Microwave Heating Distributions in Slabs, Spheres and Cylinders with Relation to Food Processing

Relatively simple mathematical models for microwave energy penetration in food products were applied, based on Lambert's law and taking into account internal reflections. With these models direct insight in microwave heating distributions in products with basic geometries and homogenous composition can be gained. Effects of changes in product composition with relation to dielectric properties and changes in product size can be predicted. The model was not designed to give an exact and detailed prediction of temperature distributions during microwave heating. However, it can simulate microwave heating of various food products and be useful for designing heat treatments.     

Microwave-Heating Temperature Profiles for Thin Slabs Compared to Maxwell and Lambert Law Predictions

Slabs of agar gel were heated in a microwave oven. Temperatures were measured at various depths into the sample to experimentally determine the internal temperature profile. These were compared to power and temperature profiles predicted from Lambert's law, Maxwell's field equations and a Combined equation. Lambert's law and the Combined equation predicted a much slower heating rate than found experimentally, while Maxwell's field equations gave a much more accurate prediction. Because of the internal standing waves that are created, a small variation in sample thickness could make a large difference in heating rate for thin samples.     

Mathematical Modeling and Simulation of Microwave Thawing of Large Solid Foods Under Different Operating Conditions

Microwaves require shorter times to increase foodstuffs temperature when compared to conventional heating methods. However, there are some problems associated to temperature distribution within the products, owing to the preferential absorption of electromagnetic energy by liquid water, caused by differences between its dielectric properties and those of ice (“runaway”). To analyze the behavior of food microwave thawing, a mathematical three-dimensional (3D) model was developed by solving the unsteady-state heat and mass transfer differential equations; this model can be applied to large systems for which Lambert’s law is valid. Thermal, mass transport, and electromagnetic properties varying with temperature were used. The numerical solution was developed using an implicit Crank–Nicolson finite difference method using the classical formulation for one-dimensional (1D) systems and the alternating direction method in two and three dimensions. The model was validated using experimental data from the literature for 1D and two-dimensional conditions and with experiments performed in our laboratory for 3D heat transfer using frozen meat. It was applied to predict temperature and water concentration profiles under different thawing conditions in meat products and to simulate the effect of a fat layer located at the surface of the meat piece on temperature profiles. For different product sizes in rectangular geometry, numerical simulations demonstrated that microwave thawing times were significantly lower in comparison to conventional thawing methods. To prevent overheating during thawing, the combination of continuous microwave power with simultaneous application of air convection and the application of microwave power cycles, using refrigerated air convection with controlled surface temperature, were analyzed.     

Mathematical Modeling of Microwave Thawing by the Modified Isotherm Migration Method

A mathematical model of microwave thawing of homogeneous food products is developed and solved numerically using the Modified Isotherm Migration Method. The model is used to predict thawing time and temperature profiles for microwave thawed meat cylinders at three frequencies (2450 MHz, 915 MHz, 300 MHz) and different power levels. Model and experimental results for thawing a lean beef cylinder heated at low microwave power using 2450 MHz frequency compare well. The advantage of using 915 or 300 MHz power over 2450 MHz power is shown by calculations. The results show that microwaves significantly accelerate the thawing rate. The mathematical model is explored as a tool for designing optimal microwave/convective heating protocols for rapidly thawing foods in desired temperature ranges.     

Microwave Assisted Thawing of Model Frozen Foods Using Feed-back Temperature Control and Surface Cooling

An apparatus was developed that combined microwave energy and cold air with different ambient temperatures to reduce thawing time and avoid run-away heating during microwave assisted thawing. Effects of microwave power level, sample thickness and surface air temperature on thawing time were investigated. The microwave power was cycled on and off using two temperature control schemes to maintain a predetermined temperature gradient based on hot and cold points. Thawing time was reduced by as much as a factor of seven compared to convective thawing at ambient temperature when appropriate conditions were used.     

A study of the power absorption and temperature distribution during microwave reheating of instant rice

Microwave heating is a rapid and convenient food‐reheating method. However, uneven heating often occurs. To study the ‘cold spot’ problem of chilled instant rice during microwave reheating, online monitor was used to determine the temperature distribution during reheating. A cylindrical model was used to simulate the microwave reheating process of instant rice, and Lambert’s law was applied to analyse the microwave power absorption of rice and to predict the reheating temperature. The results showed that the internal temperature was highest at the geometric centre during microwave reheating. The predicted data using Lambert’s law matched the experimental results with relatively good agreement, and the correlation coefficient R² is between 0.990 and 0.999.     

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.     

Physicochemical properties of thawed chicken breast as affected by microwave power levels

The main objective of this study was to determine the effect of microwave thawing at different power levels on the physicochemical properties of chicken breast. Each frozen chicken breast samples were thawed in microwave oven at microwave power levels of 125, 250, and 375W. To compare the effect of microwave thawing on meat quality, refrigerator thawing was used as a control. As the microwave power level increased, the thawing rate increased and temperature differences between the edge position and center position of the samples also increased. Treatment at a power level of 250 W resulted in a lower thawing loss and a higher moisture content, water holding capacity, and tenderness than the other treatments (p<0.05). However, there were no significantly differences of proximate composition excluding moisture content, pH, lightness, and yellowness. The use of excessive input energy during microwave thawing (higher microwave power level) damaged the physicochemical properties of the thawed chicken breast; therefore, the microwave power level of 250 W is the best thawing method on the frozen chicken breast to minimize the changes in meat quality in this study.     

Power absorption analysis of two-component materials during microwave thawing and heating: Experimental and computer simulation

Simulation of power absorption during microwave (MW) thawing is challenging, particularly for small-volume frozen samples, owing to the differences in properties of the frozen and unfrozen portions. In this study, experimental analysis was conducted on the power absorption of two-component materials including water and oil, where oil was used as an analogue to frozen materials, and the power absorption of each component was estimated based on the measured temperature. A previously developed three-dimensional (3D) model for a flatbed MW oven was modified and used to simulate the temperature profiles and distribution during MW heating at 2450 MHz, 600 W. A power absorption approach was developed by using an empirical equation considers the composition, volume, and dielectric loss factor of each component during MW thawing and heating. The simulated results were successfully tested with the experimental results for small and large volumes of the coupled water/oil samples and were validated for small and large volumes of frozen Tylose paste.     

不同解冻技术对冷冻香菇(L.edodes)品质的影响

本文研究常温空气、冷藏、微波(119、259和280 W)以及超高压(100、150和200 MPa)解冻对香菇品质(色泽、汁液流失率、硬度、离子渗透率以及总酚含量)和解冻时间的影响。结果表明:与常温空气和冷藏解冻相比,微波和超高压解冻能够显著缩短香菇的解冻时间,并且随着微波功率和压强的增大,解冻时间逐渐缩短。但超高压解冻后香菇色泽与硬度显著下降,离子渗透率显著上升,从而使得香菇品质显著降低;而微波解冻在色泽(总色差降低19.9~44.1%)、硬度(升高18.9~38.0%)、汁液流失率(降低45.7~42.0%)、总酚含量(升高21.1~25.2%)和其他生理指标方面都优于常温空气解冻技术。其中,259 W的微波功率是香菇解冻的最优条件,同时该条件的感官评价分值最高(7.1)。本文表明微波解冻能够显著提高香菇的解冻速率和品质,这对香菇加工具有一定意义。     

冻豆腐微波解冻工艺优化

为改善传统解冻耗时长、易污染的缺点,适应现代化餐饮业需要,利用微波技术对冻豆腐进行解冻。设定不同微波功率和微波时间对冻豆腐进行处理,以基本成分、质构特性、p H、色泽、感官品质为评价指标,并与新鲜豆腐进行对比,考察微波解冻对豆腐品质的影响。确定最佳解冻参数为微波功率150 W、解冻时间90 s。     

An investigation of the dielectric and thermal properties of frozen foods over a temperature from –18 to 80°C

The dielectric properties (dielectric constant [ε′] and dielectric loss factor [ε″]) for three frozen meals (basil fried chicken, green curry with chicken, and congee with minced pork) were measured at frequency 2.45 GHz from –18 to 80°C. Thermal properties (thermal conductivity [k] and specific heat capacity [c]) of each food item were characterized using their composition for improving the modeling of microwave thawing process of frozen products. For all frozen meals, similar trends in the dielectric and thermal properties values were observed as a function of temperature. In all samples of frozen foods, the dielectric properties (ε′, ε″) rapidly increased with temperature for the range from –10 to 0°C. Thereafter, dielectric properties linearly increased with temperature for basil fried chicken and congee with minced pork but linearly decreased with temperature for green curry with chicken from 0 to 80°C. The dielectric properties data bases were used to calculate power reflected (P_r), power transmitted (P_t), dielectric loss tangent (tanδ), and penetration depth (d_p). These parameters were related to the dielectric properties. The thermal conductivity values of all samples decreased with increasing temperature in the frozen stage and little changed after thawing. While the specific heat capacity values increased first and then do not change considerably. The temperature-dependent material properties can give insight into how the food product interacts with the incident electromagnetic radiation.     

Thawing Time of Frozen Food of a Rectangular or Finitely Cylindrical Shape

Through dual step simulations, theoretical regression formulae were obtained for estimating the dimensionless thawing time of rectangular or cylindrical food. These formulae contain independent variables related to an initial uniform food temperature, final central food temperature, thawing medium temperature, highest freezing point of food, empirical constant for estimating food enthalpy, shape factor of food and Biot number. The formulae were verified for their reliability by comparing thawing times estimated by these formulae with those determined experimentally. Error analyses were performed by using the formulae to examine the errors of thawing times due to uncertainty in parametric values.     

微波解冻猪里脊肉过程的优化设计研究 Key words:

猪肉在我国是最重要的肉类之一,而为了储存更长时间,通常采用冷冻的方式保存。因此,在进一步加工之前就需要经过解冻这一过程,传统的解冻方式耗时长,严重制约了生产效率。微波加热作为一种常用的加热方式,与传统加热方式相比,具有加热速度快、省时节能等突出优点。本文利用家用变频微波炉对冷冻猪里脊肉块进行解冻处理,对解冻程序进行了优化设计,并与微波炉自带的解冻程序进行了对比研究,对肉汁渗出率、色泽以及质构特性进行分析。结果表明：两种优化程序的解冻效果要优于微波炉自的解冻程序,样品能基本解冻完全,肉汁渗出率较低,能较好维持鲜肉的颜色,经过优化后的微波解冻猪里脊肉温差更小,质构特性更佳,同时也缩短了解冻时间。     

Evaluation of the effects of different thawing methods on texture,colour and ascorbic acid retention of frozen hami melon (Cucumis melo var. saccharinus)

The effects of different thawing methods (room temperature thawing, running water thawing, microwave thawing at 300 W, 500 W, 700 W and high‐pressure thawing at 100 MPa, 150 MPa, 200 MPa) on quality (drip loss, texture, colour, ascorbic acid contents and sensory quality) of frozen hami melon were investigated. The results showed that different thawing treatments affected the quality of hami melon significantly (P < 0.05). Samples thawed by microwave at 500 W best preserved ascorbic acid for nearly half of fresh hami melon. Less drip loss, texture damage and discolouring were observed for the samples thawed by microwave at 700 W than other thawed samples; however, serious ascorbic acid loss was detected. High‐pressure‐thawed samples exhibited shorter thawing time but severe quality losses. The results suggested that microwave thawing at 500 W was an appropriate method to thaw frozen hami melon with the highest sensory score (6.7).     

不同解冻方法对速冻桑葚汁液流失率、理化品质及抗氧化活性的影响

为减少速冻桑葚在解冻过程中的汁液流失率、缩短解冻时间和减少其色泽的变化,增加解冻桑葚的营养物质和抗氧化活性的保留量,采用微波功率500 W解冻、微波功率550 W解冻、超声波解冻、冷藏解冻、室温解冻进行速冻桑葚的解冻实验。结果表明:微波功率500 W解冻和微波功率550 W解冻的解冻时间较短,分别为0.58 min和0.35 min,冷藏解冻的解冻时间最长为305 min。不同解冻方法的汁液流失率存在显著差异,微波功率500 W解冻的汁液流失率最低,为1.34%;超声波解冻的汁液流失率最高,为9.24%。微波功率500 W解冻桑葚的总酚含量(151.66 mg/100 g)、花色苷含量(矢车菊素-3-O-葡萄糖苷2.27 mg/g、矢车菊素-3-O-芸香糖苷0.69 mg/g、天竺葵素-3-葡萄糖苷0.047 mg/g)及抗氧化活性((2,2’-联氨-(3-乙基苯并噻唑啉-6-磺酸)二氨盐)自由基清除能力21.87μmol Trolox/g、1,1-二苯基-2-三硝基苯肼自由基清除能力35.70μmol Trolox/g、铁离子还原能力27.85μmol Trolox/g)均显著高于其他4种解冻方法。综合考虑,微波功率500 W解冻优于其他4种解冻方法。     

Research progress and application of ultrasonic- and microwave-assisted food processing technology

Microwaves are electromagnetic waves of specific frequencies (300 MHz–3000 GHz), whereas ultrasonic is mechanical waves of specific frequencies. Microwave and ultrasonic technology as a new processing method has been widely used in food processing fields. Combined ultrasonic and microwave technology is exploited by researchers as an improvement technique and has been successfully applied in food processing such as thawing, drying, frying, extraction, and sterilization. This paper overviews the principle and characteristics of ultrasonic- and microwave-assisted food processing techniques, particularly their combinations, design of equipment, and their applications in the processing of agricultural products such as thawing, drying, frying, extraction, and sterilization. The combination of ultrasonic and microwave is applied in food processing, where microwave enhances the heating rate, and ultrasonic improves the efficiency of heat and mass transfer. The synergy of the heating effect of microwave and the cavitation effect of ultrasonic improves processing efficiency and damages the cell structure of the material. The degradation of nutrient composition and energy consumption due to the short processing time of combined ultrasonic and microwave technology is decreased. Ultrasonic technology, as an auxiliary means of efficient microwave heating, is pollution-free, highly efficient, and has a wide range of applications in food processing.     

Continuous microwave-assisted extrusion for high moisture texturized foods: A feasibility study

This study demonstrates a technical scale set-up for microwave-assisted extrusion of high moisture, texturized food systems. The set-up has been evaluated in a stationary study regarding the correlation of microwave tuner position and gap with the reflected microwave power and the product temperature, respectively. On that basis, a casein-based model food system has been plasticized by continuous microwave extrusion. The product outlet temperature and chemical composition have been investigated with varying screw speeds and power inputs.The stationary study resulted in specific tuner positions, giving an area of desired process parameters. This area was successfully transferred to a continuous product application, where desired gel-sol transition was achieved. With increasing power input, the product temperature increased, while it decreased with increasing screw speed and constant power input. The fat, protein, and dry matter content of the casein-based model system did not differ significantly throughout all trials.Industrial relevanceMicrowave heating is a promising alternative heating method for food applications. Within microwave extrusion it shows high potential for upscaling applications whilst maintaining a homogenous temperature distribution throughout the product. Moreover, microwave technology offers high efficiencies and the possibility to use green energy sources. Furthermore, the space saving design and easy application provide high potential for scale-up.     

超声波解冻与传统解冻方式的比较与竞争力评估

超声波解冻通过空化效应、机械效应和热效应对冷冻食品进行解冻,解冻效率高、对冷冻食品的损伤较小。该文简要介绍了空气解冻、水解冻和电解冻的解冻机制及研究进展,详细阐述了超声波解冻的机制及研究进展。将超声波解冻与空气解冻、水解冻和电解冻进行对比,发现超声波解冻的解冻效率最优,且将超声波与这3种传统解冻方式结合可以提高解冻效率,并且获得更好的解冻效果。超声波频率和强度以及冻结食品的解冻温度是影响解冻效果的主要参数。超声波在食品中的衰减程度受到解冻温度的影响极大,因此在调节超声波频率和强度同时调控解冻温度对解冻效果至关重要。     

Quality changes during frozen storage and thawing of mixed bread

In the present work investigations of the dependence between frozen storage time, the method of thawing (air blast at 50 degrees C and microwave), organoleptic and physico-chemical changes in bread are reported. The quality of the thawed product was analysed directly after thawing and after two days of storage at room temperature. It was found that changes in quality of bread are more affected by frozen storage than by the employed thawing method. The thawing methods had a significant (p < or = 0.01) effect on the investigated physico-chemical parameters of the product, however, their statistically significant (p < or = 0.01) effect on the sensory quality could only be revealed after two days of storage of the previously thawed bread at room temperature rather than directly after thawing. The results obtained in the present study suggest that bread which underwent microwave thawing had generally better quality in comparison with air blast thawing.