ABSTRACT
Autoxidation is a free radical-induced 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 the basic steps of initiation, propagation, and termination. 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 initiators (In∙), including light, heat, or metal ions to form a lipid free radical, also named alkyl radical (R∙). The resultant alkyl radical reacts with atmospheric oxygen to form an unstable peroxyl radical (ROO∙), which may in turn abstract a hydrogen atom from another
I. Introduction .......................................................................................................................... 519 II. Measurement of Oxidative Rancidity ................................................................................... 520
A. Primary Changes .......................................................................................................... 520 1. Changes in Reactants ............................................................................................. 520 2. Weight Gain ............................................................................................................ 521 3. Hydroperoxides ...................................................................................................... 522 4. Peroxide Value ........................................................................................................ 522 5. Active Oxygen and Oil Stability Index/Rancimat Methods .................................. 523 6. Differential Scanning Calorimetry ........................................................................ 524 7. Conjugated Dienes.................................................................................................. 524
B. Secondary Changes ...................................................................................................... 525 1. 2-Thiobarbituric Acid Value ................................................................................... 526 2. Oxirane Value......................................................................................................... 527 3. p-Anisidine Value ................................................................................................... 527 4. TOTOX Value ........................................................................................................ 528 5. Octanoate................................................................................................................ 528 6. Carbonyls................................................................................................................ 529 7. Hydrocarbons and Fluorescent Products ................................................................ 530
III. Measurement of Frying Fat Deterioration ............................................................................ 530 IV. Recent Developments for Quantitation of Lipid Oxidation .................................................. 532
A. Electron Spin Resonance Spectroscopy ....................................................................... 532 B. Infrared Spectroscopy .................................................................................................. 532 C. Chemiluminescence Spectroscopy............................................................................... 534 D. Nuclear Magnetic Resonance Spectroscopy ................................................................ 534 E. Chromatographic Techniques ...................................................................................... 536
References ...................................................................................................................................... 537
unsaturated fatty acid to form a hydroperoxide (ROOH), including conjugated and nonconjugated hydroperoxides, as well as a new alkyl free radical. The new alkyl radical initiates further oxidation and contributes to the chain reaction. The chain reaction (or propagation) may be terminated by the formation of nonradical products resulting from a combination of two radical species.
