Thermophysical, rheological, electrical, and dielectric properties of mango (Mangifera indica cv. Palmer) pulp and nectar: Temperature correlations for microwave and conventional thermal processing

Continuous flow microwave heating is an alternative technology to conventional heat exchangers in the thermal processing of liquid foods. This technology enables rapid volumetric heating while preserving product quality; however, achieving homogeneous heating is challenging. The design and analysis of microwave systems require multiphysics simulation techniques to simultaneously model fluid flow, heat transfer and electromagnetic field propagation. Since accurate simulations depend on reliable food property characterization, thermophysical, rheological, electrical, and dielectric properties of mango (Palmer variety) pulp and nectar were measured to provide temperature correlations applicable to modeling microwave thermal processing, as well as conventional heating, pulsed electric field treatment, and ohmic heating applications. Mango pulp and nectar were produced from fresh fruits and characterized by pH, acidity, total solids, soluble solids and ash content. Density was measured by pycnometry (25 - 80 °C), thermal conductivity (35 - 75 °C) was determined by the concentric cylinders method, electrical conductivity (20 - 80 °C), was obtained with a conductivity meter and heat capacity (5 - 90 °C) was assessed by DSC - differential scanning calorimetry. Polynomial correlations for temperature were adjusted. Rheological properties were evaluated using a parallel plate system for mango pulp and a coaxial cylindrical system for mango nectar. Both samples exhibited pseudoplastic behavior, which was modeled using the power law model, with temperature dependence described by Arrhenius equation (20 - 80 °C). The main contribution of this study is the characterization of the dielectric properties (relative electrical permittivity and loss factor) using an open-ended coaxial-line technique over a frequency range of 200 - 3000 MHz and a temperature range of 20 - 120 °C. Temperature correlations were adjusted for dielectric properties and penetration depth at the commercial frequencies of 915 and 2450 MHz. Relative permittivity showed a linear decrease with temperature, attributed to water polarization. The loss factor showed contributions from both ionic conduction and dipole rotation mechanisms to microwave heating. Penetration depth was greater at 915 MHz (20 - 73 mm) than at 2450 MHz (10 - 25 mm); however, at 915 MHz, it decreased with increasing temperature, which may lead to non-uniform heating.