Inactivation kinetics of apple polyphenol oxidase in different pressure-temperature domains

The impact of high hydrostatic pressure and temperature on the stability of polyphenol oxidase (PPO) was studied in cloudy apple juice. Application of 200-500 MPa near room temperature or heat treatment at 45-55 degrees C at ambient pressure caused an increase of PPO activity of up to 65% in freshly squeezed apple juice. Combined pressure-temperature inactivation experiments with fully activated PPO (5 min treatment at 400 MPa and 20 degrees C) were carried out in the range of 0.1-700 MPa and 20-80 degrees C. Enzyme inactivation kinetics followed a 2.2 order reaction scheme at all pressure-temperature conditions tested. A polynomial model was successfully applied to describe the rate of PPO inactivation as a function of pressure and temperature and was used to construct a pressure-temperature isokinetic diagram. This diagram clearly showed synergistic effects of pressure and temperature on the inactivation of apple PPO at pressures above 300 MPa and antagonistic effects at lower pressures. Compared to ambient pressure conditions, temperatures required to inactivate PPO in apple juice were increased 10-15 degrees C at 100-300 MPa. Industrial relevance: High pressure processing of fresh fruits is gaining popularity in the food industry because of its ability to inactivate microorganisms and some enzymes near room temperature with little impact on flavour or nutritional attributes of the food. However, quantitative data regarding the impact of process parameters on the target reaction are required to economically utilise this technology. This paper provides a mathematical model describing the combined effect of pressure, temperature and treatment time on the inactivation of PPO in cloudy apple juice. (C) 2009 Elsevier Ltd. All rights reserved.