280 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS This free radical can strip a hydrogen atom from another molecule of methyloleate to form a hydroperoxide and thus complete a chain of reac- tions leading to a primary oxidation product. 11 10 9 8 --CH--CH=CH--CH2-- I OOH Further reactions occur involving breakdown of the molecule with forma- tion of aldehydes, ketones, and acids. Second proposition: That the chemical methods now commonly used for the detection and measurement of rancidity constitute an art and not a science. There are several tests which reveal the presence of peroxides, for ex- ample, the liberation of iodine from potassium iodide, the oxidation of ferrous salts or stannous chloride. The hydroperoxides, however, build up to a point and then break down into secondary products as rancidity progresses. The concentration of peroxide oxygen therefore bears no pre- cise relationship to the stage of deterioration of the rancid material. The popular Kreis test in which the sample is brought into contact with an ethereal solution of phloroglucinol in the presence of hydrochloric acid is known to give a positive colour reaction with a number of aldehydic substances, for example, epihydrin aldehyde and malonic dialdehyde which are not known to be and are not considered to be autoxidation prod- ucts of fatty acids (2). The Kreis test gives a positive reaction with rancid fatty matter but what it detects is not known. Malonic dialdehyde also produces a colour reaction with thiobarbituric acid (3) and a similar reaction is produced by rancid fatty matter (4). Although this test is described as more sensitive than the Kreis test we still do not know what it denotes. Other tests are directed to distinguish a-dicarbonyl compounds (5). So tar as soaps are concerned these tests would be carried out on the fatty acids prepared by decomposition with mineral acids. I suggest as other possibly useful tests in the same category as those I have mentioned, first, a reaction with vanillin in presence of concentrated hydrochloric acid (6). A strong pink coloration is produced by a rancid fat and the test is quite delicate. What it detects is again not known. Second, hydroperoxides may be detected in soaps by adding to an aque- ous solution a dilute alcoholic solution of phenolphthalein which has been reduced by zinc dust in caustic soda. A little free caustic soda should be added. Peroxides produce the usual pink coloration with phenolphthalein which they reconstitute by oxidation (7). The tests commonly used are empirical and it is not known exactly what they mean. Their application is therefore an art and not a science. Third proposition: That the measures adopted to ensure against the develop- ment of rancidity constitute an art and not a science,

RANCIDITY IN SOAPS 281 The following points require consideration in the case of soaps: 1. Choice of suitable oils and fats. We know that there should be no significant proportions of soaps of linoleic, linolenic, and more highly un- saturated acids and that unsaponifiable matter should not have a high io- dine value. Suitable refining is required for example, to remove mucilagi- nous and protein matter which might cause microfrganisms to thrive in the soap. When hydrogenated oils form part of the mixture the characteristic hydrogenation odour must be adequately removed, because the recurrence of this odour in the soap produces a pseudo rancidity off-odour. 2. Avoidance of metallic contamination during processing especially with metals like copper, cobalt, manganese, iron and nickel which act as oxida- tion catalysts. 3. Correct processing. The practice of superfarting probably affects rancidity stability both with respect to the amount of free fat and the mode of adding it. On the mechanical side it has been considered good practice to chill soap rapidly in films on internally chilled rollers with free access of air, but it is now, I think, becoming increasingly recognised that this aggravates the liability to rancidity, and frame cooling is preferred. Under this heading milling and plodding can lead to the inclusion of much air in the soap. It appears to me to be sound practice to carry out these operations under an atmosphere of carbon dioxide so that air is excluded from the mill and plodder. 4. Colouring agents and perfumes require careful selection to avoid secondary deterioration in colour and odour during storage. 5. Soap should be carefully protected fi'om light and dampness during storage and the temperature should never be unduly high. 6. It is possible to delay the development of rancidity and pseudo rancidity by incorporating in the soap either an inhibitor of rancidity or an antioxidant. I distinguish an inhibitor as either a reducing agent such as sodium stannite, sodium hydrosulfite, or sodium formaldehyde sulfoxy- late or an antiseptic such as sodium benzoate, the esters of p-hydroxyben- zoic acid or thymol. I distinguish an antioxidant as a substance capable of breaking the chain reaction of autoxidation at concentrations as low as a few hundredths or thousanths of one per cent. Interruption of auto- oxidation can be brought about by a substance which can readily provide a hydrogen atom, more readily than, for example, methyl oleate, in the case we have examined. The peroxidic free radical will then attack the anti- oxidant rather than the oleate. For example, consider an antioxidant AH•. It reacts with the methyl oleate peroxidic free radical to form a hydroperoxide and a new free radical from the antioxidant. --CH-- + AH2-'---CH-- + AH O--O-- OOH