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

Abstract

In the processing of liquid foods, heat treatment is intended for inactivating microorganisms and enzymes that compromise the safety and product quality. Enzymes polyphenoloxidase (PPO) and peroxidase (POD) in green coconut water and pectin methylesterase (PME) in orange juice exhibit high thermal stability and the sensorial quality of fruit based products (color and taste) is sensitive to high temperatures, making it challenging to process these products without the addition of preservatives. The emerging technologies of ohmic heating and heating by focused microwaves are promising for promoting rapid volumetric heating without surface overheating and with a possible non-thermal effect on enzyme inactivation. Thus, the objective of this work is to study the inactivation of PME in orange juice and of PPO and POD in green coconut water to determine the respective kinetics in conventional and microwave heating, seeking identification of non-thermal effects in different processings and products. To study the inactivation kinetics by microwaves, batch tests will be performed in immersion thermal baths and in a chemical synthesis mini-reactor by focused microwaves with fiber optic temperature record. To study the inactivation kinetics by ohmic heating, batch tests will be performed in glass tubes with electric conductive or non-conductive caps immersed in an ohmic treatment cell filled with saline solution with thermocouple temperature record. The enzymatic activity will be evaluated by spectrophotometric methods. Kinetic models for prediction of residual activity will be adjusted and validated based on recorded temperature history using a non-linear optimization method linked to the numerical integration of lethality, thus avoiding the simplifying assumption of constant temperature. Stocastic Monte Carlo methods will allow estimating the uncertainty of the kinetic model parameters and, consequently, the prediction uncertainty. It is expected that this research will allow the elucidation of the non-thermal effects of microwaves and electricity on the inactivation of the enzymes of green coconut and orange juice. (AU)