Study and modeling of enzyme inactivation in the processing of green coconut water and orange juice by ohmic and dielectric heating.

Quality attributes of fruit-based foods are sensitive to the high temperatures required to inactivate the deteriorating enzymes polyphenoloxidase (PPO), peroxidase (POD) and pectinesterase (PE). Ohmic and focused microwave heating are promising because they promote rapid volumetric heating without superficial overheating and with a possible non-thermal effect on inactivation. The objective of this work is to study the inactivation of PPO and POD in green coconut water and that of PE in orange juice to determine the respective inactivation kinetics, aiming at identifying non-thermal effects. Batch tests were performed in thermal immersion baths, in a chemical synthesis mini-reactor by focused microwaves with temperature recording by optical fiber sensors and in a customized ohmic heating cell with refrigeration and temperature recording by insulated thermocouple. Kinetic models for the prediction of residual activity were fitted based on similar temperature histories for paired conventional and electrothermal tests, using a nonlinear optimization method linked to the numerical integration of lethality, thus avoiding the simplifying hypothesis of constant temperature. The uncertainty of the fitted parameters was quantified by Monte Carlo methods. The first order and fractional kinetic models did not adequately describe the data. The Weibull model with a constant shape factor showed convergence and fit similar or better to the two fractions model for PE and PPO. Estimates of parameter distributions depended on the heating method. Evidence of dependence on possible non-thermal effects with process temperature was observed. The POD showed overactivation and none of the studied models proved to be adequate. It is hoped that this research, by the quantifying the non-thermal effects of electric fields on the inactivation of enzymes in green coconut water and orange juice, will allow assessment of the viability and scale-up of these technologies for the processing of these products. (AU)