Two opposing factors have been found to affect the rate of autoxidation of the fresh meat pigment oxymyoglobin, MbFe(II)O-2, in air-saturated aqueous acetate buffer with increasing hydrostatic pressure to yield metmyoglobin, MbFe(III): (1) expansion in the transition state, corresponding to a volume of activation of Delta V-# = +12.7 +/- 0.6 ml mol(-1) (25.0 degrees C, 0.16 M NaCl), in effect decreasing the rate; and (2) an increased specific acid catalysis resulting from a decrease in solution pH and increasing the rate. In the pressure range investigated (up to 250 MPa) and at the pH relevant to traditionally deboned meat (pH = 5.65 at ambient pressure investigated, together with pH = 5.21 in order to obtain mechanistic information), the pressure/pH effect was found almost to negate the kinetic effect of expansion in the transition state. Red-to-brown discoloration during pressure treatment of meat is accordingly expected to be less significant for hot-deboned and other high-pH meats.
