Sohan Birla

Heat and Mass Transport during Microwave Heating of Mashed Potato in Domestic Oven—Model Development, Validation, and Sensitivity Analysis

UNLABELLED A 3-dimensional finite-element model coupling electromagnetics and heat and mass transfer was developed to understand the interactions between the microwaves and fresh mashed potato in a 500 mL tray. The model was validated by performing heating of mashed potato from 25 °C on a rotating turntable in a microwave oven, rated at 1200 W, for 3 min. The simulated spatial temperature profiles on the top and bottom layer of the mashed potato showed similar hot and cold spots when compared to the thermal images acquired by an infrared camera. Transient temperature profiles at 6 locations collected by fiber-optic sensors showed good agreement with predicted results, with the root mean square error ranging from 1.6 to 11.7 °C. The predicted total moisture loss matched well with the observed result. Several input parameters, such as the evaporation rate constant, the intrinsic permeability of water and gas, and the diffusion coefficient of water and gas, are not readily available for mashed potato, and they cannot be easily measured experimentally. Reported values for raw potato were used as baseline values. A sensitivity analysis of these input parameters on the temperature profiles and the total moisture loss was evaluated by changing the baseline values to their 10% and 1000%. The sensitivity analysis showed that the gas diffusion coefficient, intrinsic water permeability, and the evaporation rate constant greatly influenced the predicted temperature and total moisture loss, while the intrinsic gas permeability and the water diffusion coefficient had little influence. PRACTICAL APPLICATION This model can be used by the food product developers to understand microwave heating of food products spatially and temporally. This tool will allow food product developers to design food package systems that would heat more uniformly in various microwave ovens. The sensitivity analysis of this study will help us determine the most significant parameters that need to be measured accurately for reliable model prediction.

Assessment of Heating Rate and Non-uniform Heating in Domestic Microwave Ovens

Abstract Due to the inherent nature of standing wave patterns of microwaves inside a domestic microwave oven cavity and varying dielectric properties of different food components, microwave heating produces non-uniform distribution of energy inside the food. Non-uniform heating is a major food safety concern in not-ready-to-eat (NRTE) microwaveable foods. In this study, we present a method for assessing heating rate and non-uniform heating in domestic microwave ovens. In this study a custom designed container was used to assess heating rate and non-uniform heating of a range of microwave ovens using a hedgehog of 30 T-type thermocouples. The mean and standard deviation of heating rate along the radial distance and sector of the container were measured and analyzed. The effect of the location of rings and sectors was analyzed using ANOVA to identify the best location for placing food on the turntable. The study suggested that the best location to place food in a microwave oven is not at the center but near the edge of the turntable assuming uniform heating is desired. The effect of rated power and cavity size on heating rate and non-uniform heating was also studied for a range of microwave ovens. As the rated power and cavity size increases, heating rate increases while non-uniform heating decreases. Sectors in the container also influenced heating rate (p < 0.0001), even though it did not have clear trend on heating rate. In general, sectors close to the magnetron tend to heat slightly faster than sectors away from the magnetron. However, the variation in heating rate among sectors was only 2 °C/min and considered not practically important. Overall heating performance such as mean heating rate and non-uniform heating did not significantly vary between the two replications that were performed 4 h apart. However, microwave ovens were inconsistent in producing the same heating patterns between the two replications that were performed 4 h apart.

Development of a multi-temperature calibration method for measuring dielectric properties of food

In the most commonly used, open-ended coaxial probe method, dielectric properties of food products are measured after calibrating the instrument at 25°C using air (open circuit), short (short circuit) and deionized water. Measurement accuracy may be compromised when dielectric properties are measured at temperatures other than 25°C. The main objective of this study was to systematically perform calibration at multiple temperatures, quantify measurement errors and develop a method for multitemperature calibration to measure dielectric properties of materials over a wide temperature range. The dielectric properties of deionized water were measured from 1 to 90°C after calibrating the dielectric measurement system using air, short and deionized water at six different temperatures (1, 25, 40, 55, 70, and 85°C). The temperature-dependent dielectric properties of water calibrated at six temperatures were then compared with the reported values at two typical microwave frequencies of 915 and 2450 MHz. The results showed that the 25°C calibration is valid for dielectric constant measurement, but not valid for dielectric loss factor measurement at temperatures far from the calibration temperature. Multitemperature calibration is helpful for reducing errors and improving the accuracy of the temperature-dependent dielectric properties measurement, especially for low-loss materials. Calibrations at two temperatures (25 and 85°C) within the range studied were found to be suitable for the temperature-dependent dielectric properties measurement. The dielectric properties of lasagna components (ricotta cheese, pasta, and meat sauce) were measured using this multitemperature calibration method.

Modeling and Validation of Microwave Ablations With Internal Vaporization

Numerical simulation is increasingly being utilized for computer-aided design of treatment devices, analysis of ablation growth, and clinical treatment planning. Simulation models to date have incorporated electromagnetic wave propagation and heat conduction, but not other relevant physics such as water vaporization and mass transfer. Such physical changes are particularly noteworthy during the intense heat generation associated with microwave heating. In this paper, a numerical model was created that integrates microwave heating with water vapor generation and transport by using porous media assumptions in the tissue domain. The heating physics of the water vapor model was validated through temperature measurements taken at locations 5, 10, and 20 mm away from the heating zone of the microwave antenna in homogenized ex vivo bovine liver setup. Cross-sectional area of water vapor transport was validated through intraprocedural computed tomography (CT) during microwave ablations in homogenized ex vivo bovine liver. Iso-density contours from CT images were compared to vapor concentration contours from the numerical model at intermittent time points using the Jaccard index. In general, there was an improving correlation in ablation size dimensions as the ablation procedure proceeded, with a Jaccard index of 0.27, 0.49, 0.61, 0.67, and 0.69 at 1, 2, 3, 4, and 5 min, respectively. This study demonstrates the feasibility and validity of incorporating water vapor concentration into thermal ablation simulations and validating such models experimentally.

Validation of radio-frequency dielectric heating system for destruction of Cronobacter sakazakii and Salmonella species in nonfat dry milk

Cronobacter sakazakii and Salmonella species have been associated with human illnesses from consumption of contaminated nonfat dry milk (NDM), a key ingredient in powdered infant formula and many other foods. Cronobacter sakazakii and Salmonella spp. can survive the spray-drying process if milk is contaminated after pasteurization, and the dried product can be contaminated from environmental sources. Compared with conventional heating, radio-frequency dielectric heating (RFDH) is a faster and more uniform process for heating low-moisture foods. The objective of this study was to design an RFDH process to achieve target destruction (log reductions) of C. sakazakii and Salmonella spp. The thermal destruction (decimal reduction time; D-value) of C. sakazakii and Salmonella spp. in NDM (high-heat, HH; and low-heat, LH) was determined at 75, 80, 85, or 90 °C using a thermal-death-time (TDT) disk method, and the z-values (the temperature increase required to obtain a decimal reduction of the D-value) were calculated. Time and temperature requirements to achieve specific destruction of the pathogens were calculated from the thermal destruction parameters, and the efficacy of the RFDH process was validated by heating NDM using RFDH to achieve the target temperatures and holding the product in a convection oven for the required period. Linear regression was used to determine the D-values and z-values. The D-values of C. sakazakii in HH- and LH-NDM were 24.86 and 23.0 min at 75 °C, 13.75 and 7.52 min at 80 °C, 8.0 and 6.03 min at 85 °C, and 5.57 and 5.37 min at 90 °C, respectively. The D-values of Salmonella spp. in HH- and LH-NDM were 23.02 and 24.94 min at 75 °C, 10.45 and 12.54 min at 80 °C, 8.63 and 8.68 min at 85 °C, and 5.82 and 4.55 min at 90 °C, respectively. The predicted and observed destruction of C. sakazakii and Salmonella spp. were in agreement, indicating that the behavior of the organisms was similar regardless of the heating system (conventional vs. RFDH). Radio-frequency dielectric heating can be used as a faster and more uniform heating method for NDM to achieve target temperatures for a postprocess lethality treatment of NDM before packaging.

Short communication: Radio frequency dielectric heating of nonfat dry milk affects solubility and whey protein nitrogen index

The US infant formula market is estimated at over

Multiphysics Modeling of Microwave Heating of a Frozen Heterogeneous Meal Rotating on a Turntable

3.5 billion, of which 75% are dairy-based formulas. Dried dairy powders pose a significant food safety risk, with Cronobacter sakazakii and Salmonella spp. being pathogens of particular concern. Radio frequency dielectric heating (RFDH) can provide rapid, uniform heat treatment of dry powders; thus, it potentially may be used as a postprocess lethality treatment for nonfat dry milk (NDM) or powdered infant formula. Because RFDH is a heat treatment, the functionality of the NDM may be altered and should be evaluated. High heat- and low heat-NDM were RFDH processed at temperatures ranging from 75 to 90°C for 5 to 125 min. Products were then assessed for whey protein nitrogen index (WPNI), solubility, and color. In low heat-NDM, RFDH decreased WPNI and solubility if the process was done at ≥ 80°C; however, in high heat-NDM, RFDH had a greater effect on solubility than WPNI and some color properties were altered. Further investigation of RFDH is merited to validate its application as a pathogen control process for NDM across processing parameters that result in acceptable functional properties for infant formula and other food products containing NDM.

A 3-D Heat Transfer and Fluid Flow Model for Cooling of a Single Egg under Forced Convection

A 3-dimensional (3-D) multiphysics model was developed to understand the microwave heating process of a real heterogeneous food, multilayered frozen lasagna. Near-perfect 3-D geometries of food package and microwave oven were used. A multiphase porous media model combining the electromagnetic heat source with heat and mass transfer, and incorporating phase change of melting and evaporation was included in finite element model. Discrete rotation of food on the turntable was incorporated. The model simulated for 6 min of microwave cooking of a 450 g frozen lasagna kept at the center of the rotating turntable in a 1200 W domestic oven. Temperature-dependent dielectric and thermal properties of lasagna ingredients were measured and provided as inputs to the model. Simulated temperature profiles were compared with experimental temperature profiles obtained using a thermal imaging camera and fiber-optic sensors. The total moisture loss in lasagna was predicted and compared with the experimental moisture loss during cooking. The simulated spatial temperature patterns predicted at the top layer was in good agreement with the corresponding patterns observed in thermal images. Predicted point temperature profiles at 6 different locations within the meal were compared with experimental temperature profiles and root mean square error (RMSE) values ranged from 6.6 to 20.0 °C. The predicted total moisture loss matched well with an RMSE value of 0.54 g. Different layers of food components showed considerably different heating performance. Food product developers can use this model for designing food products by understanding the effect of thickness and order of each layer, and material properties of each layer, and packaging shape on cooking performance.

Effect of Water Assisted Radio Frequency Heat Treatment on the Quality of ‘Fuyu’ Persimmons

Shell eggs are cooled under forced convection in commercial egg coolers. They are prone to contamination by pathogenic bacteria such as Salmonella spp., which can grow if the eggs are not properly chilled. Transmission of Salmonella Enteritidis in eggs has been reported, requiring proper chilling of the egg to minimize the risk of foodborne illness. Accurate estimation of egg temperature under various storage conditions would assist egg processing industries to evaluate the adequacy of the egg chilling unit operation to ensure food safety. In order to determine the center temperature of a shell egg under transient forced-air convection cooling, numerical simulations were carried out using a finite volume based computational fluid dynamics (CFD) model. Conduction heat transfer was solved inside the egg, and convective heat transfer was solved at the exterior surface of egg immersed in the fluid domain. Experimental tests were conducted to determine the center temperature of an egg placed inside the test chamber of a wind tunnel by varying the air velocity (0.3 to 1.2 m s-1) and air temperature (3.7°C to 11.4°C) at the inlet of the test chamber. The CFD model was validated by comparing the results with experimental observations. The root mean square error (RMSE) of the egg center temperature predicted by the CFD model varied from 0.2°C to 0.9°C at constant inlet air temperature. Further, the model was also validated for varying inlet air temperature with time. The simulated and experimental values of egg center temperature at various boundary conditions were found to be in good agreement, with RMSE ranging from 0.23°C to 0.37°C. The predicted egg temperatures from the heat transfer model can be provided as an input to a microbial growth model to estimate the potential growth of Salmonella spp. during egg cooling for assessing food safety.

Treatment Protocols to Control Codling Moth in Apples Using Radio Frequency Energy

Water assisted radio frequency (RF) heat treatment was evaluated as a potential alternative to chemical fumigation for providing quarantine security against Mexican fruit fly (Anastrepha ludens) in ‘Fuyu’ persimmon. Three holding times were chosen for each of the three treatment temperatures (46, 48 and 50 °C), one time at, one above and another below 100% mortality. Heat treatment protocols included preheating the fruit in 40°C water, followed by RF heating in 12 kW, 27.12 MHz RF system, holding at the target temperature for the required time and then hydro cooling at 4°C for 30 min. The preheating time at 40°C was determined based on the final RF heating uniformity over the fruit cross-section. Quality parameters, including weight loss, firmness, soluble solids, titratable acidity, peel and pulp color, and calyx browning of persimmons, were evaluated after 7 days of room (22°C) and cold storage (4°C). All treatments except for one condition (48°C + 8 min holding) had no significantly adverse effects on quality attributes. Slight calyx browning was observed in the treated samples and the degree of browning increased with treatment time for each treatment temperature. Results suggested that water assisted RF heat treatments provided potential for disinfestation of persimmons with acceptable product quality.