Effect of beef systems on meat composition and quality

The fatty acid composition of ruminant products has become increasingly important in recent years, because of the association between the fatty acid composition of dietary fat (and in particular saturated fats) and cardiovascular and other lifestyle diseases in humans. Dietary advice in Europe is to decrease the fat content of the diet, maintain the ratio of the polyunsaturated to saturated fatty acids (P:S) at about 0.4 and to increase the intake of n- 3 polyunsaturated fatty acids (PUFA) relative to n-6 PUFA (WHO, 2003). The latter recommendation seeks to address the large increase in n-6 at the expense of n-3 PUFA which has occurred from the Palaeolithic period to the present time. Currently, the n-6:n-3 PUFA value is approximately 15:1 in Western Europe and USA, whereas during our evolution it was 1:1 or less (Simopoulos, 2001; Leaf et al. 2003). Although it is the fat content and fatty acid composition of the whole diet, which is important, research effort has focused on changing individual foods to make them more compatible with these guidelines. The processes of lipolysis and biohydrogenation in the rumen, which result in the conversion dietary PUFA to more saturated end products, are major reasons why ruminant fats are highly saturated in nature. However, this biohydrogenation is also responsible for ruminant fats being the major source of conjugated linoleic acid (CLA), a range of cis and trans conjugated isomers of octadecadienoic acid, some with important anticarcinogenic or antiatherogenic activities. Understanding the events surrounding fatty acid metabolism in the rumen is central to the development of effective strategies to manipulate the fatty acid composition of beef. Increasing PUFA in ruminant tissues increases the susceptibility to oxidative breakdown of muscle lipid during conditioning and retail display. A high degree of oxidation changes flavour and promotes muscle pigment oxidation, which reduces shelf life. However, some oxidation is required for optimum flavour development in beef. The extent of lipid oxidation is limited by antioxidants in tissues. These antioxidants include vitamin E (either added to the diet or present naturally) and other phenolic compounds from the diet. This paper reviews strategies (nutritional and genetic) to enhance the nutritional value of beef by increasing its content of n-3 PUFA and conjugated linoleic acid (CLA). The implications for meat quality, in particular colour shelf life and sensory attributes, are discussed.