Experimental cookery

The term rancidity is used by the homemaker to designate the develop- ment 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 ac- cording 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 o£ 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 OXIDATIVE RANCIDITY 551 « 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 vrith 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 decom- position products of rancid fats are carbon dioxide, carbon monoxide, hy- drogen, nitrogen, oxygen, and other gases. Triebold gives a good summary of the products formed in development of rancidity.