I. Nature and Sources of Fats and Oils
Chemical and Physical Characteristics
THE present study deals only with such oils and fats as are capable of serving as foodstuffs, even though in practice they are not put to such use. The common chemical characteristic of such oils and fats is that they may be decomposed into glycerin and one or more acids of the class known to chemists as fatty acids. (Chemists designate as acids a class of substances which have an acid or sour taste; contain the element hydrogen; and act upon metals, hydrogen being evolved and its place being taken by the metal. The compound thus formed with the metal is known as a salt.) The common physical properties of such oils and fats are that they float on water but are not soluble in it; they are greasy to the touch, and have lubricating properties; they are not readily volatile; and may be burned without leaving any residue, i.e., ash. No other class of substances has the chemical properties of the fats and oils; but many possess similar physical ones, e.g., mineral oils, earth-wax (ozocerite), paraffin, animal waxes like spermaceti or beeswax, vegetable waxes like carnauba or candelilla wax, volatile or essential vegetable oils like the oils of thyme, of cloves, of cedar, and attar of roses. None of these substances furnishes both glycerin and fatty acids; none of them has nutritive value; none of them will be considered further in this treatise.
Fats and oils, then, in the restricted meaning in which these two words are used hereafter, are substances which consist always of chemical combinations of glycerin with certain fatty acids, and which may serve as foods.
The distinction between a fat and an oil is purely an accidental one depending upon the environment in which the substance happens to be placed. If the substance is solid at ordinary temperatures, it is termed a fat; if fluid, an oil. This is merely a distinction of convenience, since all oils are solidified at lower temperatures and all fats melted at higher temperatures. Obviously, the dividing line that holds for a cool climate would not hold for a hot one. In each climate, however, the distinction is of importance in industrial and in culinary uses; it has also some importance in nutrition, since fats are somewhat less digestible than oils. In this study fat is often used indiscriminately for a solid or a liquid substance of the class here under consideration.
Animal and Vegetable Sources
Animal fats and oils are derived both from terrestrial and marine animals. Marine fats include liver oils, blubber oils, and fish oils. In addition, from certain marine animals waxes are obtained, e.g., spermaceti, which, because it is a wax and not a fat, need not be considered here. The different types of marine fats, which in practice are often mixed, have been of great importance in the past and still possess considerable significance. Some of these serve special purposes, such as codliver oil; others are used to some extent as foodstuffs; but for the most part they serve industrial uses.
With two important exceptions animal fats are obtained from carcasses. These two exceptions are butter and the fat of the yolks of eggs. Carcass fat is found in different locations. There is a good deal of it in the visceral cavities and in and around the viscera. More or less of it occurs in the muscles, in the connective tissue, under the skin, and in the bones. The proportions found in the different parts of the body vary from species to species and in any given species with the age of the individual animal and its condition. The fats from the different parts and organs of a given animal differ somewhat in their properties. As a rule, the fat from the interior of the animal is somewhat firmer than the fat from near the body surface, i.e., it melts at a somewhat higher temperature. Furthermore, under certain conditions the feed of the animal affects the physical properties of the carcass fat more or less. Animals fattened upon a diet containing much oil -- for example, peanuts -- tend to produce softer carcass fats than animals of the same species fattened upon a diet containing relatively little oil -- for example, corn (maize). Animals form fairly specific fats from starches but deposit in their tissues unchanged such portion of the fats and oils of the ration as is not promptly oxidized.
Vegetable fats and oils are found in greatest abundance in fruits and seeds. While fats and oils do occur in the roots, stalks, branches, and leaves of plants, they are rarely present in these organs in quantities large enough for commercial purposes. In some seeds and fruits, however, the fat content is great -- in several cases as high as 35 per cent; in dried coconuts 65 per cent -- and these are the commercial sources of vegetable fats. In some seeds the fat is practically confined to the germ or embryo; this is the fact in most of the cereals. (The germ or embryo is that part of the seed which gives rise to the plant when the seed germinates. It is usually only a small part of the seed. The remainder of the seed consists mainly of reserve food material with the help of which the embryo grows into the plantlet which, as it develops roots, is enabled to draw its nourishment from the soil.) The olive contains a large amount of fat in the pulp surrounding the kernel and only a smaller amount in the kernel itself, while in the oil palm both the pulp and the kernel contain large amounts. The fat from the pulp may have characteristics quite different from those of the fat in the kernel.
Edible and Inedible Fats
In commerce a distinction is commonly made between edible and inedible fats, based either upon external characteristics, such as unattractive color, taste, or odor, or upon sentimental considerations, such as revolting origin (from garbage, for example), decomposition, or the possibility of contamination with a poisonous substance or with the germs of disease. The distinction between edible and inedible fats is nevertheless a purely practical one, for with modern methods nearly all fats can be refined or modified to the point of physiological edibility. That the distinction exists at all is because it is either unprofitable to convert inedible into edible fat to a greater extent than is done or else because such conversion is not permitted for sanitary reasons.
Sanitary considerations are a more important factor deterring the transformation of inedible into edible fats in the case of animal than in the case of vegetable products, for animal fats may be treated as inedible, even if they are not repulsive to the senses, because their origin is revolting. This is the case when they are obtained from animals that have died otherwise than by slaughter, i.e., from disease, old age, or accident. Such fats are not permitted by health authorities to be used for food purposes because of the danger of transmitting disease, though in former times some fat of this sort was unquestionahly so used. Because of the danger of disease transmission, fat from animals killed by slaughter is not permitted to be used for food if inspection of the carcass shows that the animals were diseased. In most countries inspections have been established in slaughterhouses to protect the consumer from this danger. The degree and effectiveness of such inspections vary in different sections and in different countries. In the United States, food fats remain subject to the provisions of federal and local food laws after they are shipped out of the inspected slaughterhouse. They may, therefore, if officials deem it necessary, again be inspected at any time on their way to the consumer.
So far as is known to date, there is little or no danger of the transmission of diseases of plants to man. Therefore, governments have not thought it necessary to supervise the production of vegetable fats in the same rigid manner in which fat production from animal carcasses is controlled. The only sanitary control over vegetable oils that exists, aside from the very special control applied to the manufacture of margarin, is the general control exercised by the several food laws over vegetable oils in common with all other foodstuffs. This form of control concerns itself principally with adulteration, with decomposition (rancidity), and with various types of misbranding. Vegetable oils, in common with nearly all other foodstuffs, may transmit disease if handled by infected persons. Such occurrences are extremely rare and of a type not at present controllable by food officials.
Drying and Non-drying Oils
In commerce the distinction between edible and inedible fats is not the only one that is made. A different but equally important distinction is drawn between the drying and the non-drying oils. The two kinds of distinction are not comparable, for both non-drying and drying oils may be either edible or inedible. Thus in Europe linseed oil (a typical drying oil) is used for food, whereas in America it is not now so used -- for one reason because it is too expensive as compared with non-drying oils which are readily available in abundance.
Drying oils absorb oxygen from the air and are thereby converted into plastic, elastic, resin-like substances. Hence, when exposed in a thin layer, as in painting, they form a tough, elastic, waterproof film which adheres tightly to the painted surface and protects it from the weather. The two chief drying oils are linseed oil and tung (chinawood) oil, which find wide use in the manufacture of paints, varnishes, artificial rubber, linoleums, and other coverings.
The non-drying oils find a wide variety of industrial uses: they enter into soaps and cleansers, cosmetics, lubricants, leather dressings, and candles. They are used in the processes of wool manufacture, especially carding; they are employed in making tin plate and in foundry work. Fats and oils, whether edible or inedible, drying or non-drying, have still other industrial uses. They were the primitive illuminants and are still so used in a relatively slight degree. Some of the industrial uses of the fats and oils depend upon their physical properties, others upon their chemical character, to which detailed reference will be made later.
Between the drying oils and the non-drying oils is a group of oils which, while they possess the property of absorbing oxygen, do not do so sufficiently to qualify them as drying oils. They are changed more or less when exposed to the atmosphere, but not as completely as linseed, tung, and certain other drying oils. They are often termed semi-drying oils. All gradations are found between completely drying oils and completely non-drying oils. Soy bean oil and corn oil are examples of semi-drying oils. When exposed to the atmosphere in a thin layer they thicken but do not form a hard, dry film. The film remains sticky and somewhat runny -- tacky is the word often used. Under some circumstances appropriate amounts of semi-drying oils are mixed with full-drying oils to make paints of cheaper grades.
Deterioration of Fats and Oils
Fats and oils are quite unstable substances. When stored for any considerable length of time, especially when the temperature is high and the air has free access to them, they deteriorate and spoil. In this respect different fats differ markedly. Some spoil very much more rapidly than others. Among the various fats, spoilage takes the form of rancidity. The fat acquires a peculiarly disagreeable odor and flavor. A vast amount of scientific research has been carried on to determine the cause and nature of rancidity, but investigators are far from agreement on the subject. For present purposes it is sufficient to point out that spoilage of a fat, usually identical with rancidity, is accompanied by partial splitting of the fat into glycerin and fatty acids. The glycerin disappears, or at any rate is unobjectionable, but the fatty acids remain dissolved in the fat, give it an acid reaction, and contribute to its objectionable rancid flavor.
The rancidity of a given parcel of fat is not necessarily the result of long storage under unfavorable conditions. The fat may have been spoiled and rancid from the moment of its production. This will inevitably be true when the materials from which it was produced have undergone decomposition. Thus the fat obtained from putrefying carcasses will be rancid and so will the oil expressed from fermented cottonseed. In other words, to obtain a sound and sweet fat, the raw material must be sound and sweet; it must be worked up speedily before it has had time to decompose; and this must be done under clean and sanitary conditions. The fat thus obtained must be stored under favorable conditions and its consumption cannot be too long delayed. These conditions it is difficult to obtain in many of the less civilized portions of the world, especially in the tropics, where many fat- and oil-yielding raw materials are produced. Hence fats and oils made at the source of the raw materials may be less sound than those produced at or near the place of consumption.
The fact that so great a proportion of the fat supply, especially vegetable oils, is or becomes rancid and decomposed, necessitates refining, decolorizing, and deodorizing. Certain oils -- for example, cottonseed oil -- require refining even when they are not decomposed, because they contain certain impurities and are of dark color. The act of refining is not merely an item of expense, but in the case of decomposed fats it involves the removal of the fatty acid contaminating the fat or oil. The effect is that the yield of refined oil is less than the crude oil with which the operation was begun. This disappearance of material is known in the trade as the refining loss . Hence the trade in fats, oils, and greases specifies the amount of free fatty acids permissible in them. If the amount exceeds specifications, price adjustments are commonly demanded and conceded. If the refining loss is too heavy to make refining economical, the fat is commonly consigned to the soap kettle. The past history of a fat -- whether or not it was ever decomposed -- in some way not clearly understood, affects its keeping quality after refining. Such fats require especially great care and skill in refining, deodorizing, and decolorizing to insure reasonable keeping qualities. It may not be profitable to refine a fat even though the refining loss be only moderate.
Next: II. Properties of Fats and Oils
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