Microwave milk pasteurization system using coaxial slot radiator

Abstract

Microwave heating is a volumetric heating and free-fouling process which can save billions of dollars caused by periodic cleaning procedures of heat exchangers in the current dairy industry. The current milk microwave pasteurization method results in non-uniformity in the temperature distribution, which compromises the pasteurization quality. This thesis aims to improve the uniformity of microwave heating using a low-power coaxial slot antenna for the application of milk pasteurization. Initially, the relative complex permittivity of cows’ raw milk is measured using a Keysight 85070E dielectric probe over a temperature, T ranging from 25 ◦C to 75 ◦C with an interval of 5 ◦C and a frequency ranging from 0.2 GHz to 6 GHz. The measurement results of relative complex permittivity are modeled using a modified Debye relaxation model and their values are used in the simulation for radiator design. A coaxial slot radiator is designed and optimized using the COMSOL Multiphysics simulator by considering the radiator sunk into the 100 mL of cows’ milk for pasteurization. The radiator’s performance is optimized by adjusting the slot length and slot position on the monopole radiator. At radio frequency of 2.45 GHz, the slot length of 2.4mm and slot position at 4.7mm from the end tip of the radiator provide optimized impedance matching, Zin of 51.54 − j0.3Ω, which is close to ideal impedance, Zin of 50Ω. The monopole slot radiator is fabricated using a semi-rigid RG405U cable with a SubMiniature version A (SMA) connector. The antenna is fed with 2.45 GHz magnetron based microwave generator, which is implemented and calibrated. The reflection coefficient, |S11| of the radiator with generator system in 100 mL of cows’ raw milk, is measured across a temperature ranging from 25 ◦C to 85 ◦C using a portable radio frequency (RF) reflectometer and a lab heater. The measurements of |S11| show readings higher than −45 dB at 25◦Cand higher than−25 dB at 85 ◦C. The temperature distribution generated from the radiator in 100mL of cows’ raw milk at processing powers of 100W, 125W, and 150Ware simulated using COMSOL and measured using an infra-red (IR) thermal imaging camera and two thermocouple sensors mounted on a 3D-printed holder. The measured temperature distributions show a significant improvement in temperature uniformity with a maximum temperature difference, ΔT of 3.4 ◦C, 2.3 ◦C, and 2.2 ◦C for power usage of 100W, 125W, and 150W respectively. With a maximum temperature difference, ΔT of 24.1 ± 1 ◦C, milk microwave batch pasteurization improved by up to 89.2% compared to previous non-uniformities. The collected cows’ raw milk samples are then placed in pre-sterilized containers and processed inside biosafety cabinet II for pasteurization quality assessment based on aerobic plate count (APC) tests and to investigate the heating effects on milk’s nutrition at power usage of 100Waccording to the physiochemical properties tests using the Master Eco ultrasonic milk analyzer. The APC tests show the technique’s ability to eliminate milk micro-organisms with a 5-log reduction of the microbial population after 7 min, 6 min, and 5 min at microwave powers usage of 100W, 125W, and 150Wrespectively. The measured milk’s physiochemical properties show similar heating effects on protein, solid-non-fat, and fat and fewer effects on density, dry matter (DM), and lactose compared with previous studies on conventional milk microwave batch pasteurization.