RANCIDITY IN SOAPS* By W. W. MYDDLETON, D.Sc., F.R.I.C. County Laboratories, Ltd., Stanmore, Middlesex, England To is•'RoI)vCV. a discussion on rancidity in soap, I put forward four propositions which I shall describe and explain in turn. My first proposition is as follows: That any theory of rancidity more than twelve years old should be put upon the shelf with the Theory of Phlogiston. Rancidity in soap is recognised by the appearance of discoloured patches on the surface and by the odour better known as the odour of rancid fat. A so-called tasteless soap also develops a strong flavour when it becomes rancid. As to what goes on in the soap, little is known by direct observa- tion on soap as such little more perhaps than that atmospheric oxygen is absorbed and, as one would expect, flake or powdered soap, presenting a larger surface to the air, becomes rancid more rapidly than bar or tablet soap under similar conditions of storage. The process is described as autoxidation because it takes place at room temperature and the operative agent is molecular oxygen. For other clues as to what happens in soap we must turn to the analogous phenomena in fatty acids and their esters. It Js in this field that most of the fundamental work on rancidity has been carried out. This is partic- ularly fortunate because, in the highly purified acids and esters, impurities which act in the natural oils and fats and in the soaps made from them as pro- or antioxidants have been eliminated. From this work we know that the saturated acids and their esters are stable at room temperature unless attacked by microbrganisms. On the other hand, oleic, linoleic, linolenic, and the more highly unsaturated fatty acids and their esters, oxidise with increasing ease in the order given. Let us consider the case of methyl oleate. It has one double bond at the middle of the carbon chain and its iodine value is normal for that condition. The molecule can be broken at the double bond by disruptive oxidation with potassium permanganate in acetone and Jt then gives a mixture pro- viding the monobasic nonoic acid and the dibasic acid. * Presented at the April 9, 1954, Meeting, London, England. 278

RANCIDITY IN SOAPS 279 11 10 i 9 8 --CH2--CH--CH--CH2-- Nonoic i Azelaic The carbon atoms are numbered from the carboxyl group. The acids derived from the rupture at the double bond thus contain nine carbon atoms each. When rancidity has progressed in methyl oleate the iodine value is lowered and the false inference has been drawn that oxygen adds on at the double bond to form a peroxide. The iodine value is misleading and if saturation is measured by the volume of hydrogen absorbed in the pres- ence of a suitable catalyst the double bond is shown to be substantially in- tact. Furthermore when the rancid ester is disruptively oxidised with per- manganate not two but four acids are produced, two monobasic and two dibasic, namely octoic and nonoic, suberic and azelaic acids. They can be accounted for on the assumption that in autoxidation the attack by oxygen is directed to either of the methylene groups adjacent to the double bond at atoms 8 and 1 ] and that hydroperoxide groups become attached there. With the autoxidation effective at the 11th carbon atom further disrup- tive oxidation with permanganate will cause fission between atoms 9 and 10 and also between 10 and 11 and one carbon atom will disappear from the high molecular weight products. On the left we shall have octoic acid and on the right, azelaic. 11 10i 9 8 --CH--CH--CH--CH2-- I : OOH Octoic Azelaic In the part of the rancid ester with the hydroperoxide group at the 8th carbon atom the products will be nonoic acid (monobasic 9 carbon atoms) and suberic acid (dibasic 8 carbon atoms). The evidence I have outlined was provided by publications under the auspices of the Rubber Producers Research Association in 1942, twelve years ago, and forms the starting point for the newer theories of rancidity (1). The most satisfactory theory is that the first stage of the oxidation process is the formation of a free radical under the influence of residual free radicals or radiation, by loss of a hydrogen atom from a methylene group adjacent to a double bond. Molecular oxygen then forms a l•eroxidic free radical. 11 10 9 8 --CH•CH--CH--CH•-- Ill 10 9 8 --CH--CH•-CH--CH•-- • ß .