Effect of vitamin E supplementation and diet on fatty acid composition and on meat colour and lipid oxidation of lamb leg steaks displayed in modified atmosphere packs.

Groups of 8 lambs were allocated to one of five concentrate diets supplemented with all-rac-α-tocopheryl acetate containing 30 (C30), 60 (C60), 120 (C120), 250 (C250) and 500 (C500) mg/kg dry matter. Two other groups were fed grass silage and 400 g/day concentrate with 60 (S60) or 500 (S500) mg α-tocopheryl acetate/kg dry matter. Within diet, vitamin E level did not affect growth performance or carcass characteristics. Basal diet did not affect final live weight, conformation and fatness scores. M. semimembranosus from S lambs contained more α-tocopherol than that of C lambs on the same intake and by day 6 in MAP (75%O2/25%CO2) chroma and a* were below acceptable levels in C30 lambs. TBARS were higher in C30 and C60 muscle than in other treatments (P<0.001) after 3 and 6 days display. Muscle fatty acid composition varied with basal diet but lipid oxidation depended more on vitamin E concentration with an initial concentration of 1.9 µg/g muscle preventing significant lipid oxidation.

Influence of vitamin E supplementation and basal diet on the vitamin E status, performance and tissue fatty acid concentration in lambs.

In order to determine the effect of dietary vitamin E level and basal diet on vitamin E status, performance and tissue fatty acid content, five groups of eight Suffolk × Charollais wether lambs with an initial live weight of 28.4 (s.d. 1.6) kg were allocated to one of five concentrate-based diets supplemented with all-rac-α-tocopheryl acetate to contain 30 mg (C-30), 60 mg (C-60), 120 mg (C-120), 250 mg (C-250) or 500 mg (C-500) α-tocopheryl acetate/kg dry matter (DM), for 63 days. Two additional groups of eight lambs entered the study at 31.2 (s.d. 3.3) kg and were fed grass silage and 400 g/day concentrate for 56 days, with the whole diet providing the equivalent of 60 mg (S-60) or 500 mg (S-500) α-tocopheryl acetate/kg DM. Lambs were weighed and blood samples obtained by venipuncture weekly. Dietary vitamin E level did not affect performance (P > 0.05), but lambs fed grass silage grew more slowly (P < 0.001) and had a higher (P < 0.001) feed conversion ratio (kg feed/kg gain) than those fed concentrates. At day 0 plasma α-tocopherol concentrations were 0.8 µg/ml and did not differ between treatments (P > 0.05). Plasma α-tocopherol concentrations then decreased in all lambs except for those fed S-500, which increased, and at slaughter were (µg/ml) 0.07, 0.23, 0.39, 0.76 and 1.57 in C-30, C-60, C-120, C-250 and C-500 and 1.18 and 1.93 in S-60 and S-500, respectively. At slaughter, muscle and liver α-tocopherol concentrations were in the deficiency range for lambs fed C-30, C-60 or C-120, whereas plasma creatine kinase and tissue polyunsaturated fatty acids were unaffected by dietary vitamin E level, but creatine kinase levels were higher (P < 0.05) and glutathione peroxidise levels lower (P < 0.001) in lambs fed grass silage than concentrates alone. Muscle and liver α-tocopherol concentrations were 1.8- and 4.1-fold higher in lambs fed S-60 than C-60, but there was less of a difference between lambs fed S-500 or C-500 with muscle and liver differences of 0.4- and 0.7-fold, respectively. Tissue n-3 polyunsaturated fatty acid concentrations were higher (P < 0.05) and n-6 fatty acids lower in lambs receiving the grass silage compared to concentrate-based diets, but were not affected by dietary vitamin E level. It is concluded that lower plasma and tissue levels of α-tocopherol are present in lambs supplemented with all-rac-α-tocopheryl acetate on a concentrate compared to a mixed diet of silage and concentrates, and that normal growth can be achieved at tissue levels previously considered to represent deficiency.

Effect of dietary oil source on the flavour and the colour and lipid stability of lamb meat.

This study investigated the influence of five sources of dietary oil (linseed oil (LO), fish oil (FO), a protected lipid supplement (PLS, 18:2 to 18:3 ratio 3:1), fish oil/marine algae (FOMA) and PLSMA) on the colour and lipid stability of lamb muscle and the flavour of grilled loin chops. LO produced the highest proportion of 18:3n-3 in muscle phospholipid, the highest ratings for lamb flavour intensity and overall liking and the lowest ratings for abnormal flavour intensity. PLS increased the proportion of 18:2n-6 which reduced lamb flavour intensity and increased abnormal lamb flavour intensity. Diets containing FO or MA increased proportions of the longer chain n-3 fatty acids and similar reduced ratings for lamb flavour as the PLS diet. FO-containing diets increased fishy flavour notes, especially when in combination with MA. 'Putty' and 'fish oil' odours were recognised as being present more frequently in cooked subcutaneous lamb fat from lambs fed FO and FOMA than other diets. Lambs fed MA, FO and the combination of the two produced meat that was oxidatively less stable and had a reduced colour and lipid oxidative shelf-life, which was at least partially due to the lower vitamin E content of the muscle. These results have significant implications for the formulation of diets that may improve nutritional ratios in lamb meat but which adversely affect flavour and meat stability.

Flavour perception of oxidation in beef.

Lipid oxidation is a major factor in meat quality. In order to relate human perceptions of lipid oxidation, as determined by a trained taste panel, to a chemical measurement of oxidation, we studied meat from animals with a wide range of potential oxidation through differences in their PUFA composition and by displaying the meat in high oxygen modified atmosphere packs for varying lengths of time. Meat was obtained from 73 Angus- and Charolais-cross steers from different trials that had been raised on 10 different diets: grass silage (high in C18:3, n-3), cereal concentrate (high in C18:2, n-6), three diets with 3% added fat consisting of three levels of protected lipid supplement (high in C18:2, n-6 and C18:3, n-3, ratio 1:1), a control with Megalac(®) (relatively saturated), three diets with three levels of inclusion of protected fish oil (high in C20:5 n-3 and C22:6 n-3) plus a constant amount of unprotected fish oil and a final diet with an unprotected fish oil control. The longissimus dorsi muscle was excised from the left carcass side, aged vacuum packaged for 10-13 days depending on the projects and frozen for less than eight months. TBARS and sensory analyses were performed on steaks displayed for 0, 4 or 9 days under simulated retail conditions, exposed to light in modified atmosphere packaging (CO(2):O(2); 25:75). Meat oxidation increased throughout display for each of the diets, as shown by a rise in TBARS values. This increase was not linear, differences between 0 and 4 days of display were smaller than between 4 and 9 days of display. The lowest TBARS and lowest increment occurred in the two control diets and the grass-fed animals, probably due to the more saturated fat of meat from animals fed the control diets and the higher content of vitamin E. Sensory attributes were also influenced by time of display. Positive attributes, such as beef flavour or overall liking, decreased throughout display, whereas negative attributes, such as abnormal and rancid flavours, increased. The correlations between sensory and analytical attributes were high. TBARS were a good predictor of the perception of rancidity (Spearman's rho=0.84). Panellist preferences were related to the presence of beef flavour (rho=0.93) and to the absence of abnormal (rho=-0.88) and rancid flavours (rho=-0.83). Under the experimental conditions used, a TBARS value of around 2 could be considered the limiting threshold for the acceptability of oxidised beef.

Effects of fatty acids on meat quality: a review.

Interest in meat fatty acid composition stems mainly from the need to find ways to produce healthier meat, i.e. with a higher ratio of polyunsaturated (PUFA) to saturated fatty acids and a more favourable balance between n-6 and n-3 PUFA. In pigs, the drive has been to increase n-3 PUFA in meat and this can be achieved by feeding sources such as linseed in the diet. Only when concentrations of α-linolenic acid (18:3) approach 3% of neutral lipids or phospholipids are there any adverse effects on meat quality, defined in terms of shelf life (lipid and myoglobin oxidation) and flavour. Ruminant meats are a relatively good source of n-3 PUFA due to the presence of 18:3 in grass. Further increases can be achieved with animals fed grain-based diets by including whole linseed or linseed oil, especially if this is "protected" from rumen biohydrogenation. Long-chain (C20-C22) n-3 PUFA are synthesised from 18:3 in the animal although docosahexaenoic acid (DHA, 22:6) is not increased when diets are supplemented with 18:3. DHA can be increased by feeding sources such as fish oil although too-high levels cause adverse flavour and colour changes. Grass-fed beef and lamb have naturally high levels of 18:3 and long chain n-3 PUFA. These impact on flavour to produce a 'grass fed' taste in which other components of grass are also involved. Grazing also provides antioxidants including vitamin E which maintain PUFA levels in meat and prevent quality deterioration during processing and display. In pork, beef and lamb the melting point of lipid and the firmness/hardness of carcass fat is closely related to the concentration of stearic acid (18:0).

Manipulating meat quality and composition.

Meat quality describes the attractiveness of meat to consumers. The present paper focuses on two major aspects of meat quality, tenderness and flavour. Both aspects of quality can be influenced by nutrition, principally through its effects on the amount and type of fat in meat. In several countries, high levels of intramuscular fat (marbling fat), i.e. above 30 g/kg muscle weight in longissimus, are deemed necessary for optimum tenderness, although poor relationships between fat content and tenderness have generally been found in European studies, where fat levels are often very low, e.g. below 10 g/kg in UK pigs. Muscle lipid may be a marker for red oxidative (type 1) muscle fibres which are found at higher concentrations in tender muscles and carcasses. Nutritional treatment can be used to manipulate the fatty acid content of muscle to improve nutritional balance, i.e. increase the polyunsaturated (PUFA): saturated fatty acid value and reduce the n-6:n-3 PUFA value. Increasing PUFA levels may also change flavour because of their greater susceptibility to oxidative breakdown and the generation of abnormal volatile compounds during cooking. This situation particularly applies to the n-3 PUFA which are the most unsaturated meat lipids. In pigs, a concentration of 3 mg alpha-linolenic acid (18:3)/100 mg in muscle and fat tissue fatty acids can easily be achieved by including whole linseed in the diet. This level has led to abnormal odours and flavours in some studies, but not in others. In cattle and sheep, feeding whole linseed raised 18:3 concentrations in muscle fatty acids from about 0.7 mg/100 mg to > 1 mg/100 mg. As with pigs, this diet also increased levels of long-chain n-3 PUFA formed from 18:3, including eicosapentaenoic acid (20:5). Although this increase led to greater oxidative breakdown of lipids during storage and the generation of large quantities of lipid-derived volatile compounds during cooking, there were no deleterious effects on odour or flavour. When 18:3 levels are raised in lamb and beef because of grass feeding, the intensity of the flavours increases in comparison with grain-fed animals which consume and deposit relatively more linoleic acid (18:2). In ruminants, very high levels of 18:2 produced by feeding protected oil supplements cause the cooked beef to be described as oily, bland or pork-like.