Part 57

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°.

=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.

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.

=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. 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.

PHYSICAL PROPERTIES OF FATS.

=285. Specific Gravity.=—The specific gravity of an oil is readily determined by a westphal balance (=53=), by a spindle, by a sprengel tube, or more accurately by a pyknometer. The general principles governing the conduct of the work have already been given (=48-59=). The methods described for determining the density of sugar solutions are essentially the same as those used for oils, but it is to be remembered that oils and fats are lighter than water and the graduation of the sinkers for the hydrostatic balance, and the spindles for direct determination must be for such lighter liquids. The necessity of determining the density of a fat at a temperature above its melting point is manifest, and for this reason the use of the pyknometer at a high temperature (40° to 100°) is to be preferred to all the other processes, in the case of fats which are solid at temperatures below 25°.

When great delicacy of manipulation is desired, combined with rapid work, an analytical balance and westphal sinker may be used conjointly. In this case it is well to have two or three sinkers graduated for 20°, 25°, and 40°, respectively. Nearly all fats, when melted and cooled to 40°, remain in a liquid state long enough to determine their density. The sinkers are provided with delicate thermometers, and the temperature, which at the beginning is a little above the degree at which the sinker is graduated, is allowed to fall to just that degree, when the equilibrium is secured in the usual manner. The sinker is conveniently made to displace just five grams of distilled water at the temperature of graduation, but it is evident that a round number is not necessary, but only convenient for calculation.

=286. Expression of Specific Gravity.=—Much confusion arises in the study of data of densities because the temperatures at which the determinations are made are not expressed. The absolute specific gravity would be a comparison of the weight of the object at 4°, with water at the same temperature. It is evident that such determinations are not always convenient, and for this reason the determinations of density are usually made at other temperatures.

In the case of a sinker, which at 35° displaces exactly five grams of water, the following statements may be made: One cubic centimeter of water at 35° weighs 0.994098 gram. The volume of a sinker displacing five grams of water at that temperature is therefore 5.0297 cubic centimeters. This volume of water at 4° weighs 5.0297 grams. In a given case the sinker placed in an oil at 35° is found to displace a weight equal to 4.5725 grams corresponding to a specific gravity of 35°/35° = 0.9145. From the foregoing data the following tabular summary is constructed:

Weight of 5.0287 cubic centimeters of oil at 35°, 4.5725 grams. ” ” 5.0297 ” ” ” water at 35°, 5.0000 ” ” ” 5.0297 ” ” ” ” ” 4°, 5.0297 ”

Relative weight of oil at 35°, to water at 35°, 0.9145 grams. ” ” ” ” ” 35°, ” ” ” 4°, 0.9092 ”

=287. Coefficient of Expansion of Oils.=—Oils and fats of every kind have almost the same coefficient of expansion with increasing temperature. The coefficient of expansion is usually calculated by the formula

D₀ - D₀ʹ δ = ---------------- (tʹ - t)D₀

in which δ represents the coefficient of expansion, D₀ the density at the lowest temperature, D₀ʹ the density at the highest temperature, t the lowest, and tʹ the highest temperatures.

In the investigations made by Crampton it was shown that the formula would be more accurate, written as follows:

D₀ - D₀ʹ δ = --------------------------- (tʹ - t) × D₀ + D₀ʹ --------------- 2

The absolute densities can be calculated from the formula Δ = δ + K, in which Δ represents the coefficient of absolute expansion, δ the apparent coefficient of expansions observed in glass vessels, and K the cubical coefficient of expansion of the glass vessel. The mean absolute coefficient of expansion for fats and oils, for 1° as determined by experiment, is almost exactly 0.0008, and the apparent coefficient of expansion nearly 0.00077.

=288. Standard of Comparison.=—In expressing specific gravities it is advisable to refer them always to water at 4°. The temperature at which the observation is made should also be given. Thus the expression of the specific gravity of lard, determined at different temperatures, is made as follows:

15°.5 40° d ------ = 0.91181; d ----- = 0.89679; 4° 4°

100° and d ---- = 0.85997, 4°

indicating the relative weights of the sample under examination at 15°.5, 40°, and 100°, respectively, to water at 4°.

=289. Densities of Common Fats and Oils.=—It is convenient to have at hand some of the data representing the densities of common fats and oils, and the following numbers are from results of determinations made in this laboratory: