ABSTRACT: 13 C Nuclear magnetic resonance spectroscopy has been applied to elucidate the mechanism of lipid oxidation oc- curring during thermal treatment of fish. Effects of temperature and time of processing have been studied by means of a model system of lipids, extracted from salmon (Salmo salar) muscle, to simulate industrial conditions of canning. Unsaturated fatty acids located at the sn-2 position of the glycerol moiety were the most prone to oxidative damage. Regarding the mechanism of the reaction, results inferred from olefinic and methylenic res- onances indicated a higher susceptibility of the allylic sites clos- est to the carbonyl group, followed by those placed near the methyl terminal group. Unsaturations located in the middle of the carbon chain did not show much damage. The glyceryl re- gion provided an unusual resonance at 53.4 ppm, which could be assigned to a hydroxylic compound formed during process. JAOCS 75, 147–154 (1998). KEY WORDS: 13 C NMR spectral regions, fish lipids, oxidation, thermal processing. Many studies have described the important role that alter- ations of lipids during processing of foods have on the qual- ity of the final product (1,2). Lipid damage is often focused on the reactivity of polyunsaturated fatty acids (PUFA) be- cause it can produce a significant number of polar com- pounds, including volatile derivatives by action of heat via the process of lipid oxidation (3–5). The highly unsaturated fatty acid composition renders fish flesh extremely suscepti- ble to oxidation and rapid degradation during processes that involve thermal treatment. Although some information is available on the effects of strong thermal processing, such as canning, on lipid compo- sition of fish muscle, it is essentially related to lipolysis and the amounts of free fatty acids formed (6,7). Regarding ox- idative damage, some studies have described the volatile pro- files produced during heat treatments of fish to establish a re- lationship with the quality of canned muscle (8,9). Elevated temperatures during the canning process can stimulate and accelerate the interaction and decomposition of many inter- mediate products. In addition, the qualitative and quantitative extension of this reaction depends on the fish species processed (9). These points render fish lipid oxidation during heating extremely difficult to analyze. In the evaluation of a mechanism of fish lipid oxidation and in the assessment of its overall flavor quality, different methodologies have been employed (10). Most of these meth- ods are based on the formation of specific compounds or on the modification of others. However, their evolution is so rapid that the use of these approaches in fish thermal process- ing has not been very useful. The main advantage of 13 C nu- clear magnetic resonance (NMR) spectroscopy in lipid stud- ies over the classical procedures is the simultaneous determi- nation of compounds formed and of the acyl compounds degraded and the study of the mechanisms involved in such reactions (11,12). 13 C NMR spectroscopy has been success- fully employed in the monitoring of free fatty acid release after fish thermal processing (12,13). The oxidative reaction involves both a degradation of acyl compounds and a modification in the olefinic nuclei due to the formation of conjugated dienes and hydroperoxides. The aim of this work was to employ 13 C NMR spectroscopy to study the lipid oxidation that occurred during the heating of fish muscle. To simulate industrial conditions of canning, lipids extracted from salmon white muscle were heated in a model system at different times and temperatures. After thermal stress, 13 C NMR spectra were recorded both on raw and heated lipids. To elucidate the mechanism and extension of the oxi- dation, different spectral regions were carefully studied. EXPERIMENTAL PROCEDURES Raw material. Samples of salmon (Salmo salar) fresh mus- cle were purchased at a commercial market (kept on ice for 1 d). Lipid extraction. Lipids were extracted from raw muscle by the Bligh and Dyer (14) method. Lipid extracts were stored at -20˚C in chloroform until analysis; propyl gallate was used Copyright © 1998 by AOCS Press 147 JAOCS, Vol. 75, no. 2 (1998) *To whom correspondence should be addressed at Instituto de Investiga- ciones Marinas del CSIC, Eduardo Cabello 6, E-36208 Vigo, Spain. E-mail: medina@iim.csic.es. 1 Current address: Dipartimento di Scienza degli Alimenti, Università di Napoli Federico II, I-80055 Portici, Italy. 2 Current address: Centro Interdipartimentale di Metodologie Chimico- Fisiche, Università di Napoli Federico II, Via Mezzocannone 16, 80134 Napoli, Italy. Oxidation in Fish Lipids During Thermal Stress as Studied by 13 C Nuclear Magnetic Resonance Spectroscopy Isabel Medina*, Raffaele Sacchi 1 , Italo Giudicianni 2 , and Santiago Aubourg Instituto de Investigaciones Marinas del CSIC, E-36208 Vigo, Spain