FIG. 80.—APPARATUS FOR FRACTIONAL DISTILLATION OF
PETROLEUM ETHER.]
=281. Freeing Extracted Oils from Petroleum.=—The petroleum ether which
is used for extracting oils tends to give them an unpleasant odor and
flavor and its entire separation is a matter of some difficulty. The
greater part of the solvent may be recovered as described in paragraph
=43=. Heating the extracted oil for several hours in thin layers, will
remove the last traces of the solvent, but affords opportunity for
oxidation, especially in the case of drying oils. An effective means
of driving off the last traces of petroleum is to cause a current of
dry carbon dioxid to pass through the sample contained in a cylinder
and heated to a temperature of from 85° to 90°. The atmosphere of the
inert gas will preserve the oil from oxidation and the sample will,
as a rule, be found free of the petroleum odor after about ten hours
treatment. Ethyl ether or chloroform may be used instead of petroleum,
but these solvents act on other matters than the glycerids, and the
extract is therefore liable to be contaminated with more impurities
than when the petroleum ether is employed. Other solvents for fats
are carbon tetrachlorid, carbon disulfid, and benzene. In general,
petroleum ether should be employed in preference to other solvents,
except in the case of castor oil, which is difficultly soluble in both
petroleum and petroleum ethers.
=282. Freeing Fats Of Moisture.=—Any excess of water in glycerids will
accumulate at the bottom of the liquid sample and can be removed by
decanting the fat or separating it from the oil by any other convenient
method. The warm oil may be almost entirely freed of any residual
moisture by passing it through a dry filter paper in a jacket funnel
kept at a high temperature. A section showing the construction of such
a funnel with a folded filter paper in place, is shown in Fig. 81.
The final drying, when great exactness is required, is accomplished
in a vacuum, or in an atmosphere of inert gas, or in the cold in an
exsiccator over sulfuric acid. In drying, it is well to expose the
hot oil as little as possible to the action of the air. Wherever
convenient, it should be protected from oxidation by some inert gas or
a vacuum.
=283. Sampling for Analysis.=—It is a matter of some difficulty to
secure a representative sample of a fat or oil for analytical purposes.
The moisture in a fat is apt to be unevenly distributed, and the
sampling is to be accomplished in a manner to secure the greatest
possible uniformity. When the quantity of material is of considerable
quantity a trier may be used which will remove a cylindrical or partly
cylindrical mass from the whole length or depth. By securing several
subsamples of this kind, and well mixing them, an average sample of the
whole mass may be secured. Where the fat is found in different casks
or packages samples should be drawn from each as described above. The
subsamples are mixed together in weights corresponding to the different
casks from which they are taken and the mass obtained by this mixture
divided into three equal portions. Two of these parts are melted in a
dish at a temperature not exceeding 60°, with constant stirring, and
when fully liquid the third part is added. As a rule, the liquid fat
retains enough heat to melt the added quantity. As soon as the mixed
fats begin to grow pasty the mass is vigorously stirred to secure an
intimate mixture of the water and other foreign bodies.[232]
[Illustration: FIG. 81.—SECTION SHOWING CONSTRUCTION OF A FUNNEL FOR
HOT FILTRATION.]
In the case of butter fat the official chemists recommend that
subsamples be drawn from all parts of the package until about 500 grams
are secured. The portions thus drawn are to be perfectly melted in a
closed vessel at as low a temperature as possible, and when melted
the whole is to be shaken violently for some minutes till the mass is
homogeneous, and sufficiently solidified to prevent the separation of
the water and fat. A portion is then poured into the vessel from which
it is to be weighed for analysis, and this should nearly or quite fill
it. This sample should be kept in a cold place till analyzed.[233]
=284. Estimation of Water.=—In the official method for butter fat,
which may be applied to all kinds, about two grams are dried to
constant weight, at the temperature of boiling water, in a dish with
flat bottom, having a surface of at least twenty square centimeters.
The use of clean dry sand or asbestos is admissible, and is necessary
if a dish with round bottom be employed.
In the method recommended by Benedikt, about five grams of the sampled
fat are placed in a small flask or beaker and dried at 100° with
occasional stirring to bring the water to the surface.
According to the method of Sonnenschein, the sample is placed in a
flask carrying a cork, with an arrangement of glass tubes, whereby a
current of dry air may be aspirated over the fat during the process
of drying. When the flask is properly fitted its weight is taken, the
fat put in and reweighed to get the exact amount. The fat is better
preserved by aspirating carbon dioxid instead of air.[234] The moisture
may also be readily determined by drying on pumice stone, as described
in paragraph =26=. In this case it is well to conduct the desiccation
in vacuum or in an inert atmosphere to prevent oxidation.