Avaliação da pasteurização contínua da polpa de açaí sobre a inativação térmica das enzimas peroxidase e polifenoloxidase: estudo experimental e simulação em CFD

Abstract

Açai-berry is a fruit from Euterpe oleracea Martius palm tree, native from the Amazon, and it is known to feature several functional properties and have high energetic and nutritional value. Despite many scientific studies have shown that regular consumption of açai may provide health benefits, current bad manufacturing practices during its pulp extraction processing have warned the public health agencies, mainly because current outbreaks of Chagas Disease may be related to the consumption of açai contaminated with the microorganism that transmits this illness. A suitable pasteurization of this açai pulp destroys the microorganisms that transmit several diseases, including Chagas and also, inactivates enzymes that cause undesirable changes in sensory aspects. Among these enzymes, peroxidase (POD) and polyphenoloxidase (PPO) reacts with many organic compounds, producing dark-brown substances, which spoils the colour and flavor of the pulp. Once the heat resistance of these enzymes is higher than the resistance of most vegetative cells of microorganisms, inactivating peroxidase and polyphenoloxidase assure a product with good microbiological quality. Based on it, this work is focused on the study of the pasteurization process of açai-berry pulp. Firstly, the methods and results of determination of rheological behavior and physical properties, density, volumetric thermal expansion coefficient, specific heat, thermal conductivity and thermal diffusivity of açai are shown. Afterward, the kinetics study of thermal degradation of anthocyanin and the enzymes peroxidase and polyphenoloxidase is presented. Anthocyanin followed a first-order kinetic degradation and showed good thermal stability, presenting half-life times between 10.7 to 28.6 hours for the temperature range from 40 to 80°C. Therefore, they are stable against conventional pasteurization processes, which applies temperatures from about 80 to 90°C during 1 to 10 minutes. Thermal inactivation of enzymes POD and PPO were represented according to the first-order kinetic model, considering that there are two different enzyme fractions with different thermal stabilities. Among the two enzymes, peroxidase has shown higher resistance against pasteurization temperatures, and decimal reduction parameters were estimated in D1 = 7.10 s and D2 = 254 s for the thermolabile and thermostable fractions, respectively, at a reference temperature of 89°C. Parameters Z1 and Z2 were estimate as 4.3°C and 2.4°C, respectively. Computational fluid dynamics was applied in the numerical simulation of the continuous pasteurization process in a lab-scale pasteurizer, containing a plate heat exchanger (PHE) with a heating and cooling sections, separated by a holding tube. Distributions of temperature and residual activity of peroxidase were obtained for a series and series-parallel flows in the PHE for up to four açai pulp flow-rates (15, 20, 25 and 30 L/h). Tests were also carried out in an experimental pasteurizer and the results were compared. Experimental data of temperature distribution were better represented by the CFD simulations for the series-flow. Nevertheless, the experimental results of the residual activity of POD diverged from those achieved by CFD. The continuous thermal processing of açai pulp confirmed the results obtained in the kinetics experiments of enzyme degradations, showing that the thermal stability of peroxidase is higher than polyphenoloxidase. Pasteurization was more effective when the pulp was processed in a series-flow arrangement. In this situation, when açai pulp was experimentally processed at 15 L/h, 89.9% of POD was inactivated. CFD simulations of these conditions showed a reduction in more than 99.9% of this enzyme activity.