Development of a computer simulation model for processing food in a microwave assisted thermal sterilization (MATS) system

The microwave assisted thermal sterilization computer simulation model (MATS-CSM) was developed to improve the previous computer simulation model for the microwave assisted thermal sterilization (MATS) system. Development of the new MATS-CSM included determination of optimum heating time step, evaluation of electromagnetic field distribution and the resulting heating pattern in food, and experimental validation of heating patterns. It was determined that the minimum number of discretization that would not cause immediate divergence of the EM-heat transfer solution was 32 steps corresponding to 97 mm and 5.6 s of displacement and heating time for every step, respectively. Furthermore, this study successfully demonstrated the symmetrical electromagnetic field distribution between top and bottom microwave entry ports and a staggered electric field pattern from one cavity of the MATS to the next. In addition, MATS-CSM confirmed that incorporating heat diffusion in the simulation model reduces the difference in hot spot and cold spot temperature by 65%. It also confirmed that water circulation reduces the edge heating effect, as observed in experiments. The heating pattern generated by MATS-CSM was verified experimentally through a chemical marker method. Based on the percent areal cross section of the weighted average temperature, there were no noticeable differences between the heating zones generated by the MATS-CSM and by the chemical marker method. The percent areal cross section of the cold area 1, cold area 2, and hot area by MATS-CSM were 35%, 25%, and 40%, respectively, and the cold area 1, cold area 2, and hot area by chemical marker method were 35%, 30%, and 35%, respectively.     

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

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.     

Novel electromagnetic-black-hole-based high-efficiency single-mode microwave liquid-phase food heating system

Microwave heating exhibits a high potential for usage in liquid food processing. Current microwave heating systems are designed for a specific load. However, when the permittivity of the load changes dynamically, the heating efficiency of these systems fluctuates considerably. We proposed a novel high-efficiency microwave liquid heating system for dynamic dielectric loads to address this limitation. In this system, an electromagnetic black hole efficiently absorbs electromagnetic waves in all directions. First, an electromagnetic black hole was realized using metamaterials (which means artificially structured dielectric materials with extraordinary physical properties) with a radially continuous refractive index distribution. Next, an electromagnetic field simulation model was established to calculate the microwave absorption of various load permittivity. To discretize and simplify the parameters of the continuous distribution in an electromagnetic black hole, a concentric layered structure and a punched structure composed of uniform isotropic dielectric materials were designed. Finally, the experimental samples were processed based on the two discrete structures. The microwave system developed for experimental verification confirmed the high efficiency of the heating system; the system is simple and usable in numerous applications. Thus, the proposed method can realize high-efficiency heating of loads over large dielectric dynamic ranges. When the dielectric constant of the load changed dynamically from 10 to 80, the microwave energy utilization rate can increase by up to 90%.     

Unlocking Potentials of Microwaves for Food Safety and Quality

Microwave is an effective means to deliver energy to food through polymeric package materials, offering potential for developing short‐time in‐package sterilization and pasteurization processes. The complex physics related to microwave propagation and microwave heating require special attention to the design of process systems and development of thermal processes in compliance with regulatory requirements for food safety. This article describes the basic microwave properties relevant to heating uniformity and system design, and provides a historical overview on the development of microwave‐assisted thermal sterilization (MATS) and pasteurization systems in research laboratories and used in food plants. It presents recent activities on the development of 915 MHz single‐mode MATS technology, the procedures leading to regulatory acceptance, and sensory results of the processed products. The article discusses needs for further efforts to bridge remaining knowledge gaps and facilitate transfer of academic research to industrial implementation.     

THERMOELECTROMAGNETIC COUPLING IN MICROWAVE FREEZE-DRYING

A coupled thermoelectromagnetic model of microwave freeze‐drying was developed by introducing the reflection coefficient method of the electromagnetic theory into the sublimation‐condensation model, to describe the coupling between the electromagnetic field strength distribution and the heat and mass transfer in microwave freeze‐drying. The experiments of beef were provided and the good agreement between the predicted and experimental results validates the model. The variations of the field strength, the temperature and the moisture content distributions during the microwave freeze‐drying process were discussed based on the model. The simulated results show that the microwave distribution in the cavity is a moving standing wave and that the actual field strength inside the material is less than that in the cavity. The nonuniform heating of the microwave will increase either the temperature in the dried region or the temperature in the frozen region, depending on the microwave input direction.     

A simplified approach to assist process development for microwave assisted pasteurization of packaged food products

The purpose of this study was to develop a convenient method to assist the food industry in developing process schedules for production of ready-to-eat meals using microwave assisted pasteurization system (MAPS). An analytical model was applied to estimate the temperature increase in the cold zone in packaged foods during heating in a 915 MHz single-mode microwave system. This model was validated in a pilot-scale four-cavity MAPS using mashed potato-gellan gum model food with different thicknesses (22 to 36 mm) and salt contents (0.0 to 1.0%). Mobile sensors were placed in the packages to measure temperature at the pre-determined cold spots. For 2.48 min of microwave heating with 5, 5, and 8.7 kW 915 MHz microwave powers, the highest temperature increase at the cold spot during microwave heating was 33.2 °C in the 22 mm thick model food with 0.6% salt content, whereas the lowest temperature increase was 10.3 °C in the 36 mm thick model food with 1.0% salt content. There was a deviation of 1.9 ± 1.2 °C between experimental and predicted data with an R² of 0.89. A simplified chart was developed based on the validated analytical results to allow rapid prediction of temperature increases in MAPS as influenced by food dielectric properties and package thickness. Examples were used to illustrate how the chart could assist in process scheduling. The chart can help assess the heating rates of various pre-packaged food products in a specific industrial MAPS or guide product development for desired heating uniformity.     

Ohmic Heating of an Electrically Conductive Food Package

Ohmic heating through an electrically conductive food package is a new approach to heat the food and its package as a whole after packing to avoid post‐process contamination and to serve consumer needs for convenience. This process has been successfully completed using polymer film integrated with an electrically conductive film to form a conductive package. Orange juice packed in the conductive package surrounded with a conductive medium was pasteurized in an ohmic heater. A mathematical model was developed to simulate the temperature distribution within the package and its surroundings. A 3‐D thermal‐electric model showed heating uniformity inside the food package while the hot zone appeared in the orange juice adjacent to the conductive film. The accuracy of the model was determined by comparing the experimental results with the simulated temperature and current drawn; the model showed good agreement between the actual and simulated results. An inoculated pack study using Escherichia coli O157:H7 indicated negative growth of viable microorganisms at the target and over target lethal process temperatures, whereas the microorganism was present in the under target temperature treatment. Consequently, our developed ohmic heating system with conductive packaging offers potential for producing safe food.     

Microwave heating of cooked pork patties as a function of fat content

ABSTRACT:  In this study, the parameters heating rate, dielectric factors, and specific heat capacity, which determine the heating profiles and the dissipation of energy during microwave heating, were obtained for precooked pork patties. The pork patties studied were lean meat, pure back fat, and several lean meat/back fat mixtures. Microwave heating at 798 W power was not sufficient to accelerate the heating rates in 100% lean meat compared to 415 W. However, samples were able to heat up faster and dissipate more energy at 798 W power as fat content increased and moisture content decreased. The recorded differences in the specific heat capacity of the studied materials as a function of temperature seemed not to be the key factor to explain the observed temperature rises. Temperature rise seemed to have more to do with the interactions of fat with the electromagnetic field, and with viscosity changes during phase transitions. Trends found for the dielectric properties over microwave heating of meat products agree with data from other authors, but the influence of parameters related to the sample composition and structure should be taken into account. The dissipation factors (ɛ"/ɛ') provided a good approximation to the capacity of the samples containing lean meat and the lean meat/fat mixtures to transform the electromagnetic energy into heat. Neither the dielectric constant nor the loss or dissipation factors were able to clarify the high amount of energy transformed into heat in 100% back fat. Penetration depth and reflected power indicated that back fat allowed microwave energy to be repeatedly redirected to the material.     

MODELING AND OPTIMIZATION OF OHMIC HEATING OF FOODS INSIDE A FLEXIBLE PACKAGE

A pulsed ohmic heating system and flexible package for food reheating and sterilization were developed to minimize Equivalent System Mass during long‐duration space missions. A package made of flexible pouch materials was powered through a pair of metal foil electrodes extending out. Preliminary tests of the package within an ohmic heating enclosure show that International Space Shuttle menu items such as chicken noodle soup and black beans could be heated using pulsed ohmic heating technology. The electrical conductivities of selected samples ranged between 0.01 and 0.03 S/cm. A 2‐D thermal‐electric model was developed using commercial CFD software Fluent to optimize the design and layout of electrodes to ensure uniform heating of the material. A static system under the assumption of there being no fluid motion in a microgravity environment was implemented. A package configuration with V‐shaped electrodes with dimensionless width of 0.147 was validated to be most appropriate for uniform heating while minimizing the cold zone to 2% of total area. The effect of field overshoot near the electrode edge is expected to be crucial to determine the uniformity of heating.     

Multiphysics modeling of microwave heating of solid samples in rotary lifting motion in a rectangular multi-mode cavity

Making samples moving during microwave heating is popular way to improve the heating uniformity. However, the common methods, e.g. using turntable and conveyor belt, can only make samples move in one plane, resulting in limited effect on improving the heating uniformity. In order to improve the uniformity of microwave heating in both horizontal and vertical directions, a rotary lifting turntable is used to drive the sample to rotate and lift during heating in this work. To observe the improvement of heating uniformity caused by the rotary lifting motion, an algorithm based on level set methods and implicit function are used to model the process. The electromagnetic field distribution during the heating process obtained by the model is verified by the discrete locations. The transient-temperature of three points and the spatial temperature profile obtained by the model are verified by the experiments. With this model, the heating effect of samples in rotary lifting motion is compared to that of samples only in rotary or lifting motion. The results show that the heating effect of the samples in rotary lifting motion is better than that of most of the samples in static state and single motion mode. In addition, influences of rotating speed, lifting speed, sample properties, and diameter of spiral motion on the heating effect is also analyzed. Since the proposed motion mode is aimed to change the horizontal and vertical position of the sample in the electromagnetic field to make the sample absorb microwave uniformly in all directions, it can be applied to all kinds of microwave cavities regardless of geometric shapes.     

Understanding the heating characteristics in microwave heating moving and deforming foods by a hybrid double-layer ALE/implicit algorithm

Multiphysics modeling is an essential tool to understand the heating characteristics of microwave processing food products. However, when the heated sample is moving and deforming during the heating process (e.g. the long-term microwave drying process), the huge mesh distortions caused by the sample's motion and deformation will make the simulation impossible. To solve this problem, a hybrid double-layer Arbitrary-Lagrange-Eulerian (ALE)/implicit algorithm is proposed in this paper, where the sample's motion and deformation is tracked by a double-layer ALE framework and convert to time-varying implicit variables. By doing so, the influence of the sample motion and deformation on the electromagnetic field distribution will be characterized by the evolution of time-varying implicit variables, rather than by explicit modeling, which reduces the mesh distortion and makes the calculation possible. A two-dimensional and a three-dimensional model of microwave cavities containing a rotating and deforming food are given to describe the algorithm in detail. Experiments for the three-dimensional model are also conducted to validate the proposed method. Results show that the shrinkage of the food may have a negative impact on the heating performance (especially energy efficiency) and corresponding remedies need to be performed for long-term microwave heating processes.     

Experimental and Numerical Analysis of Microwave Heating of Water and Oil Using a Rectangular Wave Guide: Influence of Sample Sizes, Positions, and Microwave Power

The heating process of water and oil using microwave oven with rectangular wave guide is investigated numerically and experimentally. The numerical model is validated with an experimental study. The transient Maxwell’s equations are solved by using the finite difference time domain method to describe the electromagnetic field in the wave guide and sample. The temperature profiles and velocity field within sample are determined by the solution of the momentum, energy, and Maxwell’s equations. In this study, the effects of physical parameters, e.g., microwave power, the position of sample in wave guide, size, and thickness of sample, are studied. The results of distribution of electric field, temperature profiles, and velocity field are presented in details. The results show that the mathematical models are in agreement with the experimental data. Conclusively, the mathematical model presented in this study correctly explains the phenomena of microwave heating within the liquid layer.     

Environmental remediation by an integrated microwave/UV illumination technique. 8. Fate of carboxylic acids, aldehydes, alkoxycarbonyl and phenolic substrates in a microwave radiation field in the presence of TiO2 particles under UV irradiation

Thermal and nonthermal effects originating when a system is subjected to a microwave radiation field in the TiO2-photocatalyzed transformation of model substances containing various functional groups (e.g., benzoic acid, phthalic acid, o-formylbenzoic acid, phthalaldehyde, succinic acid, dimethyl phthalate, diethyl phthalate, and phenol) have been examined under simultaneous irradiation by ultraviolet (UV) and microwave (MW) radiations. Characteristics of the microwave effects and the fate of each substrate during the microwave-assisted photocatalytic process were monitored by UV absorption spectroscopy, HPLC methods, total organic carbon assays, and identification of intermediates using electrospray mass spectral techniques. Microwave thermal and nonthermal effects were delineated by comparing results from MW-generated internal heat versus conventional external heating, and at constant ambient temperature under a microwave field. Factors involved in the nonthermal component of the microwave radiation were inferred for the initial adsorption of the substrate and its subsequent degradation occurring on the surface of TiO2 particles. Microwave effects bear on the mechanism through which a model substrate undergoes oxidative degradation. A characteristic feature of these effects was briefly examined by considering the behavior of polar (dipole moments) substrates in a microwave radiation field.     

食物位置对热风微波耦合加热效果的影响

给出了微波与热风耦合同步加热物料的数学物理模型,以微波与热风耦合加热土豆为例,建立了完整的三维仿真模型,采用数值模拟法,分别得到了位于耦合加热腔内7个不同位置上土豆最终温度场的分布参数,并以实验加以验证,对数值模拟所得的仿真结果进行了分析与评定。结果表明,不同位置对热风微波耦合加热物料的效果影响极大,这主要体现在影响微波电磁场能量转化为热能功率的大小,以及密度分布及被加热食品温度场分布的不均匀性,故在采用热风微波加热食品时,应当要重视物料位置对耦合加热效果的影响。     

Modeling and Simulation of Microwave Heating of Foods Under Different Process Schedules

This paper describes the development of a simulation model for heating of foods in microwave ovens and its uses to optimize food heating strategies. The solution of the coupled energy and mass microscopic balances considers the electromagnetic energy absorption as well as temperature-dependent thermal, transport, and dielectric properties. The microscopic balances are highly nonlinear coupled differential equations, which were solved using finite element software (Comsol Multiphysics). Maxwell equations were employed in order to describe the interaction between electromagnetic radiation and food. The mathematical model allowed the evaluation of the effect of product size and composition in the temperature profiles that developed inside the food that was radiated either on one or both sides. In order to improve the nonuniform temperature profiles that occurred within foods under continuous operation, different operation schemes were evaluated: intermittent cycles, joint action of microwaves with air impingement, and the effect of interference of electromagnetic waves.     

凝胶态烟油的制备及在电子烟中的应用

  目的  避免电子烟在储存和抽吸过程中发生烟油泄漏。  方法  单因素实验研究明胶含量、反应时间、加热温度对凝胶态烟油持液力、透光率及凝胶时间的影响;采用响应面法对制备工艺进行优化;通过扫描电镜（SEM）及热分析法研究凝胶态烟油的微观形貌和热分析特性,并对其添加在电子烟后的漏油效果和感官质量进行评价。  结果  （1）制备凝胶态烟油的合理工艺参数优化范围为明胶含量2%~6%、反应时间3 h ~5 h、加热温度80℃~100℃;各因素对凝胶时间的影响程度依次为明胶含量 > 加热温度 > 反应时间。（2）最佳工艺条件为:明胶含量为5.93%、加热温度为80℃、反应时间为4.16 h,对应凝胶时间预测值为1.95 h,模型具有较好的适用性。（3）在35℃~250℃范围内,明胶具有较好的热稳定性,烟油中主要成分的雾化基本完全。与液体烟油相比,凝胶态烟油在电子烟中显示出良好的抗渗漏效果,且感官质量无明显差异。      

Feasibility of aseptic processing of a low-acid multiphase food product (salsa con queso) using a continuous flow microwave system

ABSTRACT:  Aseptic processing of a low-acid multiphase food product using a continuous flow microwave heating system can combine the advantages of an aseptic process along with those of microwave heating. Dielectric properties of 2 different brands of 1 such product ( salsa con queso ) were measured under continuous flow conditions at a temperature range of 20 to 130 °C. At 915 MHz, the dielectric constant ranged from 58.7 at 20 °C to 41.3 at 130 °C with dielectric loss factor ranging from 41.0 at 20°C to 145.5 at 130 °C. The loss tangent at 915 MHz ranged from 0.61 at 20 °C to 3.52 at 130 °C. The temperature profiles at the outlet during processing of salsa con queso in a 5-kW microwave unit showed a narrow temperature distribution between the center and the wall of the tube. The study showed the feasibility of aseptic processing of salsa con queso using a continuous flow microwave system.     

Analysis of thermal effect using Coupled Hot-air and Microwave heating at different position of potato

A 3D mathematical food model of Coupled Hot-air and Microwave (CTMW) heating was developed to understand complex microwave interactions in food. The heating process at different position of potato using CTMW heating was investigated numerically and experimentally. Simulated spatial temperature profiles were compared with experimental ones, and predict spatial temperature profiles were in good agreement with experimental ones. In this study, the effect of the potato's position inside cavity was studied. The simulation results were analyzed and evaluated. The results showed the loss power values& density distribution at different position of potato varied greatly, which directly led to the potatoes in the uneven distribution of temperature field. The results are useful in understanding complex microwave heating, designing CTMW heating systems.     

Analysis of heat transfer during ohmic processing of a solid food

To produce a safe cooked food product it is necessary to ensure a uniform heating process. The aim of this study was to develop a mathematical model of a solid food material undergoing heating in a cylindrical batch ohmic heating cell. Temperature profiles and temperature distribution of the ohmic heating process were simulated and analysed via experimental and mathematical modelling which incorporated appropriate electromagnetic and thermal phenomena. Temperature profiles were measured at nine different symmetrically arranged locations inside the cell. The material was ohmically heated imposing a voltage of 100 V, while electrical field and thermal equations were solved for experimental and theoretical models by the use of FEMLAB, a finite element software. Reconstituted potato was chosen to represent a uniform solid food material and physical and electrical properties were determined prior to the experiment as a function of temperature. The simulation provided a good correlation between the experimental and the mathematical model. No cold spots within the product were detected but both experimental and model data analysis showed slightly cold regions and heat losses to the electrode and cell surfaces. The designed model could be used to optimize the cell shape and electrode configurations and to validate and ensure safe pasteurisation processes for other solid food materials.     

Numerical analysis of survival of Listeria monocytogenes during in-package pasteurization of frankfurters by hot water immersion

ABSTRACT:  The objective of this research was to develop and validate a more accurate method to analyze and calculate the inactivation of Listeria monocytogenes in frankfurter packages during postlethality hot water immersion heating and the subsequent cooling processes. Finite difference analysis with implicit scheme was used to simulate the heat transfer process during in-package pasteurization of frankfurters. A volumetrically distributed simulation method was developed to calculate the lethality of the thermal treatment. The simulation method was validated using frankfurter packages inoculated with a 4-strain cocktail of L. monocytogenes. Experimental results showed that the numerical analysis model could accurately simulate the heat transfer process during heating and cooling of frankfurter packages. The simulated temperatures on the surface or in the middle of the package matched very closely with the experimental observations. Using the simulated temperature distribution in the packages, the integrated lethality simulation method, based on the volumetric distribution of bacteria, could accurately predict the reduction in the bacterial counts. The calculation results were on average within 0.3 log(CFU/g) difference from the experimental observations, while the General Method systematically underestimated the bacterial reductions by approximately 0.9 log(CFU/g). The study shows that the integrated lethality method is more accurate than the General Method in calculating the lethality of thermal processes for conduction-heated foods.