Effect of the Polyunsaturated Fatty Acid Composition of Beef Muscle on the Profile of Aroma Volatiles J. Stephen Elmore, † Donald S. Mottram,* ,† Michael Enser, ‡ and Jeffrey D. Wood ‡ Department of Food Science and Technology, The University of Reading, Whiteknights, Reading RG6 6AP, United Kingdom, and Division of Food Animal Science, School of Veterinary Science, The University of Bristol, Langford, Bristol BS40 5DU, United Kingdom The effect of n-3 polyunsaturated fatty acids (PUFAs) in beef muscle on the composition of the aroma volatiles produced during cooking was measured. The meat was obtained from groups of steers fed different supplementary fats: (i) a palm-oil-based control; (ii) bruised whole linseed, which increased muscle levels of R-linolenic (C18:3 n-3) and eicosapentaenoic acid (EPA, C20:5 n-3); (iii) fish oil, which increased EPA and docosahexaenoic acid (C22:6 n-3); (iv) equal quantities of linseed and fish oil. Higher levels of lipid oxidation products were found in the aroma extracts of all of the steaks with increased PUFA content, after cooking. In particular, n-alkanals, 2-alkenals, 1-alkanols, and alkylfurans were increased up to 4-fold. Most of these compounds were derived from the autoxidation of the more abundant mono- and di-unsaturated fatty acids during cooking, and such autoxidation appeared to be promoted by increased levels of PUFAs. Keywords: Aroma volatiles; beef; polyunsaturated fatty acids; lipid oxidation INTRODUCTION The main sources of volatiles in cooked meat are the thermal degradation of lipid and the Maillard reaction, which occurs between amino acids and sugars (Mottram, 1994). Heat-induced oxidation of fatty acids, particularly unsaturated fatty acids, produces degradation products, such as aliphatic aldehydes, ketones, and alcohols, which may have intrinsic flavors. These degradation products may react further with Maillard products to give other compounds that may contribute to flavor (Mottram and Edwards, 1983; Elmore et al., 1997). Altering the fatty acid composition of beef muscle can affect its flavor characteristics (Ford et al., 1976). For example, the relative levels of linoleic acid (C18:2 n-6) and R-linolenic acid (C18:3 n-3) in grain and forage are largely responsible for the differences in volatile com- position, and hence the flavor, of beef finished on these diets (Larick et al., 1987; Larick and Turner, 1990). The nutritional value of n-3 polyunsaturated fatty acids (PUFAs) in the human diet is well recognized, and increased consumption of these fatty acids, particularly eicosapentaenoic acid (EPA, C20:5 n-3) and docosa- hexaenoic acid (DHA, C22:6 n-3), has been recom- mended (Department of Health, 1994). To help meet these recommendations we have produced beef with increased levels of n-3 PUFAs through feeding dietary supplements of linseed and fish oil (Scollan et al., 1999). These supplements doubled the concentrations of di- etary n-3 PUFA in the longissimus lumborum muscle, compared with steers fed a palm-oil-based supplement. The C 20 and C 22 PUFAs are deposited in the phospho- lipids of ruminants (Ashes et al., 1992), which are important sources of lipid-derived flavor compounds during cooking (Mottram and Edwards, 1983; Mottram, 1996). Because of the low oxidative stability of these fatty acids, it seems likely that changes in their con- centration, although small, would result in alterations to the composition of the aroma volatiles produced during cooking. This paper examines the aroma profiles of cooked steaks, in relation to their fatty acid composi- tion, and discusses the implications for the eating quality of meat. MATERIALS AND METHODS Sample Preparation. Four groups of Charolais steers matched for initial live weight (436 ( 3.4 kg) were individually fed ad libitum diets, adjusted to provide 60% of dry matter from grass silage and 40% from concentrates based on barley and sugar beet. The concentrate contained one of four fat supplements: (1) Megalac, a palm-oil-based milk substitute (Velac Ltd., Royston, U.K.), used as control; (2) whole linseed, lightly bruised; (3) Fish oil, South American herring oil, containing 30% PUFA (Issac Spencer Ltd., Fleetwood, U.K.); (4) Linseed and fish oil mixture (1:1 oil weight basis). Ap- proximately 45% of the dietary fat intake was provided by the supplements. All diets had a similar vitamin E content (R- tocopherol acetate, 345 mg/kg concentrate) to prevent oxidation of PUFAs in tissues. Steers were slaughtered conventionally, with captive bolt stunning, after an average of 120 days on test. After chilling at 4 °C for 48 h postmortem, the loin was boned out, vacuum-packed, and conditioned at 1 °C for 10 days. Steaks consisting of a complete cross-section of longissimus lumborum muscle, 15 mm thick, were cut posterior to the ninth rib position, vacuum-packed, blast frozen, and stored at -20 °C for 3 to 6 months before cooking and determination of flavor volatiles. Fatty Acid Analysis. Samples of muscle tissue from all eight steers (longissimus lumborum muscle), taken 48 h postmortem, were blended separately in a small food processor. The fatty acids were extracted, methylated, and analyzed by gas chromatography, as described by Whittington et al. (1986). Duplicate 1-g samples were hydrolyzed in 6 mL of 5 M potassium hydroxide in methanol/water (1:1) at 60 °C for 2 h. * Author to whom correspondence should be addressed (fax +44 118 931 0080; e-mail D.S.Mottram@reading.ac.uk). † The University of Reading. ‡ The University of Bristol. 1619 J. Agric. Food Chem. 1999, 47, 1619-1625 10.1021/jf980718m CCC: $18.00 © 1999 American Chemical Society Published on Web 03/26/1999