The activity and kinetics of carrot peroxidase were determined by using pyrogallol, guaiacol and o-dianisidine as hydrogen donors and peroxidase inactivation was studied by thermal and microwave heating. The kinetics of peroxidase showed characteristics which were dependent upon the identity and concentration of the hydrogen donor used. With pyrogallol and guaiacol, the true K(m) was found to be 0.34 and 1.4 mM for hydrogen peroxide, respectively, whereas the apparent K(m) with o-dianisidine was 7.7 x 10(-3) mM. The lowest K(m) and highest V(max)/K(m) with o-dianisidine exhibited the greater tendency of the enzyme toward hydrogen peroxide and the specificity of the competing substrate, o-dianisidine. Thermal treatment of carrot peroxidase was done in the range of 35-75 degreesC for 0.5-180 min. Inactivation kinetics of peroxidase showed a biphasic first-order model, while at 75 degreesC, peroxidase showed monophasic first-order behaviour. Kinetic parameters, k and E(a), were determined for heat labile and heat resistant fractions of peroxidase. Biphasic behaviour of enzyme inactivation was observed for the microwave treatment at 70 and 210 W, whereas at 350 and 700 W microwave powers enzyme inactivation was monophasic. Microwave heating was found to be more effective for inactivating the enzyme than thermal treatment and additionally vitamin C retention was higher in microwave treated samples compared to heat treatment. (C) 2004 Elsevier Ltd. All rights reserved.