The remarkable growth in broiler and turkey consumption over the past 20 years has occurred largely because of the poultry industry's ability to anticipate and meet consumers' needs. Today, a growing body of research suggests the industry can make further progress in customer satisfaction by feeding birds increased supplemental vitamin E to prevent meat quality deterioration during storage. Similar research in pork and beef has shown impressive results as well, and both of these industries are now encouraging the use of vitamin E supplementation to improve their competitive position in the supermarket.
Although the benefits at the meat case vary by species, the underlying goal remains the same: to halt or reduce lipid oxidation that leads to discoloration, off flavors, off odors and potentially toxic peroxides and aldehydes. Poultry meat is especially susceptible to off flavors and odors because of its high concentration of polyunsaturated fatty acids (PUFA). These may give poultry a competitive advantage over red meat among consumers worried about saturated fat intake, but PUFAs are also more prone to oxidation. In addition, restructuring and precooking of meat products--important trends in poultry and other meat marketing--significantly increase the susceptibility of muscle tissue to oxidative deterioration.
Vitamin E helps maintain meat quality because lipid oxidation is a free-radical mediated process that begins in the highly unsaturated cell membrane structures. Vitamin E acts as a free radical scavenger, and its location within the phospholipid layer of the cell membrane makes it "the first line of antioxidant defenses," as Wen et al. (1996) put it. What researchers must now determine is the optimum level of vitamin E supplementation to preserve poultry products against lipid oxidation.
In a review of the studies to date, Sheehy et al. (1995) note that the optimum vitamin E supplementation level to prevent oxidative rancidity in broiler meat appears to be 100 to 200 IU/kg of feed (90,719 to 181,439 IU/ton) of feed during the last four weeks before processing.
This wide range in supplemental levels to preserve meat quality is necessary because the optimum level will depend on many factors. Feeding oxidized fats or oils limits oxidative stability within the bird's tissues, as does increased feeding of PUFA to enhance the meat's fatty acid profile. Environmental stress also affects vitamin E requirements and, thus, the total supplementation needed to elevate tissue levels and avoid oxidation. Sheehy et al. recommend the higher levels of vitamin E supplementation when it's necessary to compensate for increased oxidative stress placed on the tissue.
Cooking and further processing, such as grinding and restructuring, increase the optimum supplementation level because they disrupt the lipoprotein complex of cell membranes, exposing more lipid surface area to lipid attack. Jensen et al. (1995) noted that in cooked meat, off flavors due to lipid oxidation can occur after only a few hours of storage, as opposed to the slower deterioration in raw meat.
In this study, vitamin E supplementation levels were 100 or 500 IU/kg (90,719 or 453,597 IU/ton) of feed. However, the basal diet itself had a high natural content of vitamin E and also included 46 IU supplementation per kg, for an estimated total vitamin E content of 98 IU/kg (88,905 IU/ton). Thus, the vitamin E intake of even the control birds approached the minimum levels recommended by researchers to promote meat quality. The actual vitamin E intake of the birds receiving 100 IU/kg was just under 200 IU/kg (181,439 IU/ton). The supplementation regimen began when the broilers were 10 days old and continued until slaughter 32 days later.
The high level of vitamin E in the basal diet provided reasonable oxidative stability for chilled or frozen raw meat, but Jensen et al. found significantly greater stability of cooked meat with the higher levels of supplementation (Figure 1). To assess meat quality deterioration, the researchers measured TBARS (thiobarbituric acid reactive substances) levels, which increase as lipids in the muscle oxidize. The meat from birds that had received either 100 or 500 IU/kg (90,719 or 453,597 IU/ton) of supplemental vitamin E had about 50 percent lower TBARS levels by the second day of storage, and this advantage continued through the rest of the eight-day storage period.
