The role of metmyoglobin reducing activity in the maintenance of fresh meat colour

Abstract

The oxidation state of myoglobin is a major factor in fresh red meat colour and is a result of net oxidation/ reduction processes occurring in the meat. However, the lack of association between metmyolgobin reducing activity (MetMbRA) and colour stability has lead to uncertainty over its role in post-mortem meat. Also, there are conflicting reports on the level of MetMbRA in meat during storage. Therefore, it was decided to investigate the role and the potential of MetMbRA in the maintenance of the colour of fresh meat. This involved a series of experiments to test whether any manipulation in colour, colour stability or MetMbRA would be linked to each other.

Initially, the characteristics of MetMbRA in ovine longissimus were determined, and the effects of pre-rigor temperature (5°C to 35°C range), pre-slaughter stress and electrical stimulation on MetMbRA, colour and colour stability of ovine longissimus were investigated. MetMbRA was negatively affected by incubation temperatures above 30°C, but colour and colour stability were little affected at 24 h post-mortem and after 2 weeks of vacuum storage at 2°C. Neither pre-slaughter stress nor electrical stimulation affected MetMbRA or colour parameters. MetMbRA was not correlated with colour or colour stability. The results of that study indicated that MetMbRA was not the primary determinant of colour or colour stability of ovine longissimus muscle.

Secondly, the effect of antioxidants on oxidative processes and MetMbRA in beef patties were investigated in two experiments. In the first experiment colour, colour stability, TBA values and MetMb reducing activity were measured during storage at 2°C of raw beef patties treated with four antioxidants (resveratrol, carnosine, quercetin and rutin) at two concentrations (110µmol/kg meat and 550 µmol/kg meat. In the second experiment the application of these antioxidants in solution was tested. The storage period and antioxidant treatment affected colour, colour stability and TBA values. The colour-stabilising effect of the antioxidants was in the following order resveratrol> quercetin> rutin> carnosine. The performance of resveratrol was affected by the method of application. Resveratrol gave the highest protective effect against lipid peroxidation. Carnosine acted as a pro-oxidant under the specified experimental conditions. MetMb reducing activity remained relatively stable during storage. It was concluded that pH, antioxidant concentration and method of antioxidant application could affect the potencies of the antioxidants in meat systems. MetMb reducing activity was not correlated with storage time, colour parameters, MetMb (%) or lipid oxidation. Also, it was concluded that the lack of NADH and not the loss of MetMb enzymatic activity was the factor limiting the MetMb reducing activity in beef patties and, consequently, the factor dictating the colour and colour stability of beef patties.

Thirdly, the presence of MetMbRA in the myofibrillar fraction of muscle extracts was investigated as a possible reason for the observed increase of MetMb reducing activity in aged meat. The relationship between muscle MetMb reducing activities and meat colour parameters were investigated in a beef pattie system. NADH-dependent, MetMb reducing activity was found in the particulate fraction of meat following sedimentation of a meat homogenate at 35000 g. The presence of MetMbRA in the myofibrils (MMRA), defined as the activity in the sediment after centrifugation of a meat homogenate at 2000 g, was confirmed. The myofibrillar fraction contained 63% of the total MetMb reducing activity available in the meat. The particulate MetMb reducing activity (PMRA) in beef patties was found to be a good indicator of the total MetMbRA in meat. Different correlation between MetMb reducing activities and colour were obtained when beef patties were made of fresh meat (maximum 4 days post-mortem) and 5 weeks vacuum packed meat during normal cold display.

Finally, a cause-effect study was designed to clarify the involvement of MetMb reducing activities in the preservation of the colour of fresh meat utilizing the differing effects of ZnCl₂ and CaCl₂ on meat colour. Pre-rigor water and ZnCl₂ infusions improved the colour and colour stability of ovine LD muscle. CaCl₂ infusion decreased the colour of the muscle. The effects of water, ZnCl₂ and CaCl₂ on colour seem to be due to their effects on the oxidative processes in meat, decreasing the pigment's concentration and altering the physical state of the meat. The data suggests that MetMb reducing activities (especially myofibrillar MetMbRA) may be contributing to the oxidation of lipid and meat pigment.

In all the studied models, MetMbRA was not correlated to colour and colour stability of fresh meat. Thus MetMbRA is not an important factor for fresh meat colour.