Polyphenoloxidase and peroxidase activity in partially frozen systems withdifferent physical properties

Enzyme catalysed reactions can occur at significant rates at low temperatur es, thus exerting a major control on the quality of frozen foods. The tempe rature-concentration effect in partially frozen systems can make enzyme rea ctions controlled by diffusion and thus rate limited by viscosity according to the Stokes-Einstein relation. Polyphenoloxidase (PPO) activity in sucro se, glycerol and fructose water media and (POD) activity in fructose and gl ycerol water media, were assayed spectrophotometrically from 20 degrees C t o -30 degrees C. The media have at equal sub-freezing temperature marked di fferences in viscosities and others physical properties. For both enzymes, the activity data showed similar dependence on temperature and, above freez ing, on the concentration of the media. A relation with the viscosity of th e media at equal sub-freezing temperature was not evident. POD activity sho wed a "break" between -10 degrees C and -20 degrees C, suggesting a reversi ble conformational change. The temperature dependence of PPO rate constants were well described by both the Williams-Landel-Ferry (r(2)=0.998) and the Arrhenius (r2=0.985) models. A hypothesis for the non conformity of the me asured kinetics with the theoretical model is that in the temperature range considered the reactions are still activation controlled or in a transitio n zone, and should become diffusion limited only at lower temperatures and higher viscosities. (C) 1999 Canadian Institute of Food Science and Technol ogy. Published by Elsevier Science Ltd. All rights reserved.