The energy for drying agricultural products comes from various sources such as electricity, natural gas, biomass and fossil fuels. The practice of solar energy utilization in the agricultural sector ...has a potential scope to minimize the cost of operation, replace the conventional drying methods and save environment by reducing carbon foot print. In the present study, heat pipe assisted natural convection indirect solar dryer (HNISD) was used to determine the drying behaviour with respect to thickness in banana slices with three different thickness namely 2,4 and 6mm. The drying time taken for 2 and 4 mm thick slices was 12 hours whereas the slices with 6 mm thickness took 13 h of drying time. The experimental data was best fit with the Midilli-kucuk model for HNISD. For open sun drying Two-term model was found to be best model to explain the drying characteristics of banana slices.
•Determination of drying kinetics of grape pomace and evaluation of quality parameters.•Testing the drying of grape pomace with a special design heat pump dryer.•Determination of phenolic degradation ...in drying of grape pomace at different air velocities.•Despite the losses of bioactivity properties, HP dryer is the effective in grape pomace drying.•Lower energy consumption was obtained from HP dryer than Control.
Pomace of Hamburg Muscat was dried at temperature of drying air 45 °C and different air velocity (1.5, 2.0 and 2.5 m/s) in open-loop heat pump (HP) and laboratory-type closed-loop dryer (as a control). In the HP dryer, it was concluded that drying air velocity was slightly effective on drying time however there is no significant effect on the power consumption of the change in the drying air velocity at the same temperature. When comparing the energy consumption of the HP dryer and convective dryer, the energy consumption was reduced by up to 51%. In HP drying, the increase of air velocity from 1.5 m/s to 2.5 m/s caused a reduction in drying time by 69%. It was observed that part of the bioactive properties was lost in pomace samples but there were fewer losses in all bioactive properties than the others except the total anthocyanin at 2 m/s.
•Ultrasound pre-treatment before drying and ultrasound assisted drying were evaluated.•Parametric effects, drying kinetics and food quality were discussed.•Inconsistencies of the ultrasound effects ...were pointed out and analyzed in detail.
This study presents a state-of-the-art overview on the application of ultrasound technology in the drying of food products, including the ultrasound pre-treatment and ultrasound assisted drying. The effect of main parameters and ultrasound technology on the drying kinetics and food quality were discussed. Inconsistencies were pointed out and analyzed in detail. Results showed that for ultrasound pre-treatment, the food products may lose or gain water and increase of ultrasonic parameters (sonication time, amplitude and ultrasound power) promoted the water loss or water gain. When ultrasound technology was applied prior to drying, an increase in drying kinetics was always observed, though some different results were also presented. For ultrasound assisted drying, the ultrasound power always gave a positive effect on the drying process, however, the magnitude of ultrasound improvement was largely dependent on the process variables, such as air velocity, air temperature, microwave power and vacuum pressure, etc. The application of ultrasound technology will somehow affect the food quality, including the physical and chemical ones. Generally, the ultrasound application can decrease the water activity, improve the product color and reduce the nutrient loss.
This study investigated the effects of ultrasonic frequency, ultrasonic power, irradiation height and temperature on the drying characteristics, quality and microstructure of wolfberry by ...ultrasonic-assisted far-infrared drying. By fitting five commonly used thin-layer drying mathematical models, it was found that the coefficient of determination (R2) of the Weibull model was 0.99400–0.99825, the root mean square error (RMSE) was 1.2162 × 10-4–4.5209 × 10-4, and the reduced chi-square (χ2) was 0.00207–0.00663, which was the best fit. Under the application of ultrasound, the average drying rate of wolfberry increased. Compared with natural drying, the polysaccharide content increased by 33.2 % at 250 mm irradiation height, and the total phenol content increased by 44.9 % at 40 kHz ultrasonic frequency. The antioxidant activity was the strongest, and the total flavonoids content was the highest (2.594 mg/g) at 24 W ultrasonic power. By comparing the microstructure of wolfberry under different drying methods, such as a fresh sample, natural drying, hot air drying, and ultrasonic-assisted drying, we found that the ultrasonic assistance increased the number of micropores on the surface of wolfberry, reduced the damage to epidermal cells, reduced the mass transfer resistance of the drying process and accelerated the drying process. This study shows that ultrasonic-assisted far-infrared drying technology played a significant role in the heat and mass transfer of wolfberry drying, and had great potential in the commercial processing of wolfberry.
Results of an experimental study are presented and discussed for pulsed vacuum drying (PVD), infrared-assisted hot air-drying (IR-HAD), and hot air-drying (HAD) on drying kinetics, physicochemical ...properties (surface color, nonenzyme browning index, red pigments, rehydration ratio, water holding capacity, and ascorbic acid), antioxidant capacity (ferric reducing antioxidant power and 2,2-diphenyl-1-picrylhydrazyl radical scavenging capacity), and microstructure of red pepper. As expected, the drying time decreased with an increase in drying air temperature, IR-HAD needed the shortest drying time, followed by HAD and PVD. The effective moisture diffusivity (Deff) of red pepper under PVD, HAD, and IR-HAD was computed to be in the range 1.33-5.83 × 10-10, 1.38-6.87 × 10-10, and 1.75-8.97 × 10-10 m2/s, respectively. PVD provided superior physicochemical properties of dried red pepper compared to samples dried by HAD and IR-HAD. In detail, PVD yielded higher rehydration ratio, water holding capacity, red pigment and ascorbic acid content, brighter color, lower nonenzyme browning index, and comparable antioxidant capacity compared to samples dried by HAD and IR-HAD at the same drying temperature. Furthermore, PVD promoted the formation of a more porous structure, while HAD and IR-HAD yielded less porous structure. The current findings indicate that PVD drying has the potential to produce high-quality dried red pepper on commercial scale.
Pre-freezing has an impact on the drying time and physiochemical properties of cannabis. This study investigated the relationship between sample mass reduction and relative humidity during ...freeze-drying and the effects of pre-freezing and freeze-drying temperature on cannabis drying kinetics, trichome structure, and color, in addition to cannabinoid and terpene concentrations. Three cannabis accessions, Qrazy Train, Qrazy Apple, and Qrazy Angel, were dried at 10°C, and 20°C, with different pre-freezing conditions (−20°C and −40°C). Pre-freezing rates of 0.13°C min−1 for both Qrazy Train and Qrazy Angel and 0.15°C min−1 for Qrazy Apple were recorded for pre-freezing at − 20°C and significantly (p < 0.05) increased by 73.5%, 71.2%, and 72.9% for Qrazy Train, Qrazy Apple, and Qrazy Angel, respectively, when inflorescence was pre-frozen at − 40°C. Rational regression model best explains the relationship between mass reduction and relative humidity during drying, while drying kinetics can be described using the Page and Logarithmic models. Total color changes ranged from 1.16 to 9.69 total color changes measured for all dried samples compared to respective fresh, undried samples were not significantly (p < 0.05) different. THCA concentrations for fresh, undried Cannabis sativa L. accessions ranged from 214.4 mg g−1 to 257.5 mg g−1; this was higher than their CBDA concentrations, ranging from 0.03 mg g−1 to 0.1 mg g−1. Freeze-drying increased CBDA, CBGA, and CBG in dried samples ranging from 0.45 mg g−1 to 0.38 mg g−1, 2.87 mg g−1 to 4.91 mg g−1, and 0.57 mg g−1 to 1.33 mg g−1, respectively. Mean terpene concentration ranged from 17.7 mg g−1 to 40.3 mg g−1. Irrespective of the pre-freezing condition or cannabis accession, drying at 20°C reduced drying time by 10.4–31.9%. Findings could be of industrial relevance for improving post-harvest processes while maintaining quality of this regulated crop.
Display omitted
•Relative humidity can be used as an indicator for the end of a freeze-drying cycle.•SEM analyses show structural damage incurred by trichomes.•Freeze-drying increased CBDA, CBGA, and CBG concentrations in dried cannabis.•Decarboxylation of THCA to THC occurred due to the low drying temperature.
Drying is an important method to preserve food products. Although many traditional drying technologies (hot air, freeze, microwave drying) have been applied successfully to various food products, ...each drying technique has its own advantages and disadvantages. Novel drying methods such as infrared drying, have become very popular.
This study gives an overview on the application of infrared radiation in the drying of food products in the last decade. The effects of main parameters (infrared power, intensity, distance, wavelength and drying temperature) and the introduction of infrared radiation on drying kinetics and food quality were discussed. Inconsistencies were pointed out and analyzed in detail.
Key findings and conclusions: Drying kinetics was improved by introducing infrared radiation and increased with increasing infrared power, intensity, drying temperature and decreasing infrared distance. However, a very high infrared power, intensity, drying temperature and a very low infrared distance should be avoided as the food products will be overheated. The effects of infrared parameters on food quality were unpredictable. The energy consumption under infrared radiation was also variable. When the reduction of drying time was considerable, the energy consumption decreased. When the reduction was insufficient, the energy consumption increased. The application of infrared radiation will also affect the food quality. Generally, the infrared radiation can decrease the water activity, reduce the total color change and improve the nutrient retention.
•Application of infrared radiation in the drying of food products was reviewed.•Effects of main parameters on drying kinetics and food quality were studied.•Inconsistencies among different researchers were emphasized and analyzed.
We investigated the drying behavior of aqueous colloid-polymer suspensions using three different polyvinyl alcohols (PVAs) of various molecular weights (Mws). The dissolved polymer molecules were ...transported and accumulated around the drying interface. The accumulation was more dominant in the smaller PVAs. A packed layer of particles formed during drying. The small molecules were able to permeate the voids in the packed particles more easily. The rate of water evaporation decreased dramatically as the concentration of dissolved PVA near the drying interface (Cedge) increased. Although the Cedge increased more rapidly when we used smaller PVAs, the rate of water evaporation was not affected by the Mw. The decrease in the rate of water evaporation was smaller from a polymer solution without colloidal particles than it was from the colloid-polymer suspensions. We conclude that the accumulation of dissolved polymer molecules in the narrow voids of packed particles impedes water evaporation during drying. The drying kinetics of the water disturbed by the PVAs in the packed particles will be discussed.
Display omitted
•Drying rates of particle suspensions dramatically decrease by addition of polymers.•Small polymer molecules permeate packed particles and accumulate around the air-water interface•Large polymer molecules are filtrated by packed particles and they exhibit less accumulation•Drying rates are governed by the polymer concentrations near the air water interface•Precipitation of dissolved polymer molecules hardly affects drying kinetics
•Microwave drying was used to optimize parameters of drying natural TiO2.•The particle size has a significant influence on microwave drying efficiency.•Microwave can promote the growth of small ...cracks, which quickly release internal moisture.
In this paper, factors affecting the microwave drying characteristics of rutile TiO2 were discussed, and the data were fitted with nine common thin layers drying kinetic models. The results demonstrate that the drying efficiency of rutile TiO2 shows a positive relationship with microwave power. With a microwave power increase from 320W to 640W, the drying time is shortened by 35.7%. The microwave drying efficiency presents a positive relationship with initial moisture content. With a moisture content increase from 2% to 12%, the average drying rate increased from 0.009%·s−1 to 0.054%·s−1. Additionally, the microwave drying efficiency is positively correlated with the initial mass when the initial mass is below 45 g. Otherwise, the microwave drying efficiency is negatively correlated with the initial mass. Besides, kinetic models were used to understand the microwave drying process of nature rutile TiO2. The results show that the Modified Page model agrees well with the experimental data, and the fitting parameters are consistent with the law of matching degree. The demonstration of microwave drying techniques can be applied effectively and efficiently to the treatment processing of drying of the raw materials of metallurgy and chemical industry with the theoretical and scientific basis.
Model diagram of the microwave oven Display omitted .
Yacon is a prebiotic food, rich in insoluble fibers, however, highly perishable because of its high moisture content and unstable coloration after peeling, due to enzymatic browning. To overcome such ...challenges, this study evaluated the effects of two types of bleaching (chemical and thermal) to precede the pretreatment with ultrasound (US) and ethanol, to study the influence of these methods on drying kinetics, and on changes in product color. Yacon cylinders were dried in a tray oven at 50 °C and it was found that the drying time until final moisture content was reduced after the application of pretreatments. Particularly, pretreatment with ethanol combined with ultrasound provided the greatest reduction in drying time to reach 25% moisture content for both the chemically bleached vegetable (30.1% reduction), and the thermally bleached vegetable (61.2% reduction). The best drying rate was observed for pretreatment with US, yet the water which permeated into the internal matrix during this pretreatment likely limited great reduction in drying time. Regarding product color, chemical bleaching was more efficient in preserving the color characteristics of the product. These results show new perspectives regarding the use of ethanol and ultrasound associated with different pretreatments to improve the convective drying of yacon.
•Ethanol + US provided the greatest reduction in drying time.•Chemical bleaching was more efficient in reducing drying time.•Pre-treatment with US resulted in a higher drying rate.•Chemical bleaching promoted less interference in color.
PDF
