Microwave Freeze-drying Process Research Articles

Physicochemical, Pasting Properties and In Vitro Starch Digestion of Chinese Yam Flours as Affected by Microwave Freeze-Drying.

Microwave freeze-drying (MFD) is a new freeze-drying technique, which differs from single microwave treatment; it involves simultaneous effects of microwave power, time, and the moisture state applied to the materials. In this study, the effects of MFD under various microwave power densities (0.5, 1.0, and 1.5 W/g) on the drying characteristics of Chinese yam slices and the physicochemical, pasting, and thermal properties as well as the starch digestibility of the flour were investigated using conventional hot air drying (HAD) at 50 °C as a control. Compared to HAD, MFD shortened the drying time up to 14.29~35.71%, with a higher drying efficiency at a high microwave power density (1.5 W/g). MFD yam flours provided benefits over HAD products in terms of color, water/oil absorption capacity, and solubility, exhibiting high hot-paste viscosity but low resistant starch content. The content of total starch and free glucose of the yam flour and its iodine blue value were significantly influenced by the drying method and the MFD process parameters (p < 0.05). MFD processing could disrupt the short-range ordered structure of yam starch. Among the MFD flours, samples dried by MFD at 1.5 W/g presented the highest ratio of peak intensity at 1047 and 1022 cm−1 (R1047/1022) value, gelatinization enthalpy, and resistant starch content. These results gave a theoretical foundation for the novel freeze-drying method that MFD applied to foods with a high starch content, enabling the production of a product with the desired quality.

Development of a closed-loop control system for microwave freeze-drying of carrot slices using a dynamic microwave logic control

This research aimed to develop a closed-loop control (CLC) system to improve the microwave freeze-drying (MFD) process and to examine the effects of a dynamic microwave logic control (DMLC) on the drying characteristics of MFD. The development process consisted of two parts: (1) the MFD experiment to develop the DMLC, and (2) the implementation of the CLC with the DMLC on the MFD process. In the first part, the MFD process was examined to obtain the strategy for drying the carrot slices using microwave powers of 100 W, 200 W, and 300 W, with a temperature profile of the sample from −15 °C to 40 °C, and the final moisture content of 6% (wet basis). In the second part, the DMLC was strategically developed and integrated into the CLC system. The results showed that in the MFD process, the DMLC was developed based on a drying-phase configuration and dynamic control between the microwave power and real-time moisture content sensing to provide feedback to the CLC system. After applying the DMLC into the CLC system, the efficiency of the MFD process was improved by up to 62.4% by shortening the drying time, as compared with the freeze-dry (FD) process. The MFD-DMLC also resulted in the quality of carrot equivalent to that of a traditional FD process. Since the DMLC exhibited great potential to improve the MFD process, it could be developed for future industrial use for a high-performance MFD process in terms of product quality and process efficiency. • Indices from drying kinetics and moisture diffusion was analyzed and used to develop the drying strategy on microwave freeze-drying (MFD) with the Dynamic microwave logic control (DMLC). • Phase transition during MFD process and its moisture content was identified based on the drying-temperature profiles. • MFD-DMLC can improve drying efficiency up to 60% as compared to the original freeze drying (FD) process.

Mutual transformation of the water binding state and moisture diffusion characteristics of Chinese yams during microwave freeze drying

The moisture distribution changes in Chinese yam slices subjected to microwave freeze drying (MFD) were analyzed using low-field nuclear magnetic resonance (LF-NMR). During this process, the mutual transformation of the water binding state and moisture diffusion characteristics of Chinese yam were analyzed together with changes in the effective water diffusion coefficient, water content, drying rate and pore network structure. Notably, moisture migrated from a high to a low degree of freedom during the MFD process. The effective water diffusion coefficient and water migration velocity increased with increasing microwave power density, and the extent of the conversion of unbound to bound water also gradually increased. During the early stage of drying, moisture diffusion was mainly dominated by liquid phase diffusion. At later stages of drying, moisture diffusion shifted gradually from liquid phase diffusion to gas phase diffusion and was eventually dominated by the latter.

Structural characteristics and texture during the microwave freeze drying process of Chinese yam chips

Fresh Chinese yam is difficult to store and easily deteriorates during storage, and dehydrated yam chips processed by different methods are widely consumed as a kind of snack food. Freeze drying (FD) is often used to produce high-quality yam chips, but the extralong drying time limits its industrial application. Microwave freeze drying (MFD) has the potential to replace the conventional FD method. Therefore, this research used MFD to dry Chinese yam chips. In this study, the brittleness and hardness of Chinese yam chips during the MFD process were investigated. It was found that the brittleness of Chinese yam chips had a close relationship with the moisture content. The hardness of Chinese yam chips mainly depended on the porosity and moisture content. Moreover, the DoseResp model of the brittleness could effectively predict the brittleness change during the MFD process. The regression equation based on the porosity and moisture content could accurately describe the hardness of the Chinese yam chips during the MFD process.

Optimization of microwave freeze drying strategy of mushrooms (Agaricus bisporus) based on porosity change behavior

ABSTRACTFor dehydrated foods, porosity is a crucial parameter which affects mass and heat transfers, and is related to the product quality. It is important to monitor porosity change behavior to optimize the drying process. To achieve faster drying along with high product quality, microwave freeze drying (MFD) was applied to mushrooms dehydration processing. The relationship between porosity (include total, open-, and closed-pore porosity) and dried mushroom qualities was studied, and a suitable microwave loading strategy was obtained. It was found that lots of open pores could transform to closed pores while the moisture content (MC) was below 0.25 ± 0.05 g/g (db), and the closed-pore porosity was arrived at a relatively stable level at moisture content below 0.17 ± 0.03 g/g (db). Both total porosity and closed-pore porosity had a significant influence (P < 0.01) on texture of MFD mushroom, and open-pore porosity had a significant effect (P < 0.01) on rehydration ratio of MFD mushroom. According to the porosity change behavior of mushrooms, a dynamic microwave loading strategy can be used to reduce drying time and keep product quality during MFD process.

Browning behavior of button mushrooms during microwave freeze-drying

ABSTRACTIn 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.

The Effect of Glass Transition Temperature on the Procedure of Microwave–Freeze Drying of Mushrooms (Agaricus bisporus)

Freeze drying (FD) yields the best quality of dried mushroom but at the cost of a long drying time and high overall cost. Air drying (AD) gives an unacceptably poor quality product. To achieve faster drying along with a high-quality product, a microwave–freeze drying (MFD) technique was developed to dry mushrooms. The relationship between dried mushroom quality and the glass transition temperature during the MFD process was studied to optimize the MFD process. According to the change tendency of the glass transition temperature of mushroom during MFD, a step-down microwave loading scheme for the MFD process was developed to obtain good product quality.

The energy consumption and color analysis of freeze/microwave freeze banana chips

This study investigated the energy consumption of preparing banana chips by freeze drying (FD) and microwave freeze drying (MFD) methods. The results in this study showed that the energy consumption for 400g fresh banana (about 95g dried samples) by FD process and MFD process are about 35.73×106J (9.92kWh) and 21.76×106J (6.57kWh), respectively. Compared to the traditional FD process, MFD can save up to 35.7% energy and 40% drying time. Increasing the heating power in the secondary drying stage of MFD process had been confirmed to result in decrease in both the energy consumption and drying time. After increasing the microwave power in the secondary drying stage from 1.0W/g to 1.5W/g in MFD process, total energy consumption is about 18.12×106J (5.56kWh) and drying time can be reduced from 360min to 270min. The sensory evaluation of produced banana chips at different drying conditions (1.0W/g, 1.5W/g and 2.0W/g) revealed that the sensory properties are acceptable by the customers except the 2W/g microwave power dried product. Thus, the method that increased the heating powder in the secondary drying stage of the MFD process could potentially be an effective method to reduce the energy consumption without seriously sacrificing the color of the end product.

Experimental Investigation and Mechanism Analysis on Microwave Freeze Drying of Stem Lettuce Cubes in a Circular Conduit

Stem lettuces are an important vegetable due to its nutritional characteristics. Stem lettuce was used as the model in this investigation. The ice-melting and freezing point temperature as well as distribution and migration of unfrozen water in stem lettuce cubes during the microwave freeze drying (MFD) step were studied using differential scanning calorimetry (DSC) and low-field pulsed nuclear magnetic resonance (LF-NMR). MFD of stem lettuce cubes was carried out in three circular conduits with different diameters (40, 55, and 70 mm) at a microwave frequency of 2,450 MHz. The total drying time for MFD was almost the same for all circular conduits; that is, approximately 4.5 h. This was less than the 7.0 h required for conventional radiation freeze drying (FD) in a 40-mm-diameter circular conduit and far less than the 20 h required for FD in a flat slab model. The microwave energy supplied and specimen temperature profiles during MFD were strongly influenced by the tissues, unfreezable water content, moisture distribution, and ionic characteristics of the samples. It was also found that the MFD process involving the conduits can be divided into three steps based on the amount and distribution of unfrozen water in the frozen samples and how heat is supplied in order to prevent corona discharge.

Effect of blanching on microwave freeze drying of stem lettuce cubes in a circular conduit drying chamber

Effects of boiling water and microwave blanching methods on the dielectric properties, electrical conductivity as well as microstructure of stem lettuce cubes (as a model for plant tissue) were studied. The study also focused on how different blanching methods affect microwave freeze-drying (MFD) in a circular conduit drying chamber and conventional freeze drying (FD) of stem lettuce cubes. Results showed that the electrical conductivity of samples blanched by microwave was two times higher than that of boiling water blanched ones and five times higher than that of unblanched samples. Apart from that, dielectric constant (ε′) and loss factor (ε″) of the stem lettuce cubes decreased significantly after freezing. MFD duration of microwave blanched samples was approximately 4.5h, reduced by 30% compared to boiling water blanched ones. MFD process in a circular conduit yielded products of high quality compared to that in a tray.

Drying Characteristics and Quality of Restructured Wild Cabbage Chips Processed Using Different Drying Methods

The drying characteristics of restructured wild cabbage chips dried using microwave vacuum (MVD), hot air (AD), and microwave freeze drying (MFD) were compared. Some of the key quality parameters of restructured wild cabbage chips such as fracturability and color and sensory characteristics were measured. Results showed that the drying time was reduced with the increase of microwave power (MVD/MFD) and in the case of air drying by the temperature (AD). Drying time was the shortest in the MVD process. Optimal quality of dried chips was obtained with the MFD process at a microwave power level 2.0 W/g.

Optimal Control Policy of the Microwave Primary Freeze Drying of Random Solids

The optimal strategy of microwave energy input in freeze drying is proposed to keep the temperature of the material external surface, ice core, and sublimation interface as high as possible. This should ensure maximum process efficiency without violating the set constraints. The safety factors are introduced on electric field strength to avoid corona discharge. The microwave power is considered either constant or time variable during the process. A mathematical model of the microwave freeze-drying process is formulated with time-varying transport and dielectric properties of the frozen and dried layers. This model takes into account unknown a priori sublimation front temperature T S and mass concentration of water vapor C S at moving ice front. Comparative analysis of the various process operation conditions is performed to determine the optimal control policy for microwave freeze drying. It is demonstrated that applying an optimal control strategy for limiting process conditions significantly reduces the total microwave freeze-drying time in the primary stage.

Microwave freeze drying of sea cucumber ( Stichopus japonicus)

Freeze drying (FD) yields the best quality of dried sea cucumber but at the cost of long drying time and also the overall cost. Air drying (AD) gives an unacceptably poor quality product. To achieve faster drying along with a high quality product a microwave freeze drying (MFD) technique was developed to dry sea cucumbers. The relationship between corona discharge and microwave power at various pressures and initial moisture content conditions was studied to avoid the possibility of corona discharge during MFD. According to the drying characteristics of MFD, a control strategy for the MFD process was also developed. MFD reduced the drying time by about half of conventional FD process and provided a similar good product quality.

Theoretical Study on Microwave Freeze-Drying of an Aqueous Pharmaceutical Excipient with the Aid of Dielectric Material

A mathematic model of simultaneous heat and mass transfer for the dielectric material assisted microwave freeze-drying was derived and solved numerically using the finite-deference technique with two moving boundaries. Lactose, a typical pharmaceutical excipient, was used as the representative solid material in the aqueous solution to be freeze-dried. Silicon carbide (SiC) was selected as the dielectric material. Numerical results show that the dielectric material can significantly enhance the microwave freeze-drying process. Under typical operating conditions, the drying time is 43% shorter than that of ordinary microwave freeze-drying. Temperature variations at sublimation fronts were examined in order to determine the appropriate microwave power input. Profiles of temperature, ice saturation, vapor concentration, and pressure during freeze-drying are presented, and rate-controlling mechanisms are discussed.

Heat and mass transfer model of dielectric-material-assisted microwave freeze-drying of skim milk with hygroscopic effect

A new porous media mathematical model for freeze-drying was developed based on the adsorption–desorption relationship proposed in this paper. A finite difference solution was obtained from a moving boundary problem for the dielectric-material-assisted microwave freeze-drying process. Silicon carbide (SiC) was selected as the dielectric material; and frozen skim milk was used as the aqueous solution to be dried. Simulation results showed that the dielectric material can significantly enhance the microwave freeze-drying process. The drying time was 33.1% shorter than that of ordinary microwave freeze-drying under typical operating conditions. When the solid content of the solution to be freeze-dried was very low, or the solid product had a very small loss factor, microwave heating was less effective without the assistance of dielectric material. The mechanisms of heat and mass transfer during drying were analyzed based on profiles of ice saturation, temperature and vapor concentration. Drying rate-controlling factors were discussed. A comparison was made between the model predictions and the reported experimental data.

Numerical Investigation on Dielectric Material Assisted Microwave Freeze-Drying of Aqueous Mannitol Solution

The dielectric material assisted microwave freeze-drying was investigated theoretically in this study. A coupled heat and mass transfer model was developed considering distributions of the temperature, ice saturation and vapor mass concentration inside the material being dried, as well as the vapor sublimation-desublimation in the frozen region. The effects of temperature and saturation on the effective conductivities were analyzed based on heat and mass flux equations. The model was solved numerically by the variable time-step finite-deference technique with two movable boundaries in an initially unsaturated porous sphere frozen from an aqueous solution of mannitol. The sintered silicon carbide (SiC) was selected as the dielectric material. The results show that dielectric material can significantly enhance microwave freeze-drying process. For case of the dielectric field strength, E = 4000 V/m under typical operating conditions, the drying time is 2081 s, 30.1% shorter and 47.2% longer, respectively, than those for E = 2000V/m and E = 6000 V/m. The heat and mass transfer mechanisms during the drying process were discussed.