Effects of data expression, sample location, and oxygen partial pressure on initial nitric oxide metmyoglobin formation and metmyoglobin-reducing-activity measurement in beef muscle.

Methodology for measuring surface and subsurface metmyoglobin-reducing activity (MRA) of beef longissimus lumborum, semimembranosus, and psoas major was evaluated by submerging samples from the upper one half of 2.54cm thick steaks in 0.3% NO(2)(-), vacuum packaging, and incubating at 30°C for 2h to promote pigment reduction. Initial metmyoglobin formation (IMF) and post-reduction metmyoglobin (PRM) were measured and used to calculate relative and absolute MRA values. The subsurface of all muscles maintained more (P<0.05) reducing capacity after 6d of display than the surface. Subsurface MRA was not significantly (P>0.05) correlated with surface colour stability whereas IMF was most correlated with colour stability (|r|>0.77; P<0.05). Longissimus steaks packaged in 20% O(2) had more (P<0.05) surface MRA than steaks packaged in 80% O(2) although measurement of MRA for steaks packaged in high-oxygen with this methodology is questioned. Determination of IMF is a better indicator of MRA for steaks exposed to atmospheric O(2) than more traditional measurements based on the amount of pigment reduced over time.