Effects of Packaging and Temperature on Metmyoglobin Reducing Activity of Cooked Ground Beef Patties

Abstract

Premature browning (PMB) is a condition wherein ground beef will have a well-done appearance before reaching the USDA recommended 71.1�C to render meat safe for consumption. It has been estimated that 47% of ground beef is susceptible to PMB. This is a major safety concern. The mechanism of PMB is not fully understood. Therefore, the objectives of the study were to determine the role of metmyoglobin reducing activity of cooked ground beef in PMB and the effects of temperature and pH on the thermal stability of NADH-dependent reductase (NDR), lactate dehydrogenase (LDH), and oxymyoglobin (OxyMb) in vitro. Ground beef patties were packaged in high oxygen modified atmosphere (HiOX-MAP) or vacuum (VP), and cooked to 65.5 or 71.1�C. Reducing activity was measured in both raw and cooked ground beef patties. Lactate dehydrogenase, NDR, and OxyMb were prepared at pH 5.6 and 6.4 and heated at different endpoint temperatures. The results indicated that both reducing activity of raw and cooked ground beef were dependent on temperature and packaging (P < 0.05).Thermal stability of oxymyoglobin, NDR, and LDH were different and pH dependent (P < 0.05). The NADH-dependent reductase was the least heat stable (P < 0.05) and showed no activity at 65�C (45 > 50 > 55, P < 0.05). Lactate dehydrogenase was able to generate NADH even at 84�C. Percentage of OxyMb denaturation increased along the temperature (65 < 71 < 77 = 84, P < 0.05). The proteins were more stable at pH 6.4 compared to pH 5.6 (P < 0.05). The results indicate that enzymes can generate NADH at cooking temperature. Hence, strategies that can increase reducing activity have the potential to limit incidence of PMB.