This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Prooxidants and antioxidants. A prooxidant accelerates but an antioxidant retards oxidation of a fat. Because of the economical importance of prevention of rancidity many investigations have been conducted to find physiologically harmless antioxidants. The literature on rancidity is voluminous and many contradictions occur, particularly in regard to the activity of different substances as accelerators or retarders of oxidation. The contradictions can be attributed to the lack of suitable tests for minute quantities of some substances, the different conditions under which the tests are made, and the differences in degree of purity or refinement of the fats and oils. Fats and oils always contain at least traces of different substances in varying proportions, some of which can act as prooxidants and others as antioxidants. Patents have been granted for the use of many substances as antioxidants.
Metals. Traces of certain metals (King, Roschem, and Irwin) may act as catalysts, copper in particular being an active prooxidant. Tin had the least effect of any metals used in the experiments, although the authors state the results were not conclusive in that tests were not made under many conditions. The effect of metals is important, for in the manufacture, refining, and storage of fats they come in contact with metals.
Lecithin. Lecithin, found in egg yolk, soybeans, and other substances added to baked products, has been reported to be a powerful antioxidant and inactive. Olcott and Mattill say that commercial "lecithin," which is not a pure lecithin, has antioxidant action on refined cottonseed oil, little effect on lard, and none at all on lard-cod liver oil mixtures. They attribute the antioxidant properties to the cephalin found in the commercial lecithin. It is well known that egg yolk has antioxidant properties in cookies, cakes, and similar products in which both egg yolk and fat are used.
Cereals. Soybean flour (Musher) is said to be strongly antioxidant. Oat, corn, barley, or other cereal may be employed but oat flour is preferable because of its bland flavor. Wheat flour is a weak retarder of oxidation and oat flour is not as strong an antioxidant as soybean flour.
Effect of inert gases and humidity. Triebold and Bailey found that crackers as well as fats stored in an atmosphere of CO2 were as rancid as control samples stored in air. Other investigators have also reported the ineffectiveness of CO2 in retarding the development of rancidity. However, storage in an atmosphere of nitrogen or hydrogen retards the development of rancidity.
Triebold and Bailey also found that zero-per cent relative humidity favors, but some humidity retards, oxidative rancidity. However, humidity as high as 50 to 75 per cent produces mustiness. The 5- to 6-per cent moisture content of crackers aids in keeping the crackers fresh. They also found that in general the keeping quality of crackers was related to the length of the induction period of the shortenings used in them, but several outstanding exceptions to this relationship were noted, so that the keeping quality was more closely related to the induction period of the crackers themselves.
Antioxidants for fats in contact with water. Lea was interested in finding antioxidants for bacon fat. He investigated the effect of different sub-stances on fat in contact with a brine pickle. His results follow. Oxidation of lard in a glass vessel in contact with water is much more rapid at an alkaline than at an acid pH. It is suggested that this is due to dissolution of copper from the glass. At a pH below 5, nitrate is a powerful accelerator. Copper and iron in aqueous solution accelerate the oxidation of lard, copper being approximately 20 times as active as iron. Copper was active in as low concentrations as 0.01 p.p.m. Aliphatic polyhydroxy-compounds such as glycerol and the sugars are weak antioxidants. Aliphatic hydroxy-acids, e.g., lactic and glycollic, the ethanolamines and maleic acid are moderate, and polybasic hydroxy-acids, e.g., tartaric and citric, are powerful antioxidants. Aliphatic amino-acids, e.g., glycine, aspartic and glutamic acids, and asparagine are all powerful antioxidants. Protein has considerable antioxidant activity and may be partly responsible for the stabilization of crude natural oils and fats. The prooxidant effect of copper at low concentrations (up to approximately 1 p.p.m.) is completely inhibited by protein and other antioxidants. At higher concentrations of the metal, even 4 per cent of protein fails to prevent a powerful acceleration of oxidation. Orthophosphoric acid is a fairly good, and pyrophosphoric a very good, antioxidant. Phosphorous acid is also effective. These water-soluble antioxidants still retard oxidation when the water content is as low as 0.25 per cent. They are active at pH 7, certainly in more acid and probably in more alkaline solutions.
Volatilization of rancid products. Roschem and Newton found that samples of rancid lard blown with steam at 100°C. were odorless, bland-tasting, and entirely palatable, i.e., the products causing the rancid flavor and odor were volatile. Furthermore these volatile substances had no accelerating effect in oxidation of lard. This was interpreted to mean that the accelerating effect is due to a substance or substances of heavy molecular weight not volatile, with steam distillation at 200°C. and 3 to 5 mm. mercury pressure.
Protective action of green light. Coe and Le Clerc compared the inhibiting effect of excluding all light, using only green light of 490 to 580 mu wave length or 4900 to 5800 Angstrom units, with the effect of several antioxidants, including maleic acid and pyrogallol, upon oxidation of fats. They report that oils or fats stored at low temperatures and with all light excluded remained fresh longer than if treated with antioxidant or packaged in green wrappers and exposed to light at room temperature. Packaging in green glass or opaque wrappers was more effective in keeping fat fresh than the addition of the antioxidants. Green glass or opaque wrappers which exclude all light except the aforementioned wave lengths are suitable for packaging nuts, potato chips, or food products for display purposes.
Ketonic rancidity. Davies states this type of fat spoilage is caused by molds which attack the short-chained saturated fatty acids, the decomposition products producing disagreeable flavors. Since butter and coconut oil are the principal fats which contain short-chained acids in the glycerides, most ketonic rancidity is confined to these fats.
Effect of light, heat, and air. It has been known for years that exposure of fats to light, heat, and air is deleterious to keeping the fat fresh. Greenbank investigated the relative effect of each of these factors. He concludes that in the presence of air, light is the most powerful accelerator, and moisture and heat follow in the order given. "This conclusion holds only for lower temperatures." A vacuum may retard oxidation but not prevent it.
Triebold and Bailey found that all hydrogenated shortening samples stored in unopened containers showed induction periods of approximately the same length. Samples from containers once opened and then restored had shorter induction periods. A lard sample stored in the laboratory for a year with air excluded appeared in as good condition at the end as at the start of the experiment. They emphasize the practical importance of preventing undue exposure of shortenings to air and decreasing the surface area of the fat exposed to air.
Home storage of fats and oils. All the work on development of rancidity in fats and oils indicates that the fat should be stored so that light is excluded, at a low temperature, and with as little exposure to air as possible. For farm storage of lard, when 5, 10, or more gallons are stored in one container, the light should be excluded and Triebold and Bailey's results suggest a tall narrow container, rather than a wide flat one, for the former exposes a smaller surface of the fat to the air. When lard is removed from these containers it is preferable to remove a level layer rather than to dig out the center portion.