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Microwave drying is usually combined with vacuum environment in conjunction with hot air flow to draw the moisture rapidly. The moisture content of the vegetables undergoing drying is hard to measure online. This research designed a microwave vacuum drying (MVD)-low-field nuclear magnetic resonance (NMR) smart device and investigated the feasibility of NMR method for online measurement of state of moisture during MVD. The relation between the signal amplitude ( A 2 ) and the true moisture content ( M 1 ) of six kinds of vegetables (mushroom, carrot, potato, lotus, edamame, vegetable corn) was fitted to estimate if NMR can measure the M 1 of vegetables directly. Results showed that A 2 and M 1 of different fresh vegetables had no single empirical mathematical model to fit. However, for each kind of these vegetables, the A 2 and corresponding M 1 in different MVD stages showed a significant linear relationship. The predicted moisture content ( M 2 ) of mushroom: M 2  = 5.25351 × 10 −4 A 2  − 0.34042, R  = 0.996; carrot: M 2  = 5.78756 × 10 −4 A 2  − 0.14108, R  = 0.998; potato: M 2  = 3.10019 × 10 −4 A 2  − 0.10612, R  = 0.991; lotus: M 2  = 2.32415 × 10 −4 A 2  − 0.01573, R  = 0.998; edamame: M 2  = 3.13310 × 10 −4 A 2  − 0.4198, R  = 0.996; vegetable corn: M 2  = 1.69461 × 10 −4 A 2  − 0.09063, R  = 0.995. The linear models between M 2 and A 2 were able to estimate the end point ( M 1  < 8%) of MVD with a high accuracy ( P  > 0.950).

During the industrial manufacturing of pomegranate juice, large amounts of pomace are produced. The aim of this work was to find the effective method to dry pomegranate pomace to open new commercial applications for this co-product. The effects of three drying methods: (i) convective drying (CD) at 50, 60, and 70 °C; (ii) vacuum microwave drying (VMD) at 240, 360, and 480 W, and (iii) a combined method (CPD–VMFD); convective pre-drying (60 °C) followed by vacuum microwave finish drying (360 W), on drying kinetics and quality of PomP (pomegranate pomace obtained after preparing pomegranate juice by squeezing only arils) were evaluated. The shortest treatments were VMD at 240 and 360 W (52 and 33 min, respectively); besides, these treatments led to interesting values of the green–red coordinate, a*, (12.2 and 4.1, respectively), total phenolic content (4.0 and 4.1 mg eq gallic acid g−1 dry weight, respectively), and antioxidant activity (30.8 and 29.0 µmol g−1 dry weight, respectively). On the other hand, this study demonstrated that this co-product is a rich source of punicic acid (average value = 66.4%), being a good opportunity for the pharmaceutical and nutraceutical industries. Moreover, no significant changes in the fatty acid profile was observed as affected by the drying treatments, and no off-flavors were generated by any of the drying methods.

Microwave vacuum drying (MWVD) of lactic acid bacteria is a promising alternative to conventional drying techniques. To further optimize this process and inhibit diffusion-limiting shrinkage in the last drying period, it was attempted to embed bacteria into a protective aerated matrix. As a foaming agent, whey protein isolate (WPI) was used. Besides, different carbohydrates (i.e., maltose (M), maltodextrin (MD), and sorbitol (S)) were evaluated for their ability to maintain structural stability as well as for their suitability as protectant during MWVD. Foam properties at atmospheric pressure as a function of carbohydrate type and concentration, WPI concentration and concentration of the model strain Lactobacillus paracasei ssp. paracasei F19 ( L. paracasei ) were related to MWVD behavior and product quality (e.g., survival rate). Therewith, it was aimed at specifying crucial characteristics as well as guiding values regarding foam design intended for MWVD. Overall, high foam stability combined with small and homogeneously distributed bubbles was defined as prerequisite. Further, M- and MD-based foams, in contrast to S-containing matrices, were effectively dried without foam collapse. The best drying performance regarding drying time, microwave energy demand, and survival rate was achieved with MD-stabilized foams. Besides, the general benefit of foam drying as well as the protective effect of MD during MWVD was demonstrated. By comparing MD-stabilized foams to non-foamed MD-containing suspensions as well as pure L. paracasei , the shortest drying time and highest survival rates resulted for the aerated matrices. Concluding, microwave vacuum foam drying displays an innovative approach for the preservation of lactic acid bacteria.

Green bell pepper was dried under vacuum-assisted microwave drying condition and the process was optimized using response surface methodology. The effect of microwave power (100–300 W) and vacuum level (200–600 mm Hg) were observed on the responses, viz. green color ratio, rehydration ratio, hardness, apparent density ratio, drying time and specific energy consumption. A central composite face-centered design was used to develop predictive regression models for the responses. Analysis of variance showed that quadratic model best fitted the experimental data. The microwave power level had greater effect on the quality attributes of green bell pepper; nevertheless at higher vacuum level the dried products had better quality. The optimum drying conditions were determined to be 284.4 W microwave power, 600 mm Hg vacuum level and the optimized value of the responses were obtained as green color ratio of 80.70%, rehydration ratio of 10.75, hardness of 152.98 N, apparent density ratio of 76.02%, drying time of 78 min, and specific energy consumption of 6.57 MJ/kg. Validation experiment was carried out at derived optimum condition to verify the prediction and adequacy of the models. Close agreement between experimental and predicted values was obtained.

This 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. This study explored the transport phenomena of microwave freeze‐drying (MFD) of potato slices, revealing that the drying rate and moisture diffusion are higher in the sublimation phase than in the desorption phase. As microwave power increased, both drying rate and moisture diffusion increased, but they decreased with thicker samples. The simulation using finite element analysis matched well with experimental data within a 5% error margin, leading to a proposed shrinkage model for the MFD process.

Bitter melon (Momordica charantia L.) is a fruit that brings health benefits to consumers because the fruit is rich in bioactive compounds. In this work, a combination of low‐temperature convective drying and microwave radiation was used to dehydrate sliced bitter melon. One‐factor‐at‐a‐time design was performed to evaluate the influence of microwave power density (1.5, 3.0, 4.5 W/g), drying temperatures (20, 25, and 30°C), and air velocity (1.0, 1.2 and 1.4 m/s) on the change of moisture content, nutrient levels (vitamin C and total phenolics), and the antioxidant activities (DPPH and FRAP assays) of the bitter melon. The obtained results showed that all investigated factors affected the rate of moisture removal. Microwave power density output and air‐drying temperature strongly participated in the retention of nutrients. In this study, the drying process was driven by both heat and mass transfer processes, so the increase of air velocity prolonged the drying time causing more loss of nutrient levels and antioxidant activities. It was found that DPPH free radical scavenging ability directly correlated with total phenolic content, but the ferric‐reducing antioxidant power was related to the presence of reductants including phenolic compounds, vitamin C, and other phytochemicals in bitter melons. This work determined that microwave power density and the air‐drying temperature are the main two factors that should be used for further investigations. The effects of drying conditions in the convective low‐temperature microwave‐assisted drying of bitter melon were investigated. The results were used to develop an understanding of how the microwave power density, drying temperature, and air velocity affect the final quality of the dried fruit product.

Background Drying is a critical post-harvest process for medicinal plants, which are typically high in moisture and microorganisms. To prevent spoilage and quality loss, it is essential to dry these plants promptly. The drying method significantly impacts the levels of secondary metabolites and the organoleptic characteristics of medicinal plants. This study aimed to investigate the effects of various drying methods on the total phenolics, flavonoids, anthocyanins, antioxidant activity, and phenolic acids in caper ( Capparis spinosa L.) fruits. The experiment was performed using a completely randomized design with three replications and included 11 treatments: shade drying, sun drying, oven drying (at 50 °C, 60 °C, and 70 °C), microwave drying (at 300 W, 600 W, and 900 W), freeze-drying, salt-drying, and a fresh plant sample as a control. Results Among the drying methods tested, microwave drying consistently produced the highest levels of flavonoids, anthocyanins, and antioxidant activity, regardless of wattage. Specifically, the highest total phenol content was observed in samples dried at 900 W microwave, 600 W microwave, and 70 °C in the oven (5.3, 5.37, and 5.31 mg GAE/g DW, respectively). Drying at 600 W microwave yielded the highest levels of caffeic, cinnamic, ferulic, vanillic, and protocatechuic acids (13.03, 3.85, 4.28, 9.73, and 5.6 µg/g, respectively) while drying at 900 W microwave also resulted in elevated levels of caffeic, ferulic, protocatechuic, and p -coumaric acids. The 70 °C oven drying method also showed high levels of caffeic, cinnamic, ferulic, vanillic, and protocatechuic acids. Freeze-drying achieved the highest levels of rosmarinic, gallic, and m -coumaric acids (320.17, 175.3, and 12.99 µg/g, respectively), while shade drying produced high levels of p -hydroxybenzoic, cinnamic, ferulic, m -coumaric, protocatechuic, and p -coumaric acids. Conclusions Overall, microwave drying (especially at 600 W), oven drying at 70 °C, and freeze-drying emerged as effective alternatives to traditional drying methods. These methods not only preserved the color, texture, and taste of the fruits but also enhanced their bioactive compound levels.

Rhodomyrtus tomentosa berries are rich in nutrition, but easily mildew, so it is necessary to dry them with a proper drying technique in time. The effects of the combined microwave-hot-air-drying (CD) on the physicochemical and antioxidant properties of Rhodomyrtus tomentosa berry powder were evaluated in this study, compared with those of hot air drying (HD) and microwave drying (MD). The results showed that the hydration properties and oil holding capacity of CD-berry powder were better than those of HD- and MD-ones. However, CD-berry powder showed lower fillibility and liquidity than HD-one and but similar to MD-one. There was no significant difference among the thermal stability of berry powder produced by the three drying methods. Furthermore, CD was beneficial to maintain the color, nutrition, and active ingredients of Rhodomyrtus tomentosa berry powder. CD-berry powder and MD-one had similar antioxidant properties. In general, CD is more suitable for the processing of Rhodomyrtus tomentosa berry powder.

Studies were conducted focusing on the drying of chili pepper fruits (Capsicum annuum L.), cultivar Cyklon, using convective (AD), convective-microwave (AMD), vacuum (VD), and freeze-drying (FD) methods. The influence of the drying method and temperature on the kinetics of the process and selected quality attributes of the dried product were evaluated. It was demonstrated that the Midilli model best described the drying kinetics for all methods across the entire measurement range. FD and VD produced dried products with the highest brightness and the greatest value of the a* color parameter. The lowest b* color parameter was observed for the product dried using FD at 40 °C, while the highest b* value was noted for samples dried using AMD (100 W) at 60 °C. The highest carotenoid retention was achieved with the FD method at 40 °C, while the lowest carotenoid content was found in the product obtained using the AMD method (100 W) at 60 °C. The smallest losses of capsaicinoids were observed after FD drying at 40 °C, while the largest were found for AMD (100 W) at 60 °C. The analysis of chili pepper fruit extracts revealed the quantitative composition of 12 main phenolic compounds using the UHPLC-UV method. The highest polyphenol content was obtained with FD, while the lowest total polyphenol content was recorded after AD. Regardless of temperature, the total flavonoid content was highest in extracts from FD products, and the lowest flavonoid content was found after AMD at 100 W. For all drying methods analyzed, the total flavonoid content in the pepper extracts decreased with increasing temperature.

Kiwi and pepino fruits are most valuable fruits as they contains substantial amounts of nutrients and bioactive compounds. These fruits exhibited several health potentials such as antioxidant, antiinflammatory, antiobesity, antihyperlipidemia, and anticancer properties. However, studies on the effect of microwave and conventional drying methods on the antioxidant activity and bioactive compounds of kiwi and pepino fruits are limited. Therefore, this study was conducted to assess the effect of microwave and oven drying methods on antioxidant activity, total phenolic, and phenolic compounds of kiwi and pepino fruits. Drying of the fruit samples was carried out using conventional (70 °C for 20 h) and microwave (720 W for 3 min) ovens. 1,1-diphenyl-2-picrylhydrazyl scavenging and colorimetric Folin–Ciocalteu assays were used to assess the antioxidant activity and total phenolic contents, respectively, of fresh and dried fruits. Both drying methods significantly (p < 0.05) decreased the moisture contents of both fruits compared to untreated controls. Concomitantly, drying methods also enhanced (p < 0.05) antioxidant activity and total phenolic content of both fruits with the highest improvement being observed for microwave-dried fruits compared to untreated controls. In addition, a significant increase was observed in catechin and 1,2-dihydroxybenzene content of kiwi and pepino after drying process. However, microwave drying method reduced the amount of 3,4-dihydroxybenzoic acid in kiwi (ranging from 34.120 to 9.350 mg/100 g) and pepino (varied from 33.414 to 15.445 mg/100 g). Generally, the highest antioxidant activity and phenolic contents were reported in microwave oven dried samples, followed by samples dried in oven and fresh fruits. The results revealed that microwave drying could be more useful in fruit drying than conventional drying. In addition, dried kiwi and pepino fruits contains substantial quantities of phenolic compounds with high antioxidant activity compared to fresh fruits, and thus they are considered as healthy food.