2.4. Metmyoglobin reducing ability (MRA)
Metmyoglobin reducing ability was measured by the method of Lee, Cassesns, and Fennema (1981). Sample (2 g) was homogenized
with 10 ml of 25 mM PIPES buffer at pH 5.8. Five milliliters of homogenate was transferred to a 10 ml-volumetric flask and then 2 ml of
5mM K2Fe (CN)6 was added. The mixture was incubated with occasional stirring for 1 h at 2–4 °C. After incubation, 0.1 ml of 5% ammonium sulfamate and 0.2 ml of 0.5 M lead acetate were added to the mixture, and the mixture was allowed to stand for 5 min at room
temperature. After 5 min, 2.5 ml of 20% TCA was added and brought up to volume with distilled water. After another 5 min, the mixture
was filtered through Whatman filter paper No. 42 (diameter, 110 mm). The absorbance of the filtrate was measured at 420 nm,
after 30 min from the end of the incubation at 2–4 °C. Metmyoglobin reducing ability was expressed as the absorbance of 1 mM of K2Fe (CN)6 at 420 nm, minus the absorbance of the sample.