食品微波冷冻干燥技术的研究进展

介绍了微波冷冻干燥技术的原理,综述了微波冷冻干燥技术发展的历程及其应用和研究情况,进一步促进该技术从实验室研究走向工业应用领域。     

文物冷冻干燥水蒸气扩散系数的测量

采用聚乙二醇(PEG)溶液进行预处理,然后加以冷冻干燥,是保护出土饱水木质文物的有效方法。在自制的冷冻干燥装置上进行了仿木质文物的冻干实验,根据实验数据拟合得到了干燥区的水蒸气扩散系数。该数据将为文物冻干过程的时间预测和控制优化提供帮助。     

初始饱和度对微波冷冻干燥过程传热传质的影响

以非饱和含湿牛肉为例,进行了微波冷冻干燥实验研究。获得了干燥时间与物料初始饱和度近似成正比的结论。与升华面模型的计算结果比较表明,升华冷凝模型更符合实验规律,证实了非饱和含湿多孔介质微波冷冻干燥时升华冷凝区的存在。     

介电物质对微波冷冻干燥影响的理论研究（一）

根据Luikov法建立了一个介电物质强化的微波冷冻干燥质、热传递数学模型，以考察介电物质对微波冷冻干燥的影响。该模型采用更具有实际意义的柱坐标系，考虑了非饱和多孔冰区内的升华一凝华现象。干燥过程存在两个移动边界。     

真空冷冻干燥非稳态热质传递模型的应用

真空冷冻干燥非稳态热质传递模型的应用齐锡龄１杜小泽２李惟毅１（１天津大学热能工程系，天津３０００７２）（２清华大学热能工程系，北京１０００８４）关键词冷冻干燥热质传递升华压力１引言通过理论计算确定真空冷冻干燥速率或干燥时间对指导生产实践很有意义。但是...     

介电材料辅助的微波冷冻干燥的实验研究

设计、加工和装配了一套实验室规模的微波冷冻干燥装置,旨在实验验证介电材料对微波冷冻干燥液体物料的强化作用。介电材料用烧结的碳化硅(SiC),石英玻璃作为介电材料的参照物;甘露醇,一种典型的药物赋形剂被选为待干溶液中的溶质。实验结果表明使用介电材料可以有效地强化微波冷冻干燥过程。与传统冷冻干燥相比干燥速率大大加快,在试验条件下干燥时间可以节省20%。微波加热逐渐生效并且主要体现在干燥过程的后半部分。当溶液中的固含量很低或者固体物质具有很小的介电损耗因子时,如果不用介电材料,微波加热的效果不明显。     

介电材料辅助的微波冷冻干燥的数值模拟

通过数值求解一个考虑吸湿效应的带有移动升华界面的多孔介质热、质传递耦合模型,理论考察介电材料对微波加热冷冻干燥过程的影响.介电材料用烧结的碳化硅(SiC).甘露醇,一种典型的药物赋形剂被选为待干溶液中的溶质.模拟结果表明在微波冷冻干燥过程中使用介电材料可以加快冷冻干燥速率,特别是在待干溶液的固含量很低或者固体产品的介电损耗因子很小的情况下尤为有效.模型预测和实验测定的干燥曲线相比较显示了良好的一致性.通过考察冰饱和度和温度的分布侧形,研究分析了物料内部的质热传递机理,并讨论了干燥速率的控制因素.     

食品冷冻干燥过程的模型及影响因素

在合理简化的基础上,得到了只需较少参数的冷冻干燥过程一维数学模型。通过对胡萝卜和土豆食品的冷冻干燥实验,研究了加热方式、加热搁板温度和干燥室真空度对干燥时间的影响,模型的数值解与实验值吻合良好。     

微波真空干燥高粘度的灵芝浓缩液

热敏性、高粘度物料的干燥仍是现代干燥技术中的一个难题。灵芝水提后，由薄膜蒸发器浓缩至含水量70％左右，然后采用微波真空干燥（真空度3000Pa）至水分含量10％左右，改用传统的电热真空干燥（55～60℃）至含水量6％～7％。对干燥产品的主要生物活性成分——灵芝多糖和三萜酸进行了分析检测并与其它干燥方法进行了比较。结果表明：利用微波真空干燥的灵芝产品其灵芝多糖和三萜酸的保留率与冷冻干燥产品十分接近。而比传统真空干燥（60—65℃）的产品要高得多，此外采用微波真空干燥的干燥时间要短得多。     

微波辅助萃取过程中的分子扩散系数

建立了微波萃取虎杖中白藜芦醇过程中的分子扩散势垒模型;结合粘度理论建立了分子扩散系数与活化能之间的关系;利用活化能的概念和微波吸收功率密度表达式得到微波作用下的扩散系数表达式。从表达式可以看出,微波作用下的扩散系数与目标萃取物质的密度、微波频率及萃取温度有关,与未加微波的情况比较扩散系数明显增大。在微波辅助萃取有效成分过程中,若萃取温度较高,应选择频率较高的微波进行萃取,以最大限度提高扩散系数;对可能发生热失控现象的物质进行提取时,萃取温度应选择在临界温度附近,此时扩散系数较大。     

Browning behavior of button mushrooms during microwave freeze-drying

In order to achieve faster drying along with high product quality, microwave freeze-drying (MFD) was applied to dry button mushrooms. Although MFD can lead to similar product quality compared with freeze-drying (FD), the color deterioration of MFD mushroom is higher than FD ones. Therefore, two drying methods were used to investigate the browning behaviors of button mushrooms during MFD in terms of parameters including polyphenol oxidase activity, total phenolic content, moisture content, reducing sugar content, and vitamin C content. It was found that both whiteness and browning degree can be used to describe the browning kinetics of button mushrooms during the MFD process. Both nonenzymatic browning and enzymatic browning took place during MFD of button mushrooms, but the effect of enzymatic browning was more significant, which should be controlled during MFD. In order to avoid enzymatic browning during MFD of button mushrooms, a relatively low microwave power at initial stage of MFD should be adopted. Meanwhile, a low microwave power also should be used at the end stage of MFD so as to reduce nonenzymatic browning.     

Multicomponent mass transfer in films and rigid drops: The influence of concentration-variable diffusivity

The typical approach to multicomponent mass transfer calculations assumes that the multicomponent diffusivities are constant. The errors which result because of this assumption have been accepted because of the complexity of the calculations resulting when diffusivity as a function of concentration is included. The influence of this assumption, previously not demonstrated, has now been determined for two physical situations—a stagnant film and a rigid drop (a solid sphere, or liquid drop in which there is no internal circulation). Multicomponent diffusivities, concentration profiles, and mass fluxes have been computed to compare results for constant, and for concentration-variable, diffusivity. Results include the cases of flux operating against the concentration gradient and non-zero flux of components with no imposed gradient. The technique used to estimate mass transfer rates in multicomponent mixtures considers the effects of multicomponent interactions (friction) among molecules and also the thermodynamic correction to these interactions based on the Maxwell-Stefan (MS) formulation. A program written in Mathcad was used to carry out the calculations. The coupled differential equations which represent solute balances were solved numerically applying a central finite difference scheme and the Crank-Nicholson method. The resulting system of equations in matrix form were solved with a program also generated with Mathcad. Results show that concentration-variable diffusivity may give significantly different (as much as 80%) component mass transfer flux values, when determined for high fluxes and large imposed concentration differences. However, even in this regime, it may be possible to formulate suitable constant ‘average’ diffusivities to produce results which are much closer to those predicted with concentration-dependent diffusivities.     

A turbulent mass diffusivity model for analyzing the mixing characteristics in an impinging stream-rotating packed bed

In this study,the fluid flow and mixing process in an impinging stream-rotating packed bed(IS-RPB)is simulated by using a new three-dimensional computational fluid dynamics model.Specifically,the gas-liquid flow is simulated by the Euler-Euler model,the hydrodynamics of the reactor is predicted by the RNG k-ε method,and the high-gravity environment is simulated by the sliding mesh model.The turbulent mass transfer process is characterized by the concentration variance c2 and its dissipation rateεc formulations,and therefore the turbulent mass diffusivity can be directly obtained.The simulated segregation index Xs is in agreement with our previous experimental results.The simulated results reveal that the fringe effect of IS can be offset by the end effect at the inner radius of RPB,so the investigation of the coupling mechanism between IS and RPB is critical to intensify the mixing process in IS-RPB.     

Effect of microwave freeze-drying on microbial inactivation,antioxidant substance and flavor quality of Ashitaba leaves (Angelica keiskei Koidzumi)

To decrease microbial quantity and conserve antioxidant substances of Ashitaba leaves, microwave freeze-drying was conducted at 1, 1.5, and 2 W/g in pilot scale. After drying, microbial inactivation and quality parameters of color, chlorophylls, flavonoids, moisture content, and flavor of dried Ashitaba leaves were evaluated. After drying, the highest contents of chlorophylls and flavonoids maintained 14.62?g/kg and 15.75?mg/g, respectively; microbial colonies decreased by about 2-log; various tastes of dried sample were improved. It could be a suitable dehydration in processing of Ashitaba leaves.     

硫酸铵微波干燥特性及动力学模型分析

采用微波干燥技术进行了硫酸铵干燥的研究.通过系统实验分析了硫酸铵微波干燥特性,并采用薄层干燥模型进行数值分析.结果表明:修正Page模型(Ⅱ)较之其他模型更适于薄层硫酸铵微波干燥的模拟.应用Fick第二定律得到微波功率230-700 W,物料质量150-300 g条件下薄层硫酸铵微波干燥的有效扩散系数的变化范围分别为:...     

A variable diffusivity shrinking-core model and its application to the direct sulfation of limestone

A variable diffusivity shrinking-core model that allows for temporal and spatial variation of the effective diffusivity in the solid product layer was developed in this study. The model was used to analyze a set of experimental data for the direct sulfation of three limestones (that is, their reaction with SO2 under noncalcining conditions), and a parameter estimation procedure was formulated for extracting the effective diffusivity as a distributed parameter from the experimental conversion vs. time curves. Very strong effects of temperature, concentration, and depth in the product layer on the effective diffusivity were revealed by the analysis. On the basis of the obtained results, it was concluded that despite being essentially nonporous, uncalcined limestones can react with SO, under noncalcining conditions at rates comparable to those of calcined samples because they present much lower resistance for diffusion of SO2 through the product layer than the latter.     

Diffuse interface model of the freeze-drying process of individually frozen products

Abstract

Vacuum freeze-drying (VFD) is a dehydration method based on the sublimation of the liquid phase contained in a certain product, previously frozen, at low pressure and temperature. Since it is a time and energy consuming process, it is crucial to select the best processing conditions to minimize drying duration, thus reducing the energy requirement. Additionally, product temperature must be monitored since it plays an important role in preserving product quality. The aim of this study was to develop a Diffuse Interface Model (DIM) for in-silico simulation of the freeze-drying process of individually frozen products. Due to the geometrical features of the samples, and to the role of radiation in the heat transfer to the product, the usual one-dimensional approach is inappropriate. Using a DIM, each cell of the computational domain can be described as a porous solid matrix filled by ice and vapor with a time-varying composition, thus allowing the use of a fixed computational grid and making the computation effort less demanding in comparison to moving interface-based models. Drying of eggplant cubic samples was considered as case study: model parameters were estimated by fitting the experimentally measured product temperature and drying time to the calculated ones. The model was proven to be reliable in providing an accurate estimate of both the drying time and the product temperature. Therefore, it can be used for off-line process design and optimization, minimizing the experimental effort required to design and optimize the process.     

Determination of appropriate effective diffusivity for different food materials

Effective diffusivity is the most important key parameter needed in the analysis, design, and optimization of heat and mass transfer during food drying process. In general, two types of effective diffusivities are used to develop the mathematical modeling of food drying, namely, moisture-dependent effective diffusivity (MDED) and temperature-dependent effective diffusivity (TDED). However, no study has extensively investigated which effective diffusivity is more accurate in predicting drying kinetics. The main goal of this study is to determine the appropriate effective diffusivity for predicting the drying kinetics. Drying models were developed for different fruits and vegetables based on moisture-dependent and temperature-dependent effective diffusivities. COMSOL Multiphysics, a finite element-based engineering simulation software is used to solve the coupled heat and mass transfer equations. 3D moisture profiles were developed to investigate the spatial moisture distribution during drying. Extensive experimental investigation on five types of fruits and vegetables was conducted and results were compared with the simulated results. The experiments were repeated thrice, and the average of the moisture content at each value was used for constructing the drying curves. Close agreement between experimental and simulated results validates the models developed. It was observed that the moisture profile and temperature profile in case of MDED were more closely fitted with the experimental results. For all fruits and vegetables, the moisture ratio with MDED was significantly lower than moisture ratio with TDED. This finding confirms that the MDED is more accurate for predicting kinetics in food drying. Moreover, the moisture ratio of apple was lowest whereas pear showed the highest moisture ratio. On the other hand, carrot showed a considerably lower moisture ratio compared to potato.     

Combined effects of additives and power levels on microwave drying performance of lignite thin layer

The microwave drying performance of lignite thin layer in a bench-scale setup was highlighted in terms of three additives with 10% dosage. The dielectric loss for dried lignite, raw lignite, lignite/coal fly ash, lignite/Na₂SO₄, and lignite/Na₂CO₃ at 2,450?MHz were 0.06, 0.13, 0.14, 0.15, and 0.18. In comparison with raw lignite, the average temperature rising of the thin layer at 385?W for lignite blending with sodium carbonate, sodium sulfate, and coal fly ash was about 10, 7, and 2°C. The apparent activation energies of both falling rate periods for lignite blending with three additives were less than that of raw lignite. Sodium carbonate among three additives could be preferable one, followed by sodium sulfate and coal fly ash. The energy efficiency increased with the addition of sodium carbonate, sodium sulfate, and coal fly ash. The required electricity energy for lignite/Na₂CO₃ blend at 385?W was reduced by about one half compared with the raw lignite.     

Mathematical model for intermittent microwave convective drying of food materials

Intermittent microwave convective drying (IMCD) is an advanced technology that improves both energy efficiency and food quality in drying. Modeling of IMCD is essential to understand the physics of this advanced drying process and to optimize the microwave power level and intermittency during drying. However, there is still a lack of modeling studies dedicated to IMCD. In this study, a mathematical model for IMCD was developed and validated with experimental data. The model showed that the interior temperature of the material was higher than the surface in IMCD, and that the temperatures fluctuated and redistributed due to the intermittency of the microwave power. This redistribution of temperature could significantly contribute to the improvement of product quality during IMCD. Limitations when using Lambert’s law for microwave heat generation were identified and discussed.