Vegetable oils have been used for food purposes for centuries. Until the onset of modern civilization, these oils were expressed from seeds with crude apparatus as they were required. This oil was used directly and contained the maximum amount of flavor and associated nutritional factors. Today, in the United States, all vegetable oils are refined before being placed on the market. Contrast this with Spain, where, although huge amounts of olive oil are processed and refined, they are used only for export, the natives preferring the freshly pressed unrefined oil because of its flavor. The whole purpose of refining of oils is the preservation of the oil and the alteration of its appearance, lighter colors being associated with purity under the influence of American advertising. The snow white hydrogenated oils represent the ultimate in this color-purity association.
There can be no doubt that rancidity is a very bad thing. Besides detrimentally altering the odor and flavor of an oil, the products of the oxidation that causes rancidity are detrimental to health. It has been found that the digestion of oils by pancreatic enzymes is clearly retarded by rancidity.1 The products of auto-oxidation of linseed fatty acids were found to be lethal to rats.2 The degenerative diseases caused by rancid oils are undoubtedly brought about by the destruction of vitamins E, F, and carotene, both in the oil itself and in the body.
What is rancidity, and how does it come about? Rancidity is the combination of oxygen from the air with unsaturated fats* in the oil and the subsequent variety of products which occur as a result of this combination. Oxygen is absorbed avidly by oil, where it breaks a double bond to unite with the fatty acid in the oil to form what are termed peroxides. These double bonds are very important as they confer on certain fatty acids the properties which give them vitamin F activity. (See Annual Review of Biochemistry, 1949 for a discussion of vitamin F and its significance, also see Lee Foundation Report Numbers 1, 2, and 3.) The peroxides formed are important too, because they are very reactive, especially in the presence of moisture, and break up into the substances, termed aldehydes, which give the characteristic odor and flavor to rancid oils.
Rancidity in oils follows a specific pattern. First, there is an induction period during which the oil slowly but steadily takes up oxygen to form the peroxides. Then there is an accelerated period during which oxygen is taken up more rapidly as the break down of the oil occurs and the aldehydes are formed. Cutting off the supply of oxygen after the induction period does not prevent rancidity, as the peroxides already formed eventually break up to form the products responsible for rancidity. Therefore, rancidity can form in any oil which has been exposed to the air. As with most chemical reactions, elevated temperatures increase the rate of the reaction.
The problem, then, is to prevent the rancidity which develops in the freshly pressed oils. The conventional approach to this problem is to treat the oils with various chemicals to remove the most easily oxidized materials and to lighten the color. Further treatment with steam and vacuum processes are required to remove the malodorous products formed during the chemical treatment, or already present in inferior oils. A typical refining process may be described as follows. The seeds are steamed and flaked in rollers to break up the fat bearing cells. These are then dried to various degrees, depending on the method of pressing. For edible oils they are usually pressed in hydraulic presses, or expeller presses. During the steaming, flaking, and drying processes, free fatty acids are formed from the action of enzymes contained in the seeds. After the oil has been removed it is treated with alkali to remove these fatty acids. The oil is then washed with water and filtered through an adsorptive material such as charcoal or special clay to remove most of the water and lighten the color. At this stage, the oil is neutral and has a light color, but it is more easily oxidized than the freshly pressed oil3 (because of the loss of vitamin E and other natural stabilizers) so that an anti-oxidant of some kind must be added to improve the keeping qualities. Many chemical substances have been proposed for this use.
Is this extensive processing the only way that vegetable oils can be prepared for food use? Nature stores oil in seeds for long periods of time without any signs of rancidity unless the seeds have become bruised or broken. In addition to the natural antioxidants in the seed, the seed provides the function of keeping light and air from reaching the oil. The stability of oils in peanut butters toward rancidity is reported to be high at the time of manufacture and remains satisfactory even after storage at room temperature, in the absence of light, for two years.4 Oil-bearing seeds contain substances that decompose the peroxides formed by oxidation and which remain in the cake after the oil has been pressed out.5 After the oil has been extracted, it still contains substances which act as antioxidants. Carotenes, vitamin E and phospholipids have been reported to act as antioxidants.6 Sesame oil contains a substance which gives it unusual stability. The destruction of carotenens and vitamin E is accelerated during the steaming process in commercial processing, and the phospholipids are extracted by the alkaline and adsorption treatment.
The oil may contain substances which accelerate oxidation as well as those which retard it, the most common being the presence of aldehydes, free fatty acids, and uncombined metals. Also, ultraviolet rays from the sun cause rapid deterioration of oils. Oil stored in galvanized containers gradually dissolves zinc from the galvanizing which accelerates the development of rancidity.7 It has been found in this laboratory that slight traces of moisture in the oil promotes rancidity, probably because it helps the transformation of peroxides into the noticeable products of rancidity, the aldehydes.. Because it is the aldehydes which are the most responsible for the odor and flavor of rancid oils, and have the most harmful physiological action, they were used as a basis for determining the degree of rancidity in the form of a Fat Aldehyde Value.8 Using this method of analysis, it was found that removing all the moisture from the oil did not prevent the absorption of oxygen, but did delay the rancidity. Storing the oil at lowered temperatures did not prevent oxidation from occurring, but the oxidation process was slowed down to a negligible rate; the amount of oxidation occurring over a period of two months was less than that in oil at room temperature during the first week. It was found that phosphoric acid, which is a preservative for many foods, reacted differently with different oils. With peanut oil it delayed rancidity threefold, with sesame oil it delayed rancidity twofold, and with linseed oil it considerably hastened rancidity. Caution must be used in treating oils with phosphoric acid because during warm weather they pick up moisture rapidly, followed by invasion with bacteria. This happens with the phosphoric acid content as low as 0.01%. The addition of liver fat, containing vitamins E and A, along with phospholipids, proved exceptionally capable of reducing or preventing rancidity in linseed oil. It was found that oil filtered through oat flour was capable of picking up the powerful natural antioxidant present in oats. It was found that absolutely fresh peanut oil, treated in this manner, was kept free from rancidity for a period of three months in the presence of light and air at room temperature. If the oil was allowed to remain in contact with air for a few hours before treatment, the protection afforded by the oat flour was less. If the peanut oil was older than one day, the oat flour treatment afforded no protection at all. The further along the induction period the oil has gone, the less protection antioxidants will afford, because they cannot reverse the oxidation reaction once it has occurred. Although further oxidation may be delayed, the peroxides already formed break down, and the aldehydes formed catalyze further oxidation. Citric, tartaric and ascorbic acids have been used effectively as antioxidants.
From the preceding discussion, it will be seen that the addition of antioxidants cannot make up for careless handling of oils. Vegetable oils, cold pressed from whole high quality seed, and de-moisturized immediately, have excellent keeping quality which can be further enhanced by refrigeration. Of course it is possible to stabilize poor quality or refined oil by the addition of relatively large amounts of chemical inhibitors, but no one has investigated the physiological dangers of the long term use of these chemicals. The use of natural antioxidants may be permissible, but they must be added the minute the oil is removed from the seed to be effective. The last traces of moisture may be removed by filtering through completely dehydrated cellulose, which adsorbs nothing but the moisture–down to 0.01%.
Bottles should be filled completely to the top, stored in the dark under refrigeration,and kept under refrigeration during use if they are not to be used up within one month. Sesame oil can be stored at room temperature for two or three months before any noticeable alteration in flavor takes place.The oil may not be noticeably rancid for quite a long time after these periods, but our aldehyde tests have shown that rancidity is a steady development, and a considerable amount of these products may build up before the rancidity is actually noticeable.
To produce a high grade oil, containing all the nutritional factors originally present in the oil, is not easy, but, we feel, well worth while.
Note:
*Unsaturated fats are those in which the chemical bonds present are not fully satisfied. These bonds are very reactive toward oxygen and many other elements. They are designated as double bonds.
References Cited:
- Comp rend 239, 1483-9 (1954). (C.A. 50, 4742i)
- J Biochem (Japan) 42, 561-73 (1954) (C.A. 50, 1141g)
- Suomen Kemistilehti 28B, No. 3, 113-16 (1955), (C.A. 49, 15113c)
- Food Technol. 8, 101-4 (1954)
- Comp rend soc biol, 150, 216 (1956), (C.A. 50, 16139d)
- Chem Zentr, 1951, I, 2890 (C.A. 48, 9081g)
- Bull Central Food Technol Research Inst, Mysore 3, No. 1, 17-18 (1953) (C.A. 48, 7320i)
- Ind Engr Chem, Anal Ed, 4, 204-8 (1932).
