Thermal processing of tomato juice and pulp by microwave technology: determination of dielectric properties and inactivation of pectinesterase

Abstract

In the tomato and tomato products industry, there are several heat treatments, whose purpose of microbiological or enzymatic inactivation depends on the type of product. The treatment is carried out in tubular heat exchangers, or by a direct system, through the application of steam, in order to guarantee the safety and stability of the product. The main problems related to these conventional treatments are: loss of nutritional and sensorial quality, in addition to the great dependence and consumption of steam and water. In this sense, continuous flow microwave heating is a promising technology for minimizing these problems. To design and evaluate a microwave applicator, it is necessary to know the dielectric properties of the product as a function of temperature (e' - relative electrical permittivity, e'' - dielectric loss factor), as these define the penetration depth of waves and the rate of heating. One of the objectives of this work is to determine and correlate these properties for tomato pulp (with and without added salt and sugar) with the soluble solids content (5 to 13 °Brix) and temperature (10 to 120 °C). The technique used will be open coaxial cable technique. Another important factor for the thermal process is the kinetics of microbial or enzymatic inactivation, which can be affected in the case of heating by electric fields. The second objective of this research is to adjust a 1st order kinetic model and two fractions to represent the thermal inactivation of the pectinesterase enzyme (PME) present in tomato juice, considering conventional heating in a thermal bath and microwaves in a chemical synthesis reactor. Similar heating, holding and cooling conditions will be used in both processes and the entire temperature history of the sample will be considered for model adjustment. The fitted curves will provide suitable conditions to achieve a desired level of inactivation, as well as allowing analysis of possible non-thermal effects. Results of this research will be evaluated for their reliability, coherence and applicability and will be disseminated at scientific events.