chapter  14
Methods for Measuring Oxidative Rancidity in Fats and Oils
Pages 22

I. Introduction.......................................................................................................................... 387 II. Measurement of Oxidative Rancidity.................................................................................. 388

A. Primary Changes.......................................................................................................... 388 1. Changes in Reactants............................................................................................ 388 2. Weight Gain.......................................................................................................... 389 3. Hydroperoxides..................................................................................................... 390 4. Peroxide Value...................................................................................................... 390 5. Active Oxygen and Oil Stability Index=Rancimat Methods ................................ 391 6. Conjugated Dienes................................................................................................ 391

B. Secondary Changes...................................................................................................... 393 1. 2-Thiobarbituric Acid Value................................................................................. 393 2. Oxirane Value ....................................................................................................... 394 3. p-Anisidine Value ................................................................................................. 395 4. TOTOX Value ...................................................................................................... 395 5. Carbonyls .............................................................................................................. 396 6. Hydrocarbons and Fluorescent Products .............................................................. 396

III. Measurement of Frying Fat Deterioration ........................................................................... 397 IV. Recent Developments for Quantitation of Lipid Oxidation ................................................ 398

A. ESR Spectroscopy........................................................................................................ 398 B. Infrared Spectroscopy .................................................................................................. 399 C. Chemiluminescence Spectroscopy............................................................................... 399 D. NMR Spectroscopy...................................................................................................... 400 E. Chromatographic Techniques ...................................................................................... 402

References ..................................................................................................................................... 403

Autoxidation is a natural process that takes place between molecular oxygen and unsaturated fatty acids. Autoxidation of unsaturated fatty acids occurs via a free radical chain mechanism consisting of basic steps of initiation (Equation 14.1), propagation (Equations 14.2 and 14.3), and termination (Equations 14.4 through 14.6). Initiation starts with the abstraction of a hydrogen atom adjacent to a double bond in a fatty acid (RH) molecule=moiety, and this may be catalyzed by light, heat, or metal ions to form a free radical. The resultant alkyl free radical (R.) reacts with atmospheric oxygen to form an unstable peroxy free radical, which may in turn abstract a hydrogen atom from another

(ROOH) and a free radical. The new alkyl free radical initiates further oxidation and contributes to the chain reaction. The chain reaction (or propagation) may be terminated by formation of nonradical products resulting from combination of two radical species.