Microwave Freeze Drying Research Articles

Simulation of Transport Phenomena for Microwave Freeze‐Drying of Potato Slices Using Finite Element Analysis

ABSTRACTThis research intended to investigate the transport phenomena that occur during microwave freeze‐drying (MFD) of potato slices using drying kinetics and finite element analysis (FEA). The impacts of microwave power levels and potato slice thickness on drying rate constant (DR) and average moisture diffusion (DAVG) were analyzed using MFD kinetics and were then incorporated in the simulation. It was found that the DR and DAVG were in the range of 301.3 × 10−3–775.4 × 10−3 min−1 and 1.045 × 10−10–3.336 × 10−10 m2/s, respectively. In the sublimation phase, the DR and DAVG were higher than those in the desorption phase. The DR and DAVG increased as the microwave power level increased but decreased as the thickness increased. The FEA of temperature and moisture distribution within the potato slices demonstrated the outward transfer of heat and mass from the center to the exterior and closely matched the experimental data with an error margin of within 5%, leading to the proposed schematic shrinkage model corresponding to the MFD simulation.

Microwave freeze-drying characteristics and crosslinking behavior of wheat starch-laurel acid complex

This article investigates the effect of different microwave powers on the crosslinking behavior and microwave freeze-drying characteristics of wheat starch-lauroyl arginate complex during the microwave freeze-drying process. During microwave freeze-drying, as microwave power increased from 0.1 W/g to 0.9 W/g, the freeze-drying time of WS-LA was reduced by 50 %, while the uniformity of freeze-drying was not affected by its composition. In the research results obtained from DSC, Raman spectroscopy, Fourier-transform infrared spectroscopy (FTIR), XRD, and SEM analyses, with the microwave power increased from 0.1 W/g to 0.9 W/g, the enthalpy value of the melting peak of the WS-LA (wheat starch-lauric acid) composite decreased from 1.15 J/g to 0.62 J/g. The full width at half maximum (FWHM) value increased from 25.6 to 30.79. The ratio of absorbance at 1022/995 cm−1 increased from 1.0111 to 1.0707. The recrystallization (RC) value decreased from 8.77 % to 0.07 %. Additionally, in the microstructure, the size of WS-LA composite particles decreased accordingly. The above findings indicated that the increase in microwave power during microwave freeze-drying had a negative impact on the formation of the WS-LA complex and the ordering of its structure in the sample.

Effects of freeze–thaw pre‐treatment with different freezing methods on the microwave freeze drying of carrots

SummaryThree freeze–thaw (FT) pre‐treatments using different freezing methods that freezing at −80 °C (FT‐80), freezing at −25 °C (FT‐25) and vacuum freezing (VFT) were used for carrot slices, the effects and mechanisms of FT pre‐treatment on the drying characteristics and product quality of carrots during microwave freeze drying (MFD) were studied. The results showed that FT pre‐treatment under different freezing methods affected the water state of carrots, resulting in varying degrees of moisture loss and shortening drying time by 14.3% to 42.8%. VFT treatment caused the greatest water loss (35.33%) during FT and significantly increased the effective moisture diffusivity in the MFD process. FT treatments did not cause any significant degradation in the colour of the carrots. The VFT and FT‐80 treatments resulted in an elevated rehydration ratio, increased crispiness of MFD carrots and products with a porous microstructure and higher porosity. In contrast, the FT‐25 treatment led to a decrease in porosity due to destructive damage to the carrot cell structure. However, this damage favoured the extractability of carotenoids, resulting in a higher carotenoid content than the other treatments. This study provides a theoretical foundation for the practical implementation of FT pre‐treatment using various freezing methods on fresh foods prior to drying. It is recommended to employ VFT pre‐treatment before MFD due to its efficiency and effectiveness.

The influence of thermal radiation during microwave-assisted freeze-drying of pharmaceutical unit doses

New drying technologies for biologicals have recently been developed to accelerate the time-consuming batch freeze-drying (BFD) process. Among others, microwave-assisted freeze-drying (MFD) has been suggested as a faster and more effective drying technology. In this study, MFD cycles with the microwave radiation switched on and off were performed to assess the contribution of the microwave radiation to drying acceleration. It was found that thermal radiation emitted by the drying chamber walls was predominantly accelerating the drying of monodose placebos rather than microwave radiation. The combination of ultra-low chamber pressure, high thermal heat transfer and a short primary-to-secondary phase transition reduces drying times by more than 80 % compared to conventional BFD. In a second step, a design of experiment approach was used to assess the effect of thermal radiation versus microwave radiation and their combination, together with dosage properties such as fill volume and excipient concentration upon drying rate. The outcome showed the importance of high fill volume and high excipient concentration for an effective microwave contribution to the drying rate. Nevertheless, the drying acceleration for small pharmaceutical dosages with restricted solutes was mainly driven by thermal radiation rather than 2.45 GHz microwave radiation. The inability of ice to convert microwave energy into heat hampers the potential use of microwave freeze-drying for single-dose vaccines.

Improvement of 3D printing and subsequent microwave freeze-drying solidification accuracy of pineapple gel-based inks: Infill percentage control and internal models design

The purpose of this study was to improve the 3D printing accuracy of pineapple gel and its subsequent microwave freeze drying (MFD) solidification precision based on infill percentage control (30%, 50%, 70%, and 90%) and internal models design (Hilbert curve, honeycomb, and rectilinear). Through comparing the designed models with the physical dimensions of the printed and dehydrated products, the optimal regulation strategy was obtained. Results showed that the printing deviation of samples decreased initially and then increased with higher infill percentages, with the 70% infill percentage having the lowest deviation. Among all internal models, the rectilinear infill pattern showed the best printing accuracy. Porosity and the shape deformation rate of MFD solidification products initially decreased and then increased with higher infill percentages. The 70% infill percentage products had the lowest shape deformation rate. The honeycomb infill pattern performed best in reducing solidification sample shrinkage and deformation. Additionally, micro-CT scans revealed that the honeycomb infill pattern helped maintain the integrity of each layer's lines of MFD solidification products, closely matching the model's line distribution. In the 50% and 70% infill percentage, honeycomb and rectilinear samples exhibited better crispness. A 70% honeycomb infill pattern was recommended for optimal printing and post-solidification accuracy.

Design of a pilot-scale microwave freeze dryer for in situ neutron imaging.

The gentle yet cost-effective drying of sensitive products in the food and pharmaceutical industries is becoming increasingly important. To maintain sensitive ingredients, color, structure, and viability of micro-organisms, often freeze-drying is the only possible way to preserve the product. As many products come in as bulk material, they are dried on heated shelves resulting in poor heat and mass transport through the bed. Resulting in a very time and cost intensive process. Therefore, efforts are being made to improve the mass and heat transport of the process. The outer mass transport through the bulk can be improved by continuous mixing of the pellets, facilitating the removal of water vapor from the condenser. In addition, the issue of limited heat transport can be addressed by using volumetric energy input from microwaves. This process is called dynamic microwave freeze-drying. As dynamic microwave freeze-drying is a combined drying and mixing process, with particle properties continuously changing during drying, it is necessary to gain a more detailed insight into the process. For this purpose, a drier is designed that is capable of in situ neutron imaging, a method sensitive to a material's hydrogen content. This paper presents the design of a pilot-scale microwave freeze dryer for in situ neutron imaging and shows the first images taken during the dynamic microwave freeze-drying of bulk particles at the Center for Energy Research, Budapest Neutron Center in Budapest, Hungary.

Study on Microwave Freeze-Drying of Krill

Antarctic krill (Euphausua superba) need to undergo freeze-drying to facilitate lipid extraction, but freeze-drying is time-consuming and energy-intensive, resulting in high processing costs. Microwave heating technology can reduce freeze-drying time and lower energy consumption costs. The objective of this study was to establish a drying kinetic model to help the microwave freeze-drying process by predicting krill drying time and evaluating the impact of the drying process on krill quality. The results showed that changing the microwave power did not alter the total energy requirement to complete drying when the sample weight was fixed. The total energy requirement for microwave drying increases with the sample weight. Comparing the three methods of freeze-drying (FD), microwave freeze-drying (MWFD), and hot-air drying at 55 °C (HAD) showed that they took 18, 0.67, and 16 h, respectively, to reach the drying endpoint for krill. Overall, HAD resulted in browning, shrinkage, and quality degradation of krill due to its high temperature and long duration. While the appearance and active ingredient contents of FD krill are slightly better than those of MWFD krill, FD requires a longer process and more energy. MWFD can reduce drying time by 20 times and energy consumption by 95% compared to FD.

Effects of multiphase microwave drying on the microstructure of Chinese yam: A study based on X-ray micro-computed tomography technology

To address the issues of long drying time and high equipment costs present in freeze drying and freeze drying-microwave vacuum combined drying techniques, as well as to obtain higher-quality dried products, multiphase microwave drying (MMD) has been proposed. This study investigated the effects of MMD at different transition point moisture content levels (0.36, 0.59, and 0.79 g/g d.b.) on the microstructure and texture of dried Chinese yam. Using freeze drying (FD), microwave freeze drying (MFD), and microwave vacuum drying (MVD) as controls, X-ray micro-computed tomography (micro-CT) visualized and quantified Chinese yam’s structure across drying methods. The results indicated that as the transition point moisture content increased, the porosity and connected pore area and volume decreased, while non-connected pore area and volume increased. Chinese yam samples obtained from MMD scheme I (transition point moisture content of 0.36 g/g d.b.) exhibited a uniform and porous structure with a high total porosity (48.19 ± 0.32%), similar in hardness and shrinkage to freeze-dried yams, preserving microstructural integrity. MMD reduced drying time and maintained the quality of dried Chinese yam compared to FD, MVD, and MFD, highlighting its potential as a cost-effective method for producing premium dried Chinese yam products.

Enhancing Microwave Freeze Drying: Exploring Maximum Drying Temperature and Power Input for Improved Energy Efficiency and Uniformity

AbstractThe pursuit of energy-efficient and uniform processing drives ongoing research in microwave-assisted freeze drying (MWFD). While microwave application is acknowledged for its potential to reduce energy consumption of freeze drying applications, it introduces new challenges to gentle processing, particularly in achieving uniformity of processing due to the inherent uneven microwave field distribution in the drying chamber. This study investigates the impact of maximum drying temperature (Tmax) and microwave power input on energy consumption and uniformity in temperature-controlled MWFD. Experimental results reveal that shorter equilibration times associated with higher Tmax significantly amplify the inhomogeneity of temperature distribution. Further, higher Tmax was associated with a significant reduction in total energy demand of MWFD. Despite noticeable trends, microwave power input did not yield statistically significant differences in energy consumption or uniformity. The limited range of explored values, combined with the temperature-controlled nature of the process, may have rendered a potential influence of microwave power input negligible. This research elucidates the extent of inhomogeneity in MWFD, with implications for achieving uniform, gentle drying. It highlights the critical role of temperature control in MWFD. The study contributes to advancing the understanding of optimal processing in temperature-controlled MWFD.

Microencapsulation of fish oil by spray drying, spray freeze-drying, freeze-drying, and microwave freeze-drying: Microcapsule characterization and storage stability.

The objective of this paper was to evaluate the effect of spray drying (SD), spray freeze-drying (SFD), freeze-drying (FD), and microwave freeze-drying (MFD) on the characteristics of fish oil (FO) microcapsules. The physicochemical properties, morphology, fatty acid composition, and stability of the microcapsules were analyzed. The encapsulation efficiencies of microcapsules dried by SD, SFD, FD, and MFD were 86.98%, 77.79%, 63.29%, and 57.89%, respectively. SD microcapsules exhibited superior properties in terms of effective loading capacity, color, and flowability. Conversely, SFD microcapsules demonstrated improved solubility. Microencapsulation positively affected the thermal stability of FO, but the content of unsaturated fatty acids decreased. The findings from the storage experiment indicated that the oxidative stability of SD fish oil microcapsules was marginally lower compared to microcapsules produced through three alternative drying techniques, all of which were based on the FD concept. The comparison of various drying methods and their effects on the quality of FO microcapsules offers valuable insights that can serve as a foundation for the industrial production of high-quality microcapsules.

Wave-absorbing material-assisted microwave freeze-drying of initially unsaturated coffee frozen material: Numerical simulation and experimental verification

A multi-physics model of multiphase transport was formulated in describing freeze-drying. Traditional and microwave freeze-drying experiments were conducted using initially saturated and unsaturated coffee materials. The results demonstrated that microwave freeze-drying time of initially unsaturated material assisted by a wave-absorbing material was decreased by 40.4 % compared with traditional freeze-drying time of saturated material. The simulated drying curves achieved excellent agreements with the measured ones. The enhancement mechanisms were analyzed in terms of saturation level, temperature, Darcy’s velocity and energy utilization. Wave-absorbing material-assisted microwave freeze-drying of unsaturated material realized a simultaneous enhancement of mass and heat transfer for the freeze-drying process.

Effects of freezing and drying programs on IgY aggregation and activity during microwave freeze-drying: Protective effects and interactions of trehalose and mannitol

The acquisition of high quality lyophilized IgY products, characterized by an aesthetically pleasing visage, heightened stability, and a marked preservation of activity, constitutes an indispensable pursuit in augmenting the safety and pragmatic utility of IgY. Within this context, an exploration was undertaken to investigate an innovative modality encompassing microwave freeze-drying (MFD) as a preparatory methodology of IgY. Morphological assessments revealed that both cryogenic freezing and subsequent MFD procedures resulted in aggregation of IgY, with the deleterious influence posed by the MFD phase transcending that of the freezing phase. The composite protective agent comprised of trehalose and mannitol engendered a safeguarding effect on the structural integrity of IgY, thereby attenuating reducing aggregation between IgY during the freeze-drying process. Enzyme-linked immunosorbent assay (ELISA) outcomes demonstrated a discernible correlation between IgY aggregation and a notable reduction in its binding affinity towards the pertinent antigen. Comparative analysis vis-à-vis the control sample delineated that when the trehalose-to-mannitol ratio was upheld at 1:3, a two-fold outcome was achieved: a mitigation of the collapse susceptibility within the final product as well as a deterrence of IgY agglomeration, concomitant with an elevated preservation rate of active antibodies (78.57 %).

Comparative evaluation of quality characteristics of fermented napa cabbage subjected to hot air drying, vacuum freeze drying, and microwave freeze drying

Fermented napa cabbage (FNC) is a traditional probiotic-enriched food. In this study, the potential of different drying techniques, including hot air drying (HAD), vacuum freeze drying (FD), and microwave freeze drying (MFD), for the production of dehydrated FNCs was evaluated. To reach a moisture content of 0.12 g⋅ g−1 (d.w.b.), MFD took 2 h, which was half of the time needed for HAD. FD process required the longest drying time, which was 18 h. Therefore, MFD consumed significantly less power compared to FD and HAD, which was 12.63% of FD power usage and 30% of HAD power usage. Both MFD and FD products exhibited better rehydration ability than the HAD product due to their honeycomb-like structure with less shrinkage. MFD and FD were able to preserve the living probiotics, bioactive components, and antioxidant capacities of FNCs better than HAD. The taste and aroma of dehydrated FNC products were maintained better by microwave-induced oxidation reaction and Maillard reaction during MFD treatment, based on the quantitative results regarding amino acids, volatile compounds, and sensory evaluation. In general, MFD is a promising method for obtaining high-quality dehydrated FNCs as probiotic foods.

Effect of Microwave Freeze-Drying at Different Heating Rates on the Quality and Nutrient Content of Strawberries

Effect of Microwave Freeze-Drying at Different Heating Rates on the Quality and Nutrient Content of Strawberries

An analytical study of coupled convective heat and mass transfer with volumetric heating describing sublimation of a porous body under most sensitive temperature inputs: Application of freeze-drying

Sublimation heat-mass transfer has great applications in the pharmaceutical and food industries, such as the preservation of biological products, accelerated freeze-drying (AFD), energy storage systems, microwave freeze-drying, and enabling long-time active covid like vaccines. The current technological demand encourages investigators to provide new knowledge for freeze-drying so that it reduces the high economic cost, prevents materials from being denatured, and remains stable for a long time. In connection with this, it is of key interest to analyze the impact of convective heat and mass transfer, the rate of water vaporization, and the volumetric heating source under the most realistic temperature inputs. Despite the available works on sublimation, there is still a lack of mathematical modeling that accounts for these informations together and is presently being considered. This paper presents a heat and mass transfer problem describing sublimation in a half-porous space. The mathematical model accounts for convective heat/mass transfer and a volumetric heat source within dried and frozen regions. In addition, convection driven by the mass transfer of ice crystals within the dried region is also considered. Three different types of temperature input are placed at the surface x=0 to obtain the rapid sublimation process without harming the material properties. The exact solution to the problem is obtained successfully by using similarity transformation. The impact of various problem parameters on sublimation is comprehensively studied. In this study, it is found that with a volumetric heating source term G0, material sublimates faster than without one. Furthermore, convective heat transfer in terms of Pe1 and Pe2, enhances the temperature within the porous medium, and as a result, material sublimates faster than usual. As the value of the convective term β goes up, a reduction in the temperature field is observed. The temperature of the medium rises as the value of the Kirpichev-like number Ki increases. Similar observation is found in the case of Biot like number Bi. The concentration profile decreases as the value of the Luikov number Lu increases. It is also found that Newton-type temperature input offers a faster sublimation rate in comparison to constant and flux-type temperature input. The analytical results obtained in this study show excellent agreement with previous available results. These results provide a comprehensive theoretical and mathematical understanding of sublimation heat/mass transfer and are expected to be useful in energy storage systems, food technology, and accelerated freeze drying.

Effects of different drying methods on the properties, stability, and controlled release of Cornus officinalis flavonoids microparticles.

To improve the stability and solubility of Cornus officinalis flavonoid (COF), spray drying (SD), freeze-drying (FD), and microwave freeze drying (MFD) were used to encapsulate COF using whey isolate protein (WPI) and gum arabic as wall materials. The characterization of COF microparticles was performed with encapsulation efficiency (EE), particle size, morphology, antioxidant activity, structure, thermal stability, color, stability during storage, and in vitro solubility. The results showed that COF was successfully encapsulated in the wall material with an EE between 78.86% and 91.11%. The freeze-dried microparticles had the highest EE (91.11%) and the lowest particle size (12.42±16.73µm). However, the particle size of COF microparticles of SD and MFD was relatively large. The 1,1-diphenyl-2-picrylhydrazyl (DPPH) scavenging capacity of the microparticles obtained from SD (89.36mgVc /g) was higher than that of MFD (85.67mgVc /g), but the drying time and energy consumption of microparticles dried by SD and MFD were lower than those of FD. Furthermore, the spray-dried COF microparticles had higher stability compared to FD and MFD when stored at 4°C for 30 days. In addition, the dissolution of COF microparticles prepared by SD and MFD was 55.64% and 57.35%, respectively, in simulated intestinal fluids, which was lower than that of FD (64.47%). Therefore, the application of microencapsulation technology showed significant advantages in improving the stability and solubility of COF, and the SD can be used for the preparation of microparticles considering energy cost and quality. PRACTICAL APPLICATION: COF is an important bioactive ingredient, but its poor stability and water solubility decreases its pharmacological value. COF microparticles can improve the stability of COF, enhance the slow-release effect, and expand its application in the food field. The drying method will affect the properties of COF microparticles. Thus, the structures and properties analysis of COF microparticles by different drying methods can provide a reference basis for the preparation and application of COF microparticles.

Effect mechanism of different drying methods on the quality and browning for daylily

The influence of various drying methods, namely hot-air drying (HAD), heat pump drying (HPD), freeze-drying (FD), and microwave freeze drying (MFD), on the structural characterization and browning substrates of daylily was investigated. The chemical structure as determined by Fourier Transform infrared spectroscopy of the products from different drying methods were similar and the microphysical structure (microstructures and texture) were significantly different. High-quality daylily products with a bright appearance and a lower degree of browning were produced by FD and MFD. Higher contents of browning substrates (phenolic acid, total flavonoids, and total phenols) were found in products with lower browning (color, browning degree and Maillard fluorescent compounds). FD products kept the highest contents of gallic acid (27.39 mg/100 g), chlorogenic acid (35.92 mg/100 g), and quercetin (15.03 mg/100 g), and had a high nitrite scavenging activity. MFD products kept the highest contents of catechin (144.90 mg/100 g), rutin (71.5 mg/100 g), and kaempferol (33.87 mg/100 g). The results showed that the final products produced by FD and MFD had superior quality than those produced with HAD and HPD. FD and MFD are suitable for daylily powder with high nutritional value and HAD and HPD are suitable for large-scale production and affordable dried daylily products.

The Effect of Hybrid Drying Methods on the Quality of Dried Carrot

The study investigated the effect of a combination of drying techniques: convection, microwave, and freeze-drying, on selected physical properties of the dried material (carrot) to determine which form of hybrid drying is the best alternative to traditional freeze-drying. Carrots were dried by freeze-drying, convection-drying, and microwave-drying as well as in hybrid methods: freeze-drying-convection, freeze-drying–microwave as well as convection–freeze-drying or microwave–freeze-drying. The color, porosity, shrinkage, water activity, dry matter content, and internal structure of carrots dried using various methods were examined. The dried samples obtained with the hybrid method were compared with those obtained with a single drying technique. Freeze-drying–microwave-drying (F-M) as an alternative drying method for freeze-drying allowed us to obtain dried material with a water activity similar (p < 0.05) to that of freeze-dried samples, at the same time reducing the duration of the process by 20 h. The combination of convection-drying methods with freeze-drying (K-F) and microwave-drying with freeze-drying (M-F) allowed us to obtain dried material with lower shrinkage than in the case of convection (K) or microwave (M) drying.

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.

Investigation of the process of microwave freeze drying of plums

In this work, the advantages and disadvantages of freeze drying and microwave freeze drying were considered and analyzed. Experimental data on drying plums are given. The chemical composition of plums before and after freeze-drying was analyzed. The results of moisture change, freeze drying and microwave freeze drying of plums were compared.