Vegetable Oils

products extracted from oil-yielding raw materials and consisting of 95-97 percent triglycerides (colorless, odorless, and tasteless complete esters of glycerol and fatty acids) in combination with waxes, phosphatides, free fatty acids, lipochromes, tocopherols, vitamins, and other substances that impart color, odor, and taste. The group includes the oils of apricot, peanut, watermelon, beechnut, grape, cherry, mustard seed, muskmelon, castor bean, cedar, coconut, hempseed, coriander, corn, sesame, linseed, poppyseed, cocoa, Crambe, Lallemantia, almond, spurge, olive, walnut, palm, heart of palm, Perilla, peach, sunflower seed, rapeseed, rice, saffron, safflower, plum, soybean, winter cress, tomato, tung, pumpkinseed, and cottonseed.

Properties. The properties of vegetable oils are determined principally by the composition and fatty-acid content of their triglycerides, which usually are saturated and unsaturated (with one, two, or three double bonds) monobasic fatty acids with an unbranched carbon chain and an even number of carbon atoms (mostly C₁₆ and C₁₈). In addition, small quantities of fatty acids with an odd number of carbon atoms (from C₁₅ to C₂₃) have been found in them. The consistency of the oils and their solidification points vary with the unsaturated fatty-acid content.

In liquid oils, which contain more unsaturated acids, the solidification point is usually below zero; in solids, it reaches 40°C. Only the oils of a few tropical plants, such as palm, are among the solid group.

Upon contact with air, many liquid oils undergo oxidative polymerization (“dry out”) to form a film. The oils are grouped according to this tendency to dry out and according to the content of the given predominant unsaturated acid. For example, siccative oils similar to linseed contain mostly linoleic acid.

Castor oil, which contains mostly ricinoleic acid, does not form a film at all.

The density of vegetable oils ranges from 900 to 980 kg per cu m; the refractive index, from 1.44 to 1.48. Vegetable oils are capable of dissolving gases and of absorbing volatile substances and essential oils. An important property of all but castor oil is the capacity to mix, in any proportion, with most organic solvents (hexane, gasoline, benzene, dichlorethane). This property is associated with the slight polarity of the oils (permittivity at room temperature, 3.0-3.2; for castor oil, 4.7). Ethanol and methanol dissolve the oils to a limited extent at room temperature and more so upon heating. The oils are practically insoluble in water. The heat of combustion of the oils is (39.4-39.8). 10³ joules per g, which makes them important high-caloric food products.

The chemical properties of vegetable oils are determined mainly by the reactivity of triglycerides, which can decompose along their ester bonds to form glycerol and fatty acids. This process is accelerated at high temperatures and pressures (reagentless decomposition) and under the influence of an aqueous solution of sulfuric acid and certain sulfonic acids (Twitchell reagent) or petroleum sulfonates (Petrov’s contact). In the body, it is accelerated by the enzyme lipase. Triglycerides undergo alcoholysis, saponification (with aqueous alkaline solutions), acidolysis, ester interchange, and ammonolysis.

An important property of triglycerides is their capacity to add hydrogen at the unsaturated bonds of fatty-acid radicals in the presence of catalysts (nickel, copper-nickel). This property is the basis of the production of hardened (hydrogenated) fats. Vegetable oils are oxidized by atmospheric oxygen to form peroxides, hydroxy acids, and other products. High temperatures (250°-300°C) decompose the oils to acrolein.

Biological value. Vegetable oils are valuable biologically by virtue of the polyunsaturated fatty acids, phosphatides, and tocopherols in them. The greatest amounts of phosphatides are found in soybean oil (to 3,000 mg percent), cottonseed oil (to 2,500 mg percent), sunflower oil (to 1,400 mg percent), and corn oil (to 1,500 mg percent). A high phosphatide content is found only in raw and unrefined vegetable oils.

Sterols are another biologically active component of vegetable oils. The sterol content differs from oil to oil. Wheat-germ oil and corn oil contain as much as 1,000 mg percent and more of sterols; sunflower, soybean, rapeseed, cottonseed, linseed, and olive oils contain up to 300 mg percent; peanut oil and cocoa butter contain up to 200 mg percent; and palm and coconut oils, 60 mg percent or less.

Vegetable oils are completely free of cholesterol. A very high tocopherol content (100 mg percent and more) is characteristic of wheat-bran, soybean, and corn oils. Tocopherols constitute as much as 60 mg percent of sunflower, cottonseed, and rapeseed oils; as much as 30 mg percent of peanut oil; and as much as 5 mg percent of olive and coconut oils. However, the total tocopherol content is not an index of the vitamin value of an oil. Sunflower oil, all of whose tocopherols are a-tocopherols, has the greatest vitamin activity; cottonseed and peanut oils have less vitamin-E activity. Soybean and corn oils are almost completely devoid of vitamin activity, since 90 percent of their tocopherols are antioxidants.

Preparation. The principal means of obtaining vegetable oils are pressing and extraction. The preparatory stages common to both methods are the cleaning, drying, and hulling of the seeds (for example, for sunflower seed, cottonseed). The kernels or seeds are then ground to a paste. Before pressing, the paste is heated to 100°-110°C while mixing and humidifying. The heated mixture, a vegetable pulp, next goes to the screw press. The completeness with which the oil is pressed from the solid residue, or oil cake, depends on the pressure, the thickness of the material to be compressed, the viscosity and density of the oil, and the duration of the operation.

The extraction of vegetable oils is performed in a special apparatus called an extractor, with the aid of organic solvents (most often, extraction gasolines). As a result, a solution of the oil (in its solvent, called the miscella) and a defatted solid residue or oil-seed meal is obtained (saturated with the solvent). The solvent is removed from the miscella and oil-seed meal in distillers and screw-conveyor evaporators, respectively. The oil-seed meal of the principal oil-yielding crops (sunflower, cotton, soy, flax) is a valuable high-protein fodder product. Its oil content depends on the structure of the oil-cake particles, the duration and temperature of the extraction process, the properties of the solvent (viscosity, density), and the hydrodynamic conditions. A combined removal process involves pressing on screw presses before extracting the oil from the oil cake.

Vegetable oils obtained by any method are purified. According to the degree of purification, edible vegetable oils are classified as raw, unrefined, or refined. Oils that are only filtered are called raw. These are the most valuable, since they offer superior flavor and their phosphatides, tocopherols, sterols, and other biologically valuable components are wholly preserved. The unrefined oils are those that are partially purified, by standing, filtering, hydrating, and neutralizing. These oils are of lesser biological value, since some of the phosphatides are removed during the hydration.

Refined vegetable oils undergo a complete system of refining, including mechanical purification (removal of suspended impurities by standing, filtering, and centrifuging), hydration (treatment with a small amount of hot water [to 70°C]), neutralization or alkaline purification (the influence of alkali on oil heated to 80°-95°C), and adsorption refining (in which by treating the oils with adsorptive substances, such as animal charcoal, gumbrin, or fuller’s earth, the pigment matter is adsorbed and the oil becomes lighter and decolorized). Deodorization, that is, the removal of aromatic substances, involves treating the oils with steam under a vacuum.

Refining ensures transparency and the absence of sediment, odor, and flavor. However, refined vegetable oils are less valuable biologically. A considerable amount of the sterols and almost all of the phosphatides are lost. For example, refined soybean oil retains only 100 mg percent phosphatides; raw, it contains 3,000 mg percent. In order to compensate for this deficit, refined vegetable oils are artificially enriched with phosphatides.

The notion that refined vegetable oils better survive prolonged storage has not been confirmed by research. Deprived of their natural protective substances, they have no advantages over unrefined oils in terms of storage. On the other hand, certain vegetable oils require the elimination of harmful impurities. For example, dry defatted cottonseeds contain 0.15-1.8 percent gossypol, a poisonous pigment. Gossypol is entirely removed by refining.

The principal vegetable oils produced in the USSR are (percentage of total fat and oil production, 1969) sunflower (77), cottonseed (16), linseed (2.3), soybean (1.8), mustard-seed, castor, coriander, corn, and tung oils.

Use. The uses of vegetable oils are diverse. The oils are very important food products (sunflower, cottonseed, olive, peanut, soybean oils) and are used in the manufacture of canned goods, confectionery products, and margarine. In industry, they are used in manufacturing soaps, siccative oils, fatty acids, glycerol, and varnishes.

Vegetable oils cleansed of impurities, decolorized, and condensed (predominantly linseed, hempseed, walnut, and poppyseed oils) are used in oil painting as the principal color binder and in tempera (casein) emulsions. They are also used as paint thinners and are components of emulsion primers and oil lacquers. Slow-drying vegetable oils (sunflower, soybean) and those that do not form a film when exposed to air (castor) are used as additives to retard the drying of paints on a canvas (for protracted work on a picture, creating the possibility of clearing and repainting parts of a paint layer) or palette (for prolonged storage).

In medical practice, liquid vegetable oils are used in preparing emulsions (castor, almond) and ointments and liniments (olive, almond, sunflower, linseed). Cocoa butter is used in suppositories. Vegetable oils are also the base of many cosmetics.