Fats are added directly to many products such as crackers, cookies, and cake because of their shortening power; they are added to other products in the form of such ingredients as nut meats, coconut, fatty seeds, milk, and cheese. Prevention of rancidity may become a problem in such baked goods as crackers, cookies, and fruit cake which may be stored for longer or shorter periods; in the cereal industry; in the confectionery industry; in dairy products; in the fat and oil industry; in salad dressings; and in storage of meats, particularly cured meats such as bacon and ham, and in stored frozen fresh meats. Meat, the fat of which contains more unsaturated glycerides, is subject to the development of rancidity more rapidly than fat containing greater percentages of saturated glycerides. Thus frozen poultry and pork present more problems when stored for long periods than beef and mutton.

Types of fat spoilage. The chemical change that needs to occur in a fat before taint is detectable organoleptically is very small. Davies states that butyric and capric acids are detectable by smell and taste in concentrations of less than 80 parts per million parts of fat. Changes in fats can be detected by smell and taste before they can be detected by chemical tests. An isolated fat possessing excellent keeping qualities may show deterioration rapidly after it is combined with other ingredients, and vice versa; but, in general, the fresher the fat and the better its keeping quality, the better the keeping quality of the product with which it is combined.

Different types of spoilage may occur in fats. Davies says "mustiness" is due to microorganic breakdown of higher fatty acids and is accelerated by moisture. Mustiness is common in cereal products, particularly maize. Davies states it is accompanied by an increase in acidity of water extracts, matting of the product because of mold growth, and local spontaneous heating. It is prevented by keeping the humidity sufficiently low.

"Fishiness," according to Davies, is due to the production of trimethyl-amine in the presence of catalysts from lecithin, before the auto-oxidation of the fat proper. Fishiness occurs most commonly in butter. The presence or absence of other products can alter the fishy odor.

The term rancidity is used by the homemaker to designate the development of any disagreeable odor and flavor in fats and oils. But in the fat and oil industry the term is often restricted to the oxidative changes in fats and oils. Different investigators classify the disagreeable odors and flavors according to their production in different ways. Davies gives three types of rancidity as follows: (1) acid, (2) oxidative, and (3) ketonic. Triebold's classification is (1) hydrolytic, (2) oxidative, and (3) ketonic.

Hydrolytic rancidity. Hydrolytic or the acid rancidity of Davies is brought about by the action of lipase enzymes which by hydrolysis split the fat into glycerol and fatty acids. Davies adds that free fatty acids may also be liberated by a relatively high hydrogen-ion concentration in contact with the fat. Lipases are associated with fats in their natural state, i.e., nuts, seeds, milk, and fat of meat. Since lipase enzymes are destroyed by heat, this type of rancidity is encountered in products which are not heated to a high enough temperature to destroy the enzyme. The flavors developed by lipase action depend upon the composition of the fat. Thus flavors caused by butyric acid will be found only in products containing butter fat. Davies states that lipase activity in itself is of no great economic importance, except in the fats rich in the lower fatty acids, but secondary reactions associated with oleic acid introduce another aspect. The free fatty acids act as catalysts for oxidative changes. Greenbank says that lard with a low free fatty acid content keeps well, even when stored for long periods, and butter from sweet cream does not become rancid as rapidly as butter from sour cream. The better-keeping quality of the sweet-cream butter is attributed to its lower free fatty acid content.

Chemical and physical changes in fats with development of oxidative rancidity. Among the changes which occur when a fat or oil becomes rancid are the following: the iodine value decreases, whereas the specific gravity, acid value, and peroxide value increase. Coe states that numerous investigations have shown that when an oil or fat is protected from light by means of a green wrapper or container it may have a peroxide value equal to or even greater than an unprotected fat that has become rancid and still be organoleptically free from rancidity. From this Coe concludes that the reaction that gives rise to the rancid taste and odor has no connection with formation of peroxides.

Oxidative rancidity. Oxidative rancidity occurs through the taking up of oxygen at the double bonds of the unsaturated glycerides. Many oxidative decomposition products may be formed, though Kerr states the exact nature of these changes is not always clear. These products include aldehydes, ketones, fatty acids of lower molecular weight, hydroxy acids, oxy acids, and gases. Andrews has reported that among the gaseous decomposition products of rancid fats are carbon dioxide, carbon monoxide, hydrogen, nitrogen, oxygen, and other gases. Triebold gives a good summary of the products formed in development of rancidity.

Induction period. There is a period before the uptake of oxygen by a fat becomes appreciable which is known as the induction period. During this period the fat is still fresh and "sweet." The induction period varies for different fats and oils and for different samples of the same fat or oil. But oxidation products act as catalysts so that oxygen uptake receives increased momentum as these products are formed. Reports in the literature indicate that the first compounds formed in oxidation of unsaturated fatty acids are not oxides or peroxides. These compounds have never been isolated and have been given the name "moloxides." Exclusion of air or oxygen from a fat may retard but not inhibit its oxidation. It has been suggested that the source of this oxygen is oxygen in loose combination with the fat. It is sometimes suggested that loosely bound oxygen is the source of oxygen for the moloxides.