Microwave Freeze Drying Research Articles

Effect of microwave freeze drying on moisture migration and gel characteristics of egg white protein

This article mainly explored the influence of microwave freeze drying (MFD) power on water migration and gel characteristics of egg white. Fourier transform infrared spectroscopy (FT-IR), Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and scanning electron microscopy (SEM) were used to determine the water migration law and gel characteristics of egg white under different microwave power drying conditions. The results showed that the free water of egg white was removed first, and the water content decreased exponentially at the initial stage of drying. The weakly bound water and bound water accounted for a large proportion, thus, the drying rate decreased in the later stage. SEM showed that the gel strength of egg white protein enhanced with the drying time and reached a maximum of 399.11 g at 5 h. The gel network structure was also denser and smoother than the initial drying stage. FT-IR analysis implied that in the early stage of drying, protein molecules were unfolded under the influence of drying conditions, the hydrogen bonds maintaining the α-helix were broken, and hydrogen bonds content continually decreased. SDS-PAGE showed that the drying treatment had no effect on the peptide chain structure of the protein, and several protein molecules gathered at 6 h. This study provides a reference for MFD of egg white powder.

Microwave freeze-drying of coffee solution frozen with initial pores assisted by wave-absorbing medium

Wave-absorbing medium (WAM) assisted microwave freeze-drying (FD) of initially unsaturated frozen material was experimentally investigated in an attempt to simultaneously enhance heat and mass transfer of conventional FD. Silicon carbide (SiC) was utilized as the WAM and coffee solution was selected as the raw material. The results showed that microwave FD of the unsaturated sample at 22 Pa, 35 °C and 1 W with quartz pad saved 30.8% of drying time compared with conventional FD of the saturated one, which achieved a simultaneous enhancement of heat and mass transfer. SEM images revealed that the unsaturated sample skeleton was porous and tenuous, which profited the vapor migration and bound moisture desorption. WAM assisted microwave FD of the unsaturated sample with SiC pad shortened 41.0% of time compared with conventional FD of the saturated one, which realized a thorough enhancement of heat and mass transfer. The retentions of chlorogenic acid in all dried products were more than 92%. WAM assisted microwave FD of initially unsaturated material realized a simultaneous enhancement of heat and mass transfer in the conventional FD process while maintaining a high product quality.

Evaluation of the uniformity, quality and energy cost of four types of vegetables and fruits after pilot-scale pulse-spouted bed microwave (915 MHz) freeze-drying

The homogeneity of dried products produced via pilot-scale pulse-spouted bed microwave (915 MHz) freeze-drying (pilot-scale PSBMFD) was investigated using sweet corn, potato, banana and hawthorn as experimental materials. The quality and energy cost of the dried products were also investigated. Regarding the moisture content, color difference and shrinkage ratio, pilot-scale PSBMFD achieved high drying uniformity (⩾90%, d.b.). Compared with the conventional freeze-drying (CFD) method used in industries, pilot-scale PSBMFD effectively increased the nutrient retention (d.b.) in dried samples. It increased the retention of flavonoids in sweet corn, potato cube, banana slice and hawthorn by approximately 11.68%, 15.82%, 5.76%, and 15.94% and the total phenolic content by 11.85%, 23.58%, 28.34%, and 24.17%, respectively. Further, pilot-scale PSBMFD resulted in a larger honeycomb porous structure, greater hardness and brittleness as well as shrinkage and lower apparent density than CFD. Regarding the energy cost, pilot-scale PSBMFD can save more energy than CFD (⩾39%, d.b.). This pilot-scale experimental study provides a theoretical basis and technical support for the commercial application of the PSBMFD technology.

Efficient drying of laser-treated raspberry in a pulse-spouted microwave freeze dryer

Pulse-spouted microwave freeze drying (PSMFD) has been shown to be more energy efficient than the conventional vacuum freeze dryer. PSMFD yields higher drying rate and better energy efficiency due to volumetric heating. Because of the mass transfer resistance due to the berry’s epidermal barrier during drying, the raspberry skin was subjected to laser pretreatment so as to perforate the surface with pre-selected number of fine holes generated by a CO2 laser. The perforations allow vapor generated within the bulk of the berry to escape with no resistance of the waxy skin. In addition to studying the effect of perforation parameters on drying time; therefore, quality indicators of the dried product (shrinkage ratio, hardness, rehydration capacity, color, flavor, and anthocyanin content) were also explored. The micropore parameters in experiments were set as follows: P-0 (no perforations), P-3 (12 perforations), P-6 (24 perforations), and P-9 (36 perforations). As expected, greater the number of perforations per berry shorter is the drying time, by up to 23.08%. Moreover, the shortened drying time has a positive effect on product quality as well. The shrinkage rate could be reduced by about 3.95% and the retention rate of anthocyanin could be increased by 20.02%.

Comparative study on the resveratrol extraction rate and antioxidant activity of peanut treated by different drying methods

AbstractResveratrol is a phenolic compound and has significant benefits for human health. Peanut, rich in resveratrol and with considerable biological activity, is generally considered as the raw material of functional food. Because of the low output of fresh peanut extracts, dried peanut powder often be used for extracting functional components. The effects of hot‐air drying (HD), infrared radiation drying (IRD), and microwave‐freeze‐drying (MFD) on the resveratrol extraction income and antioxidant activity of peanut were study. The most valid and economical method was IRD. The IRD peanut resveratrol extraction ratio was 33.5% and the DPPH free radical eliminate rate was 42.5%. The MFD peanut resveratrol extraction ratio was 40.9% and the DPPH free radical eliminate rate was 40.3%. However, the MFD energy consumption was the most.Practical ApplicationsThis study investigated the effects of different drying methods on the drying characteristics of peanuts and the extraction rate of the antioxidant active substance resveratrol. It was found that MFD could be used to obtain peanut powder with high resveratrol extraction rate and antioxidant activity, and IRD had excellent energy‐saving performance, and the relatively high extraction rate of the corresponding product is more favored by the industry. The results obtained in the current work would help the comprehensive utilization of peanut and provide new ideas for the food industry to extract natural compound.

Effects of Different Drying Methods on the Characterization, Dissolution Rate and Antioxidant Activity of Ursolic Acid-Loaded Chitosan Nanoparticles.

To improve the water solubility of ursolic acid (UA), UA-loaded chitosan nanoparticles were firstly prepared by the ionotropic gelation method and dried by freeze drying (FD), microwave freeze drying (MFD) and spray drying (SD). The characterization of UA-loaded chitosan nanoparticles was performed with particle size, drug loading (DL), scanning electron microscope (SEM), fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), dissolution studies and antioxidant activity. The results demonstrated that UA was successfully encapsulated into chitosan nanoparticles using sodium tripolyphosphate (TPP) as a cross-linker, with a 79% encapsulation efficiency. The spray-dried, UA-loaded chitosan nanoparticles had the lowest drug loading (11.8%) and the highest particle size (496.9 ± 11.20 nm). The particle size of UA-loaded chitosan nanoparticles dried by MFD and FD was lower, at 240.8 ± 12.10 nm and 184.4 ± 10.62 nm, respectively, and their antioxidant activity was higher than those nanoparticles dried by SD. Moreover, the drying time and energy consumption of UA-loaded chitosan nanoparticles dried by MFD and SD were lower than that of FD. The dissolution rates of UA-loaded chitosan nanoparticles prepared by FD and MFD were 60.6% and 57.1%, respectively, in a simulated gastric fluid, which was a greater value than SD (55.9%). Therefore, the UA-loaded chitosan nanoparticles encapsulation method, combined with MFD technology, showed a promising potential to improve the water solubility of UA.

Innovative applications of freeze-drying to produce compound formula instant foods: A review

Due to the fast pace of life around the globe and the increasing demand for high-quality convenience foods by consumers, instant foods have attracted widespread attention. By adjusting the process and formula, the quality of compound formula foods can be optimized. Drying is a common way. However, conventional drying methods have some shortcomings. Freeze drying can overcome most of them. Foam mat freeze-drying can increase the drying area of materials, and the drying efficiency of it is also higher. For thermal-sensitive materials, it is a suitable drying method. Microwave freeze drying, infrared drying, and others are relatively new processing technologies that are suitable for the production of instant foods. This paper provides an overview of recent developments in processing technologies for the production of compound formula instant foods.

Effect of ultrasound-assisted osmotic dehydration pretreatments on drying and quality characteristics of pulsed fluidized bed microwave freeze-dried strawberries

In order to improve the shortcomings of low heat and mass transfer efficiency and uneven heating of freeze-drying technology, microwave heating and pulsed fluidized bed technology have been introduced, and ultrasonic-assisted osmotic dehydration (USOD) has been used for pretreatment of strawberries in this study. Using different ultrasonic power (180W, 240W, 300W) and different osmotic sugar solutes (sucrose, trehalose, sucrose and maltodextrin combination) to pretreat strawberries, and determine the dielectric properties, mass transfer, and water status and distribution of strawberries. The drying performance is evaluated by the drying rate, and the drying quality is determined by the color, texture, and total polyphenols and total flavones content. It was found that compared with freeze-drying, pulsed fluidized bed microwave freeze-drying (PFBMFD) can reduce the drying time by 45%, while USOD treatment helps to further increase the drying rate and reduce drying time by 10% because it increases water loss. In addition, dried strawberries pretreated with USOD may have better texture and nutritional quality, and sugar with low hygroscopicity and low sweetness value is more suitable for the USOD process of strawberries.

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.

Water Vapor Pathways during Freeze-Drying of Foamed Product Matrices Stabilized by Maltodextrin at Different Concentrations

This study aimed to identify the water-vapor transport mechanisms through an aerated matrix during microwave freeze-drying. Due to the larger surface area and lower water vapor transport resistance of an aerated product compared to the solution, foam structures dry faster. Different foam structures were produced with different maltodextrin (MD) concentrations (10–40%) as a foam-stabilizing agent. Depending on the initial viscosity of the solution prior to foaming, the samples differed in overrun (41–1671%) and pore size (d50 = 58–553 µm). Experiments were partially performed in a freeze-drying chamber of a light microscope to visualize structural changes in-situ. Different mechanisms were identified explaining the accelerated drying of foams, depending on the MD concentration, above or below 30%. At lower MD concentration, high overruns could be produced prior to freezing with big bubbles and thin lamellae with short diffusion pathway length. At 40% MD concentration, the viscosity was too high to integrate much air into the product. Therefore, the foam overrun was low and the bubble size small. Under these conditions, the water vapor generates high pressure, resulting in the formation of channels between bubbles, thus creating the pathways with low resistance for a very fast water vapor mass transfer. In addition, microwave freeze-drying experiments using a pilot plant unit were conducted to validate the findings of the freeze-drying microscope. A reduction of the drying time from 150 min (10% MD) to 78 min (40% MD) was achieved.

A novel combination of LF-NMR and NIR to intelligent control in pulse-spouted microwave freeze drying of blueberry

In order to improve the drying efficiency while maintaining product quality, pulse-spouted microwave freeze-drying (PS-MFD) was applied to the dehydration process of blueberries. The present study was carried out to establish a strategy for intelligently determine sublimation/desorption drying transition point (TPS-A) and drying endpoint of blueberry during PS-MFD process. Back-propagation artificial neural network (BP-ANN) model was used to establish the relationship between low field nuclear magnetic resonance (LF-NMR) spectrum and moisture content (MC) as well as between near infrared (NIR) spectrum and MC. Then the two models were applied to predict the moisture content of blueberry during PS-MFD process. Model fitting results showed that BP-ANN models based on LF-NMR spectrum and NIR spectrum could accurately predict the moisture content of blueberries. BP-ANN model of LF-NMR-MC couldn't capture the signal of free water during the early stage of PS-MFD (about 90 min), while BP-ANN model of NIR-MC had moisture content output values of blueberries during the whole PS-MFD process. The TPS-A and drying endpoint of blueberry during PS-MFD process could be accurately determined by comparative analysis of PS-MFD blueberry samples moisture content output values obtained from two models.

Multiphysics Modeling for Microwave Freeze-Drying of Initially Porous Frozen Material Assisted by Wave-Absorptive Medium

To investigate the enhanced impact of initially porous frozen material and wave-absorptive medium-assisted microwave heating on traditional freeze-drying, a multiphysics model coupling temperature,...

Pulse-spouted microwave freeze drying of raspberry: Control of moisture using ANN model aided by LF-NMR

Abstract To achieve intelligent control of the drying terminal point in pulse-spouting microwave freeze drying (PSMFD) of raspberries, low-field nuclear magnetic resonance (LF-NMR) was used to monitor the moisture status and moisture content changes during drying. As expected, three states of water were observed: bound water, immobile water and free water (around 18, 187, and 1304 ). From the T2 spectrum, the bound water content was observed not to change significantly (p > 0.05), while its proportion gradually increased, and the contents of non-flowing water and free water decreased significantly (p

Effects of different pretreatments on the pore structure of Chinese yam during microwave freeze drying.

In order to explore the effects of different pretreatments on the pore structure of Chinese yam during Microwave Freeze Drying (MFD), the samples were treated by typical drying pretreatments including osmosis, blanching and ultrasound, and compared with the untreated samples. The results showed that the pretreatment had significant impacts on the porosity of MFD Chinese yam. Ultrasonic pretreatment could obtain the highest open porosity, and the internal pore network structure of the material showed a uniform sponge shape. The shape of pores obtained by blanching pretreatment was the most irregular. The pore network structure obtained by different pretreatments could result in various product texture and rehydration, which implied that pretreatment methods should be carefully selected according to actual requirements. All these pretreatments could improve the open porosity of MFD Chinese yam, which result in a better rehydration capability. Among them, ultrasonic treatment is worth further investigating. Keywords: microwave freeze drying, pretreatment, pore network structure, Chinese yam, porosity DOI: 10.25165/j.ijabe.20201304.5605 Citation: Wang H, Duan X, Zhao L J, Duan L L, Ren G Y. Effects of different pretreatments on the pore structure of Chinese yam during microwave freeze drying. Int J Agric & Biol Eng, 2020; 13(4): 232–237.

Wave-absorbing material aided microwave freeze-drying of vitamin C solution frozen with preformed pores

To explore the effect of wave-absorbing material aided microwave heating on freeze-drying of initially porous frozen material, a lab-scale microwave freeze-dryer was designed with solid-state source selected as the microwave source instead of the traditionally used magnetron. Vitamin C was employed as the solute in aqueous solution. “Soft-ice” freezing technique was adopted to prepare solid and porous frozen materials. Experimental results showed that the solid sample by soft-ice freezing can successfully avoid structural collapse during freeze-drying, and the porous frozen material can significantly intensify mass transfer of the freeze-drying process. Under the operating conditions of 35 °C and 20 Pa, the drying time of the porous sample with the initial saturation of 0.25 decreased by 30.4% compared with the solid one, and the residual moisture content of dried product decreased with the initial saturation reducing. SEM images of the dried products revealed that the porous material had a loose pore structure and a tenuous solid skeleton, which is favorable to the migration of sublimated/desorbed vapor and the desorption of bound moisture. Evidences also showed that appropriately increasing radiation temperature led to reducing the drying time, while changing chamber pressure had an insignificant effect on freeze-drying. With sintered silicon carbide (SiC), which is a kind of wave-absorbing material, as the supporting pad of sample, microwave heating can greatly improve heat transfer of the freeze-drying process. Under the same operating conditions, the drying time of the porous sample with the microwave input power of 5 W was 28.1% shorter than that of the same sample without microwave heating, and 50.0% shorter than that of the solid sample for conventional freeze-drying. Wave-absorbing material aided microwave freeze-drying of the porous material frozen with preformed pores is a feasible and effective way to achieve the simultaneous intensification of heat and mass transfer in freeze-drying.

Ultrasound-assisted osmotic dehydration pretreatment before pulsed fluidized bed microwave freeze-drying (PFBMFD) of Chinese yam

The effect of ultrasound assisted osmotic dehydration (USOD) with 5% salt solution as the osmotic solution with pulsed fluidized bed microwave freeze-drying (PFBMFD) of Chinese yam (Dioscorea opposita Thunb.) was studied. Yam cubes were subjected to osmotic dehydration with different ultrasonic power (1.52, 2.28 and 3.04 W/g) or without ultrasound for 10 min, and the mass transfer, water status and distribution, dielectric properties and microstructure were measured. The drying characteristics and quality attributes (color and total soluble polyphenols content, total flavones content and antioxidant capacity) of the pretreated and dried yams as well as texture, flavor and microstructure of dried yams were also studied. The USOD treatment increased the mass transfer, increased loss factor and made the water more mobile. Cell size increases were observed on the micrograph because of the application of higher ultrasonic power. Thus, the acceleration of drying and energy savings were achieved. The Page model was successfully fitted to the drying curves. Furthermore, PFBMFD dried yams with USOD pretreatment had good color and texture, reduced astringency and sourness, as well as improved antioxidant capacity when compared with fresh dried samples.

Structural and functional properties of hydrolyzed/glycosylated ovalbumin under spray drying and microwave freeze drying

Ovalbumin (OVA), the main protein in egg white, affects most of the functional properties of egg white protein in food processing. The aim of this study was to investigate the effects of spray drying (SD) and microwave freeze drying (MFD) on the preparation of hydrolyzed/glycosylated ovalbumin (HGOVA) and provide useful information on the applications of egg protein powders in the food industry. Results demonstrated that the structure of HGOVA was considerably changed, and its functional properties were improved compared with those of native OVA. SD and MFD processing did not lead to dissociation of HGOVA subunits. SD-HGOVA exhibited higher protein solubility, emulsifying activity, foaming capacities, and gel hardness than MFD-HGOVA. However, MFD-HGOVA was better than the SD-HGOVA in terms of color, emulsion stability, foam stability, water/oil absorption capacity, and thermal stability. Selection of an appropriate drying method could enhance the potential applications of HGOVA in the food industry.

Drying treatments on Chinese yam (Dioscorea spp.) prior to wet milling influence starch molecular structures and physicochemical properties

The molecular structures and physiochemical properties of starches isolated from Chinese yam (Dioscorea spp.) slices with various drying treatments (including hot air-drying at different temperatures, microwave freeze drying and ultrasonically enhanced hot air-drying) prior to wet milling were examined. The average whole molecular size (Rh¯) and debranched amylopectin chain length (X¯) were significantly decreased whereas the amylose chains and content presented negligible changes after drying. These structural modifications are consistent with the observed higher pasting viscosity, lower gelatinization temperatures and enthalpy changes, and greater digestion rate and extent for the dehydrated yam slices compared to that for the fresh yam. Among the drying techniques applied, the microwave freeze drying (MFD) resulted in the greatest amylopectin degradation with the smallest Rh¯ and X¯ along with the largest amount of short amylopectin chains (DP 6–12) due to the highly intensive moisture removal process. Correspondingly, the MFD contributed to the lowest gelatinization temperatures and enthalpy changes while the highest digestibility. The molecular degradation generally increased with increasing hot air-drying temperature, agreement with the corresponding functional changes. The structure-property correlations indicates the significant roles of starch molecular structures in controlling the drying-induced physicochemical changes. This study can be used to suggest ways in which some improved functional properties of yam starch-based products can be obtained through optimal drying techniques.

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.

A promising pulse-spouted microwave freeze drying method used for Chinese yam cubes dehydration: quality, energy consumption, and uniformity

Pulse-spouted microwave freeze drying (PSMFD) used for Chinese yam cubes was evaluated at three microwave power levels (400, 600, and 800 W). The drying characteristics, energy consumption, and quality of dried yam cubes were assessed and compared with freeze dried samples. Moreover, the drying uniformity of PSMFD and static microwave freeze drying (MFD) was contrasted. Results showed that drying time decreased and drying rate increased with increasing microwave power levels, furthermore drying rate curves contained three stages. Compared to the freeze drying (FD) process, PSMFD (800 W) could shorten drying time by 55%, and appropriate microwave power (400 and 600 W) contributed to the improvement of crispness. The retention of ascorbic acid and total phenol content of yam cubes dried at 600 W was higher than that of samples dried at 400 and 800 W. In addition, the drying uniformity of PSMFD was improved compared with that of MFD. Considering the high efficiency and comparable quality compared with FD evaluated by principal component analysis (PCA), PSMFD is a promising drying technique.