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To overcome long drying time, low energy efficiency and poor product quality associated with conventional drying, a radio frequency (RF) vacuum technology is proposed for drying kiwifruit slices using a 27.12 MHz, 3 kW RF-vacuum drying system. The results demonstrated that the process variables, electrode gap, vacuum pressure, and sample thickness, had major effects on the RF-vacuum drying. The RF-vacuum drying was associated with internal heating and rapid drying resulting in 65% reduction of hot air drying (60 °C) time. Moreover, kiwifruits dehydrated by RF-vacuum drying were associated with better color stability, higher vitamin C retention, and higher rehydration capacity ( p  < 0.05) as compared with hot-air-dried samples. Based on acceptable drying rate, stable temperature and avoiding arcing, a RF-vacuum drying protocol with the electrode gap of 60 mm, vacuum pressure of 0.02 MPa, and sample thickness of 8 mm was identified. Despite some differences observed in individual fruit slices, the RF-vacuum drying technique achieved better and more uniform drying patterns among the samples. Overall, the RF-vacuum drying process may provide a more effective and practical method for high-quality dehydration of kiwifruits.

The present study aimed to investigate the effect of rotary microwave vacuum drying (RMVD), radio frequency vacuum drying (RFVD), vacuum far infrared drying (VFID), vacuum drying (VD), hot air drying (HD) and natural drying (ND) on the drying characteristics, active ingredients and microstructure of Codonopsis pilosulae slices. Compared with the fitting results of the four models, the Weibull model is the most suitable drying model for Codonopsis. The RFVD and HD color difference values were smaller compared to ND. The effective moisture diffusivity (Deff) under different drying methods was between 0.06 × 10−8 m2/s and 3.95 × 10−8 m2/s. RMVD-dried products had the shortest drying time and retained more active ingredients. The microstructure analysis revealed that the porous structure of RMVD is more favorable for water migration. RMVD is a promising dehydration method for obtaining high-value-added dried Codonopsis products.

Drying is an important post-harvest process to preserve seaweed as they are highly susceptible to spoilage due to their high moisture content. Drying can be performed in multiple ways by changing the temperature, pressure, air flow, and humidity. Therefore, the choice of drying method can affect the quality of the product in terms of sensory, chemical, and physicochemical properties. Seaweeds contain nutrients (protein, lipids, carbohydrates, vitamins, and minerals) and bioactive compounds. The compounds impact properties such as texture, taste, odor, and appearance. However, there is currently limited knowledge about how different drying methods affect the quality of seaweed products. In this paper we demonstrate, how different drying methods: i) convective drying (52 °C), ii) microwave-vacuum drying (-40 to 40 °C at 10 Pa), and iii) freeze-drying (-20 to 20 °C at 20 Pa) influence the food quality of Fucus vesiculosus and Ulva sp. by investigating physico-chemical properties such as water holding capacity, water absorption, and color, the changes in some of the chemical compounds such as macronutrients, fatty acids, amino acids, antioxidants, and pigments, as well as the taste, odor, appearance, and texture within sensory attributes. This study found that different drying methods have a species-dependent influence on the quality of seaweed, with Ulva sp. showing more similarities of using microwave-vacuum and freeze-drying methods, while the drying method for F. vesiculosus should be selected based on the desired food quality due to significant variations between the drying methods.

This research investigated the effects of drying temperature and vacuum pressure duration (t-VPD) on drying characteristics, rehydration ratio (RR), microstructure, color, vitamin C (VC), and citric acid (CA) during the pulsed vacuum-drying (PVD) process of hawthorn slices. An extreme learning machine (ELM) model was developed to predict the change of moisture content of hawthorn slices during PVD process. Experimental results indicated that both drying temperature and t-VPD significantly affected the drying characteristics and quality attributes of dried hawthorn slices. As the drying temperature increased from 60 to 70 °C, the drying time decreased from 274 to 181 min. Compared to hot air drying (HAD), the dried hawthorn obtained by PVD had higher retention rates of VC and CA and lower color differences. The highest retention rates of VC and CA of dried hawthorn slices occurred at a drying temperature of 60 °C and a t-VPD of 8 min, i.e., 80.65% and 94.82%, respectively. PVD promoted the formation of pore structures and improved the rehydration capacity of dried products. The correlation analysis showed that the values of a* and b* were in association with the VC content of hawthorn. Besides, an optimal ELM model with a 3-12-1 topology structure was obtained to predict the change of moisture ratio of hawthorn slices during PVD process. The obtained R2 and RMSE values between the actual and predicted values were 0.997 and 2.07e−2, respectively. Overall, these results indicated that PVD could be a desirable technique for the drying of hawthorn slices processing.

The aim of this study was to compare the effectiveness of microwave-vacuum pretreatment conducted at 100, 500 and 800 W on the drying kinetics of whole cranberries ( Vaccinium macrocarpon ) during hybrid osmotic and microwave-vacuum drying. Additionally, the effect of microwave-vacuum pretreatment and subsequent osmo-microwave-vacuum drying on selected quality indicators of dried cranberries, including phenolic content, antioxidant activity and color, was studied, and the quality of samples was compared with freeze dried, microwave-vacuum dried and osmo-microwave-vacuum dried samples. Irrespective of microwave power, the initial pretreatment accelerated mass transfer during osmotic dehydration of cranberries, and the Weibull model well fitted the experimental data. Final microwave-vacuum drying of cranberries was a two-stage process involving a relatively long phase with a constant drying rate, followed by a short period with a decreasing drying rate. Microwave-vacuum and osmo-microwave-vacuum drying resulted in similar retention of polyphenols and similar antioxidant activity, both of which were relatively higher than in freeze-dried cranberries. However, microwave-vacuum pretreatment at low microwave power (100 W) before dehydration also resulted in high retention of phenolic compounds, high antioxidant activity and attractive color, which were consistent with the high content of total anthocyanins and flavonoids. Microwave-vacuum, osmo-microwave-vacuum and osmo-microwave-vacuum drying combined with microwave-vacuum pretreatment at low microwave power (100 W) were the most suitable methods for the production of high-quality dried whole cranberries.

In this study, drying kinetics and quality of purple-fleshed sweet potato (PFSP) subjected to microwave-vacuum drying were investigated. The effects of hot water and steam blanching pretreatment on physicochemical characteristics of the dried products were also considered. The samples were dehydrated in a custom-made microwave-vacuum system at different power levels including 450, 600 and 850 W. Hot air drying at 70 °C was also conducted for comparison. The results showed that drying time of PFSP under microwave-vacuum conditions ranged from 6 to 12 min, significantly reduced as compared to that of hot air drying (600 min). The improvement of drying rate was also evidenced by increased effective moisture diffusivity (2.22 × 10−7–4.05 × 10−7 m2/s) of the samples. Drying kinetics of PFSP was best fitted by Page and logarithmic model with R2 ranging from 0.991 to 0.998, and RMSE from 0.016 to 0.030. PFSP dried under microwave-vacuum condition had lower water absorption index and swelling capacity than hot air drying. Color, antioxidant activity and total phenolic content of dried PFSP were also improved under microwave-vacuum drying. The effects of blanching pretreatment on quality of dried PFSP were more dominant in hot air than microwave-vacuum dried samples.

In this paper, intelligent technology of combined low field NMR (LF-NMR) and back propagation artificial neural network (BP-ANN) was used to monitor moisture content in carrot during microwave vacuum drying. The relationship between different drying powers (200, 300, and 400 W) and NMR signals (A 21 , A 22 , A 23 , and A total ) was investigated. Results show that as the drying time elapsed, the NMR signals of A total and A 23 decrease all drying conditions, A 21 and A 22 tend to increase at high moisture content and then decrease, which is consistent with the state of water while changes during drying. NMR signals can be used as indicators for online monitoring of moisture and control of the drying process. With NMR signals as input variables, a BP-ANN model was built optimized by transfer function, training function, and the number of neurons to model the moisture content (output). Compared with a linear regression model and multiple linear regression model, the BP-ANN model with the topology of 4-25-1, transfer function of tansig and purelin, and training function of trainlm outperformed the fitting performance and accuracy. This shows that the combined approach of utilizing LF-NMR and BP-ANN has great potential in intelligent online monitoring and control applications for carrot drying.

Shiitake mushrooms were dehydrated by two different drying methods, i.e., microwave-vacuum drying (MVD) and microwave-vacuum combined with infrared drying (MVD + IR). MVD was operated at microwave powers of 56, 143, 209 and 267 W under absolute pressures of 18.66, 29.32, 39.99 and 50.65 kPa, whereas infrared radiation was added in MVD + IR at 100 and 200 W. The effects of microwave power, absolute pressure and infrared power on drying characteristics, qualities and specific energy consumption were investigated. It was found that drying rate increased with lower absolute pressure, higher microwave power and higher infrared power. In particular, the results also indicated that drying undergoing MVD + IR could provide better qualities in terms of color of dried shiitake mushroom, rehydration ratio and texture of rehydrated ones. Furthermore, the drying characteristics were described by fitting data to six different drying models. Based on their coefficient of determination, root mean square error, residual of sum square and chi-square, Modified Page model could accurately predict moisture ratio for all drying conditions. Within the range of this study, the suitable drying condition with respect to the product qualities and energy consumption was MVD + IR drying at 267 W of microwave power, 18.66 kPa of absolute pressure and 200 W of infrared power.

Radiant Energy Vacuum (REV)-dried broccoli, oranges, and carrots prepared by the optimal drying protocols determined in this study were compared to the freeze-dried and air-dried samples based on the nutritional values before and after drying. An accelerated shelf-life study for REV-dried broccoli, oranges, and carrots was also conducted. For all the samples, REV drying significantly shortened the processing time. The REV-dried samples had much higher retention of the nutritional values (vitamin C, β-carotene) compared to the conventional air-drying process, and the values were also competitive to those of the freeze-dried samples. Although freeze-drying resulted in the best rehydration property, the REV-dried samples still earned the highest scores in the sensory test. In the accelerated shelf-life study conducted on the REV-dried samples, the moisture content and water activity stayed at the same level, but the nutritional values showed a downward trend. The sensory properties fluctuated in the shelf-life but still gained positive feedback from the panelists. Moreover, the testing method for β-carotene content was uniquely designed in this project and could be a semi-quantitative method to refer to.

In this study, the effect of an ultrasound (US) pre-treatment on the process of drying Cistanche slices through far-infrared vacuum drying was investigated with various experimental factors, including the US treatment time (25, 35, 45 min), frequency (20, 40, 60 kHz) and power (150, 180, 210 W). The results showed that compared with the samples without US, the material drying time after the US treatment was reduced by 16–36.8%. The effective moisture diffusion coefficients of Cistanche slices under different US conditions ranged from 1.61122 × 10−8 to 2.39274 × 10−8 m2/s, which agreed with food processing ranges. In addition, the phenylethanoid glycoside, iridoid, polysaccharide, total phenol and total flavonoid contents in Cistanche were significantly increased after US pre-treatment. However, the dried products obtained with the 45 min US treatment had greatly damaged internal structures, collapsed and seriously deformed surfaces, and low contents of active ingredients. Overall, the US pre-treatment could significantly improve the drying quality of Cistanche slices.