Fresh and osmotically pretreated (using a 35% isomaltulose solution) strawberries are highly perishable. This work aimed to dry them by intermittent microwave drying (IMD) and heated air drying (HAD) ...and evaluate their qualitative parameters. The sample temperature was monitored during IMD, adopting a setpoint temperature of 60 °C and continuous airflow, at room temperature. The HAD was performed by exposing the samples to air at 60 °C. IMD stood out for being faster (up to 64.91%) and consuming less energy (up to 73.16%) than HAD. During IMD, the magnetron was off for most of the time (up to 74.13%). The isomaltulose impregnation reduced drying rates in both methods, increasing drying time. Dried samples showed water activity (aw) within the safe range (aw<0.6). The impregnation process was carried out to a reduced shrinkage product besides the enrichment with isomaltulose. Non-pretreated strawberries had lower aw, hardness, and total color difference, and higher levels of bioactive compounds and antioxidants, especially with the IMD method. Retentions of up to 47.46%, 70.26%, and 81.18% of anthocyanins, phenolic compounds, and antioxidant capacity were obtained by IMD, respectively, concerning fresh strawberries. Therefore, IMD was an interesting alternative for fresh and impregnated with isomaltulose strawberries drying.
•Intermittent microwave drying (IMD) was better to preserve the anthocyanins.•IMD showed a high drying rate, and required low drying time and energy consumption.•Isomaltulose impregnation decreased the shrinkage of dried strawberry.
Hot-pressing drying shows promise in sludge treatment. Optimization often focuses on temperature and external load. However, the impact is restricted by sludge inherent properties, which can be ...changed by sludge conditioning. Due to coupling effects, altering individual sludge properties is difficult. Key sludge properties that significantly affect heat and mass transfer remain unclear, hampering identification of specific conditioning techniques. To address this gap, the effects of sludge specific heat, thermal conductivity, water activity, moisture diffusivity, and moisture permeability on drying performance were explored using sensitivity analysis. Sawdust conditioning and kinetics were employed to improve drying performance, validate sensitivity analysis findings, and elucidate drying mechanisms. Sensitivity analysis revealed that liquid water permeability significantly positively influenced drying performance by enhancing liquid water convection. Sawdust conditioning and kinetics demonstrated that adding 20 % sawdust increased drying flux from 31 to 62 kgwater·m⁻²·h⁻¹. The drying process included warm-up and constant rate, the first falling rate, and the second falling rate periods. Enhanced moisture transfer was dominated by breakthrough of interfacial gas film, transformation from liquid water diffusion to convection, and facilitated water vapor diffusion, respectively. Hence, this study provided novel insights, strategies, and direction of guidance for revealing sludge drying mechanisms and improving drying performance.
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In this study, the effects of the electrohydrodynamic (EHD) drying method with needle and wire, the hot air method and a combination of the EHD-hot air methods on the drying kinetics, color, ...rehydration ratio and scanning electron microscope images of apricot samples were investigated. In addition, the experimental drying data were modeled using ten different mathematical thin layer drying models. It was determined that the rehydration ratio values of the samples dried with the combination of EHD and hot air were generally higher than the rehydration ratios of the samples dried with hot air or EHD only. Samples from all the drying methods were compared with fresh samples; the color parameters were affected, and scanning electron microscopy images revealed the different physical changes in the microstructure of the products. As a result, it is determined that EHD technology can be used as an alternative drying method suitable for apricot samples.
Radio frequency-assisted hot air drying (RF-HAD) was carried out to produce carrot powders. Effect of electrode gap, sample thickness and sample weight on RF-HAD efficiency was investigated. 90 mm of ...electrode gap, 2.5 cm of sample thickness and 600 g of sample weight were selected as RF-HAD parameters in order to obtain acceptable drying rate and avoid excessive deformation. RF-HAD kinetics were studied and compared with hot air drying (HAD). RF-HAD increased drying rate and reduced drying time by 45%. Also, the quality of carrot powders obtained after RF-HAD was evaluated and compared with those produced by air and freeze drying. The results showed that the samples dried with RF-HAD had greater contents of β-carotene, ascorbic acid, and total phenols, higher antioxidant activity and improved rehydration capacity compared to HAD. The better retention of cellular microstructure after RF-HAD also resulted in better rehydration capacity. Beta-carotene, ascorbic acid, total phenolic content and antioxidant activity retention rates were 80.26%, 45.33%, 72.42% and 67.49%, respectively, for RF-HAD. These values were 89.36%, 47.02%, 79.98% and 70.61% for freeze dried samples. The retention of bioactive compounds after RF-HAD was comparable to those achieved by freeze drying.
•Radio frequency-assisted hot air drying (RF-HAD) was used to produce carrot powder.•RF-HAD improved drying rate of carrots and significantly reduced the drying time.•RF-HAD approach enhanced the retention of bioactive substances.•RF-HAD can be an alternative technology in industry to produce high-quality powders.
•Hot air convective and microwave-vacuum drying (HACD+MWVD) was investigated.•Effect of drying on the drying kinetic and quality of blueberries was evaluated.•The highest content of total polyphenols ...was noted after HACD at 90°C.•HACD at 90°C+MWVD resulted in the highest content of anthocyanins.•HACD at 90°C+MWVD resulted in the strongest antioxidant capacity.
The aim of the study was to evaluate the effect of hot air convective drying (HACD), microwave vacuum drying (MWVD) and their combination (HACD+MWVD) on the drying kinetics, colour, total polyphenols, anthocyanins antioxidant capacity and texture of frozen/thawed blueberries. Drying resulted in reduction of total polyphenols content and antioxidant capacity (69 and 77%, respectively). The highest content of total polyphenols was noted after HACD at 90°C. Lower air temperature and prolonged exposure to oxygen resulted in greater degradation of polyphenols and antioxidant capacity. Drying processes caused a significant decrease (from 70 to 95%) in the content of anthocyanins. The highest content of anthocyanins and the strongest antioxidant capacity was found in blueberries dried using HACD at 90°C+MWVD. Among drying methods, HACD at 90°C+MWVD satisfied significant requirements for dried fruits i.e. short drying time and improved product quality.
Yacón is a tuberous root from the Andean region, and a functional food due to the presence of fructo-oligosaccharides (FOS). Due to its high moisture in natura, the product is prone to microbial ...degradation. The evolution of yacón cylindrical slices under convective drying is here studied from experimental work and theoretical considerations about bulk properties. The operation of a pilot-scale convective dryer allowed monitoring the moisture and dimensions of yacón slices. Evolution of moisture under controlled temperature and relative humidity conditions is described through a two-parameter drying kinetics model. A novel approach for estimating shrinkage effects is proposed, comparing measured dimensions with predictions based upon hypothetical shrinkage behaviors. The drying kinetics and the shrinkage approach are finally combined with the global energy balance for a single slice, in order to describe the evolution of surface temperature and other yacón slice properties along the drying period.
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•The evolution of yacón slices under convective drying was studied at pilot scale. 81.•Experimental data were collected under controlled air temperature and humidity. 79.•Moisture loss observations were approximated through a novel two-parameter model. 81.•Shrinkage of yacón slices was described by a combination of hypothetical models. 80.•The global energy balance allowed to predict surface water activity and temperature. 84.
This study provides a comprehensive analysis of the vacuum drying process for sludge drying, with a focus on optimizing energy efficiency and emission control. The study used both lab-scale static ...and pilot-scale vacuum drying systems to test various parameters like vacuum levels, heat source temperatures, and sludge thicknesses. The results indicated that optimal drying conditions were achieved at a vacuum level of −0.06 MPa, a heat temperature of 140 °C, and a sludge thickness of 3.4 mm, where the drying rate reaches 0.13278 g·g−1·min−1. The study underscores the significant influence of vacuum level, temperature, and sludge thickness on drying rates. The Page model was used to analyze drying kinetics, elucidating how changes in these parameters affect drying characteristics. Furthermore, the study also examined the pollutant emissions and energy efficiency at the pilot scale. It found that high vacuum environments could efficiently dry sludge using low-temperature heat source, leading to average energy consumption per unit evaporation of 3020.29 kJ/kg, which is lower compared to traditional methods. By harnessing low-grade industrial waste heat, this can be further reduced to 875.76 kJ/kg. This study offers valuable insights for sustainable sludge management systems, highlighting the environmental and economic benefits of vacuum drying technology. The detailed experimental approach and thorough analysis make a significant contribution to the field of the sludge drying.
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•Identified varying drying rates influenced by operational parameters•Demonstrated reduced energy consumption in vacuum drying•Highlighted the impact of operational conditions on moisture diffusivity in sludge•Provided insights into pollutant emissions variations during the vacuum drying
An integrated solar system was examined and worked for drying chamomile during the summer season 2013 in Germany. The system consisted of collector, heat exchanger, reflector, main drying chamber ...below collector, additional drying chamber and supplementary electric heaters immersed in water tank. It could also storage of solar energy into water during the time of sun-shine and reuse this energy at cloudy weather or off sunshine time to raise the temperature of drying air inside the system. The capacity of main drying chamber ranged 32–35 kg of fresh chamomile and 10–12 kg for the other separate drying chamber. Air temperature inside the dryer could be maintained as desired range for drying chamomile using a temperature controller. The integrated dryer was operated about 30–33 h to reduce the moisture contents of chamomile from 72-75%–6% (wb) compared to 60 h to reduce it to 9–10% (wb) using open sun drying method.
Nine mathematical models for drying kinetics of chamomile were tested to determine the parameters of the best suitable models for those plants. It is found Midili model was the best model to define drying kinetics of chamomile for the main and additional drying chambers in solar system.
•Performance of the integrated system for hybrid solar dryer for drying chamomile.•Drying Kinetics for thin layer solar drying of Chamomile.•Experimental data was fitted to empirical models for thin layer of Chamomile.•Validation of the best model that present best results solar drying of chamomile.
The drying kinetics and quality properties of mulberries dried in hot-air were investigated at four temperatures (40, 50, 60, and 70 °C). A convenient tool, Low-field nuclear magnetic resonance ...(LF-NMR) and MRI techniques were used to study the water state and distribution during the drying process. The results showed that the water reached equilibrium with drying times of 83, 38, 29, and 17 h at drying temperatures of 40, 50, 60, and 70 °C, respectively. The Page model fantastically described the drying process of mulberries. The transverse relaxation time (T2) curves showed that T2 times of free water decreased to the equilibrium points corresponded exactly with the first stage of drying kinetic curves. MRI showed that water diffused from the internal to the external during drying. Optimum color, texture, microstructure and higher total anthocyanin content were found in mulberries dried at 50 °C. Notably, higher correlations between LF-NMR parameters and quality properties were found by partial least squares regression (PLSR), with the analysis results being credible.
•LF-NMR and MRI can directly describe the movement of water in different states.•Page model was the best model for describing the drying kinetics.•The first stage of drying related to the time point when free water disappeared.•Good Correlations were found between the LF-NMR parameters and quality properties.
Hot-air assisted radio frequency (RF-HA) heating was utilized for tilapia fillets drying to achieve rapid, uniform drying and improved sample physical properties. Effects of sample thickness (5, 8 ...and 10 mm) and with or without wrapping material (skin or gauze) were investigated under both HA and RF-HA drying treatments. The drying characteristics and quality parameters of tilapia fillets in hot air (HA) and RF-HA drying were then determined. Results show the drying rate of RF-HA drying is 1.1–1.4 times of that of HA drying for tilapia samples with different thicknesses. RF-HA dried samples are with higher rehydration rate and lower volume shrinkage rate, especially for fillets with larger thickness. The wrapped gauze did not show significant influence on the color of tilapia sample in RF-HA drying but improved the rehydration rate, while the samples with preserved fish skin resulted in a lower overall quality under both HA and RF-HA drying treatments. Page model and Henderson-Pabis models were suitable to be used describing the drying kinetics of RF-HA drying and HA drying tilapia, respectively.
•RF-HA combined drying improved drying rates of tilapia fillets.•Wrap the fish sample surface with gauze improved the dehydrated sample quality.•RF-HA combined method showed more advantages in thicker fish fillets drying.
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