Reduction In Drying Time Research Articles

Modelling the Thin-Layer Drying Kinetics of Marinated Beef during Infrared-Assisted Hot Air Processing of Biltong.

Biltong is a dried meat product that is widely consumed in South Africa. The marinated meat is traditionally dried under ambient winter conditions while commercial biltong producers use hot air driers. Hot air drying is time-consuming and energy-intensive. A combined infrared and hot air drying (IRHAD) is an alternative method of drying meat during biltong processing. The aim of this study was to establish the effect of the infrared (IR) power, the temperature, and velocity of the drying air on the drying kinetics of marinated beef and subsequently select the best thin-layer drying model for IRHAD during biltong processing. Marinated beef samples were dried at IR power levels of 500, 750, and 1000 W; drying air temperatures of 30, 35, and 40°C; and air velocity of 1.5 and 2.5 m∙s−1. Results indicate that increasing the IR power and the drying air temperature increased the IR emitter temperature and the core temperature of the marinated beef sample. Consequently, increasing the drying rate thus reduced drying time. The air velocity had an inverse relationship with the IR emitter temperature, the core temperature of the marinated beef sample, and the drying rate. The drying process was characterised by a rising rate period in the first half an hour, followed by a falling rate period which implies that moisture transport occurred partly by surface evaporation and predominantly by diffusion. The effective moisture diffusivity ranged from 4.560 × 10−10 to 13.7 × 10−10 m2∙s−1, while, the activation energy ranged between 40.97 and 59.16 kJ∙mol−1. The IRHAD of marinated beef during its processing to biltong was best described by the two-term model since it had the highest R2 (0.9982-0.9993) and the lowest RMSE (0.0062-0.0099). The power level of the IR emitter of 1000 W combined with a drying air temperature and velocity of 40°C and 1.5 m∙s−1, respectively, showed the highest improvement in the drying kinetics and the lowest drying time of 5.61 ± 0.35 hours; hence, it is recommended as a possible drying alternative for the processing of biltong.

Effect of pulsed‐vacuum, hot‐air, infrared, and freeze‐drying on drying kinetics, energy efficiency, and physicochemical properties of Ginkgo biloba L. seed

AbstractIn this research, Ginkgo biloba L. seeds (GBS) were freeze‐dried, infrared‐dried, hot‐air dried, and pulsed‐vacuum dried. A detailed analysis of their drying kinetics (drying curve, drying rate curve, effective moisture diffusivity, mathematical modeling), shrinkage and energy efficiency (specific moisture extraction, specific energy consumption), and physicochemical properties (soluble solids, total sugar, reducing sugar, ascorbic acid, phenolics, and tannins) were performed. Results showed drying methods had various effects on different indexes and were confirmed by principal component analysis. The drying process occurred in the falling‐rate period. The effective moisture diffusivity values amongst various drying methods ranged from 3.5 × 10−8 to 7.6 × 10−8 m2/s, which were within the normally expected range for food materials. The exponential model best fitted the experimental results for drying with coefficient of determination (R2 ) > .981 and root mean square error, sum of square error, and reduced chi‐square closer to zero than the parabolic and inverse‐logarithmic (Log3p1) models. The freeze‐dried GBS had the highest nutritional (statistically the same with the fresh GBS) and lower shrinkage; nevertheless, it had the lowest moisture extraction rate and the highest energy consumption. Hot‐air dried GBS had the highest shrinkage and lowest nutritional retention. Infrared‐dried GBS was superior with the highest non‐tannic phenolic, proanthocyanidins content, effective moisture diffusivity, drying rates, and energy efficiency. The current findings indicate that infrared drying is a promising drying technology for GBS and has the potential to be applied on a commercial scale.Practical applicationGinkgo seeds (GBS) are utilized as functional food across Asia. In China, ginkgo trees' extensive cultivation has surpassed market requirements, as GBS production could reach 60,000 tons per year. Therefore, there is an urgent need to find new opportunities to take advantage of the availability and usage of GBS regardless of the season and overcome the situation that GBS usage only remains at incipient phases of home‐cooked recipes. Drying is a common approach to preserve foods'. Nonetheless, there are some drawbacks to conventional drying method, such as low energy efficiency, lengthy process periods, and a decline in quality attributes. The infrared dryer provided a reduced drying time, highest drying rate, energy efficiency, and good quality; thus, it could be applied in the food industry to hot air and freeze‐drying, allowing the drying method for GBS faster. The research provided a predictive mathematical model for the parameter control GBS drying process.

Optimization of microwave assisted multilayer drying of bittergourd

Bittergourd rings were dried to a moisture level of 0.1 g water/g dry matter using a multilayer-cummicrowave drying technique. Response surface methodology was used to optimize the drying conditions based on specific energy consumption and quality of dried bittergourd viz. change in color, rehydration ratio, shrinkage ratio, ascorbic acid content and hardness of texture. A second-order polynomial model fitted well to all responses and high R2 values (>0.8) were observed in all cases. The change in color of the dried bittergourd was found higher at high microwave power and exposure time combination, whereas 80% retention of ascorbic acid and maximum rehydration ratio was observed at 504W. The specific energy consumption decreased with an increase in microwave power due to reduced drying time. The optimum operating conditions for drying bittergourd were PL: 504W and ET:24s, resulting in dried product with maximum rehydration ratio, ascorbic acid retention, overall acceptability, minimum shrinkage ratio, hardness and color change. The overall desirability was 0.725. Use of microwave application in the last stage of drying saved 39% energy. Therefore, microwave-assisted drying should be considered for improved heat and mass transfer and produce dried bittergourd with better quality.

Application of Analytic Hierarchy Process in Selection of an Appropriate Drying Platform for Maize Drying in a Solar Bubble Dryer

The solar bubble dryer (SBD) is a convenient and better option to traditional open sun drying commonly practiced by smallholder farmers in Ghana and other developing countries. However, to operate the SBD, one is faced with the challenge of frequent mixing of grains using a rake to enhance uniform drying and prevent over-heating of the top-layer exposed to the sun. Additionally, condensation in the SBD can compromise the quality of grains during drying under humid conditions. In the present study, the Analytical Hierarchy Process (AHP) was used to select an appropriate drying platform, tarpaulin, or elevated drying rack for maize drying in the SBD. Using the AHP, the elevated drying rack was chosen as a suitable platform for drying maize in the SBD. The SBD with the elevated drying racks was used to dry 270 kg of maize from a moisture content of 18.2% to 12.6% (w.w.b) within 6 hours compared to 11 hours for the same quantity of grains dried on the tarpaulin of the SBD to reach an MC of 12.5% (w.w.b). An average drying rate and moisture extraction rate of 0.93%/h and 2.88 kg/h were achieved with the elevated drying rack system compared to 0.52%/h and 1.60 kg/h when drying on the tarpaulin of the SBD. An average temperature of 44.6°C recorded in the SBD was 9°C more compared to the ambient temperature. The thermal efficiency of the SBD collector was determined as 36.2%. Drying grains with the elevated racks showed advantages of reduced drying time, improved airflow through the drying bed, and prevention of condensation, which occurs when drying on the tarpaulin of the SBD.

The impact of pulsed electric fields on quality parameters of freeze‐dried red beets and pineapples

AbstractThe effect of pulsed electric fields (PEF) on the cell disintegration index (Zp), the freeze‐drying process and the final product quality of red beet and pineapple tissue was studied. Red beets and pineapples were PEF‐treated at an electric field strength of 1.07 kV/cm and specific energy inputs of 1 kJ/kg and 4 kJ/kg (PEF1 and PEF2, respectively). Freeze‐drying was performed at a pressure of 1 mbar and heating plate temperature of 50 °C for red beet and 40 °C for pineapple. The quality of freeze‐dried tissue was evaluated by the analysis of residual moisture content, macrostructure, texture, colour and rehydration properties. For freeze‐dried red beets, the betalain content from extracts was studied. It could be stated that PEF pre‐treatment had no significant influence on drying time reduction, however significantly improved the final quality of freeze‐dried product. Moreover, the effect of PEF was strongly depending on type of plant material used.

Ultrasonic-Microwave and Infrared Assisted Convective Drying of Carrot: Drying Kinetic, Quality and Energy Consumption

In this study, the drying time, effective moisture diffusivity (Deff), specific energy consumption (SEC), and quality (color, shrinkage, and rehydration) of the ultrasound-pretreated (US) carrot slices were compared when dried by hot air drying (HD), microwave drying (MWD), infrared drying (INFD), and hybrid methods of MW–HD and INF–HD. Five mathematical models were considered to describe the drying kinetics in the carrots. The results show that US+MW–HD and INFD were the fastest and the slowest drying techniques compared to the HD technique with a 73% and 23% drying time reduction, respectively. The Deff ranged from 7.12 × 10−9 to 2.78 × 10−8 m2/s. The highest and lowest SECs were 297.29 ± 11.21 and 23.75 ± 2.22 MJ/kg which were observed in the HD and US+MWD, respectively. The color variation indices indicated that the best sample in terms of color stability was the one dried by US+MW–HD with the color variation of 11.02 ± 0.27. The lowest and highest shrinkage values were also observed in the samples dried by US+MWD and HD (31.8 ± 1.1% and 62.23 ± 1.77%), respectively. Samples dried by US+MWD and HD possessed the highest and lowest rehydration, respectively. Although the carrot slices dried at a higher pace by US+MW–HD (compared to US+MWD), the shrinkage and SEC of the samples dried by US+MWD were significantly lower than the US+MW–HD (p < 0.05). Therefore, it can be concluded that the application of the US+MWD method can be considered as a proper alternative for drying the carrot slices when compared to the HD, MWD, INFD, and hybrid methods.

Effects of Microwave Treatment on Microstructure of Chinese Fir

Microwave (MW) treatment is an effective method to increase refractory wood permeability, thereby reducing drying time and defects. The extent of modification depends on the damage extent of the wood microstructure. In this study, MW intensities of 43 kWh/m3 (low intensity) and 57 kWh/m3(high intensity) were adopted to treat Chinese fir lumber. Microstructural changes in wood samples were observed using scanning electron microscopy (SEM) and pore structure was characterized using mercury intrusion porosimetry (MIP). Results were as follows: After low-intensity MW treatment, parts of the bordered pit membranes in tracheids were damaged, and micro-fibrils on the margo were ruptured, while the torus basically remained intact. Micro-cracks were observed at both ends of the cross-field pit apertures, propagating to the cell walls of tracheids. The middle lamellar between ray parenchyma cells and longitudinal tracheids cracked, and the width of cracks was in the range of 1–25 μm. After high-intensity MW treatment, damage to the wood microstructure was more severe than that in the low-intensity MW treatment, with macro-cracks having a width range of 100–130 μm being generated. In addition, on the fracture surface of macro-cracks, the bordered pit membranes in tracheids fell off, cross-field pit membranes disappeared and the ray parenchyma cells were seriously damaged, exhibiting fracture of the tracheid walls. Both low-intensity and high-intensity MW treatment can increase the pore diameter corresponding to the margo capillaries (peak value increased from 674.7 nm to 831.8 nm and 1047.6 nm, respectively). The number of pores in the tracheid lumen diameter range also significantly increased. These results provide a theoretical support forMW treatment processes’ improvement and high-value utilization of Chinese fir.

Quartz halogen radiated fast and energy‐efficient convective vacuum drying of green tea fortified Himsagar mango: Optimization, kinetics, and quality assessment

AbstractThe nutritional attributes of Himsagar mango (HM; Mangifera indica L.) were enriched through fortification with green tea (GT; Camellia sinensis) extract followed by an energy‐proficient rapid drying technique. The HM fortified by GT (HMGT) was dehydrated employing vacuum‐drying under quartz halogen radiation (QHRVD) and the optimal process parameters were determined corresponding to a minimal moisture content of the final dehydrated HMGT deploying Taguchi orthogonal design. Additionally, an efficient periodic convective‐drying protocol was coupled with QHRVD (PQHRVD) to further escalate the dehydration of HMGT through cyclic injection of CO2 to stimulate forced diffusion, thus facilitating drying time reduction by approximately 50% compared to QHRVD. Faster drying rate and superior energy‐efficiency could be achieved in PQHRVD owing to higher effective moisture diffusivity (4.71 × 10−10 m2 s−1) and lower activation energy (2.13 kJ mol−1) in comparison with QHRVD (2.03 × 10−10 m2 s−1; 3.65 kJ mol−1). Thus, the periodic CO2 convection exhibited a remarkable reduction in processing time and energy consumption and produced a functionally enriched HM with significantly higher antioxidant properties, phenolics, ascorbic acid, and flavonoid contents.Practical applicationsHimsagar mango (HM), a tropical high‐demand fruit has been fortified with green tea (GT) (Camellia sinensis) extract for quality improvement. The amalgamated Himsagar mango was subsequently vacuum‐dried to improve the product shelf‐life and to augment the nutritional attributes. The energy‐efficient quartz halogen radiated vacuum‐drying (VD) technique was optimized to render a superior quality foodstuff. The VD operation was further intensified through periodic convection by CO2 to make the operation more beneficial. The cyclic CO2 convection resulted in significant augmentation in drying rate and rendered a lucrative foodstuff. The nutritionally enriched “HM fortified by GT” (HMGT) product displayed improved nutritional attributes (higher antioxidant properties, phenolics, ascorbic acid, and flavonoid contents) and enhanced shelf life for potential application in preparation of candy, cake, sweet, or dietary supplements. Therefore, the energy‐proficient and rapid drying method providing the functionally improved HMGT product can lay a prospective foundation for bakery and related food industries.

Color evaluation of white spot lesions treated with resin infiltration after water or grape juice storage

Aim: To evaluate the color stability of bovine enamel with artificial white spot lesions treated with resin infiltration (ICON) or remineralization with fluoride using two storage methods. Methods: Sixty incisors were submitted to artificial white spot lesion induced by demineralization-remineralization (DE-RE) cycling. Initial color was evaluated with CIE-Lab to measure ΔEab. Demineralized teeth were divided according to the treatment of the white spot lesion (n = 20): 1) Remineralization with 2% neutral fluoride gel for 4min (control); 2) ICON application following manufacturer’s recommendations; and 3) ICON with decreased drying time after the application of ethanol. After 24h, color was evaluated and samples were subdivided (n = 10) according to storage: 1) distilled water for 1 month; 2) grape juice for 10min daily. After storage, color was evaluated. L*, a* and b* data were analyzed by one-way ANOVA and ∆Eab data by two-way ANOVA followed by Tukey’s HSD (α = 0.05). Results: L* was affected by juice storage, and decreased when ICON was applied with decreased drying time after the ethanol application. The same behavior occurred with a* (increase with reduced drying time), while b* was not affected. For ∆Eab significant differences were observed between groups (p = 0.0219) and storage methods (p = 0.0007). There was no interaction effect (p = 0.1118). Remineralization with fluoride presented the lowest color changes after storage in water. Conclusion: Treatment of artificial carious lesions with resin infiltration presented greater color changes than fluoride remineralization after storage in both solutions in vitro.

Ethanol pre-treatment to ultrasound-assisted convective drying of apple

For the first time, the ethanol as pre-treatment to the ultrasound-assisted convective drying of food was evaluated. Pre-treatments were performed by immersion of apple slices in ethanol (0–30 min). Pre-treated samples were convectively dried (50 °C, 1 m s−1), without/with ultrasound (21.77 kHz, 20.5 kW/m3). As results, if both technologies were considered, conventional drying time reduction reached 70 ± 2%. From drying kinetics modelling, it was identified that ethanol pre-treatments mainly reduced the external resistance to mass transfer, while ultrasound had a greater influence on the internal one. In dried samples, as the ethanol pre-treatment time increased, the shrinkage decreased, and their rehydration capacity was greater. After rehydration, samples showed a decrease of >85% in viscoelastic characteristics. The antioxidant capacity and total phenolic content were better retained with ultrasound application. The obtained results corroborate that the proposed technologies are complementary significantly accelerating the drying without negative effects on physical properties.

Effects of drying temperature of red popcorn grains on the morphology, technological, and digestibility properties of starch

Popcorn consumption is becoming increasingly attractive, and the effects of post-harvest operations are essential for the improvement of industrial processes. In this context, the objective of this study was to evaluate the effects of drying temperature on the morphology, technological, and digestibility properties from the isolated starch from red popcorn grains. Red popcorn grains were dried in oven-drying (30 °C) and fixed-bed dryer at 40, 70, and 100 °C. The morphology properties of grains and starch, the technological and digestibility properties of the starch, were evaluated. The drying temperature of 100 °C reduces the extraction yield by 13.14%, changes the internal structure of the popcorn grains and the native morphology of the starch granules, increases the pasting temperature and the gelatinization enthalpy, and reduces digestibility compared to drying temperature of 40 °C, which is more indicated due to reduced drying time.

Simulation of Microwave Assisted Finish Drying and Inactivation of Escherichia Coli, Bacillus Cereus and Staphylococcus Aureus of Onion Powder

The onion powder with initial moisture content of 10% (db) in form of thin infinite layer of thickness 5mm was dried using a laboratory microwave oven at 50W/g microwave power density to final moisture content of 2% (db).The temperature, moisture change pattern of the experimental process was monitored. Computer simulation of the predicted temperature pattern with drying time was obtained by using the mathematical model governed by energy balance equation and using data of the thermo-physical properties of the onion powder obtained either experimentally or through literature review. The usage of high microwave power to mass ratio results in raised temperature with reduced drying time. The decrease in drying rate is due to initial moisture content. Effective moisture diffusivity increases with decreasing moisture content due to the application of high power density. Lethality values for Escherichia coli, Bacillus cereus spores and Staphylococcus aureus, which are more prominent in onion powder, were then evaluated from the simulated temperature curve. DT and z values were used to estimate the sterilization values for each individual microorganism. It was observed that significant sterilization of E.coli and S.aureus start after 35 s at a minimum temperature of 89oC with 0.047 and 0.044 log reduction while, for B.cereus it starts after 75 s at 117oC with 0.431 log reduction. The maximum temperature of 117ᵒC was limited due to unacceptable sensory quality of end product.

Optimization of Solar Dryer using Taguchi Method

Solar dryers are one of most efficient devices for drying food and other products. These devices are eco-friendly and need solar energy for their operation and which is available freely and abundantly at almost every place. These are simple devices which do not need use any fossil fuel for their operation, which are expensive, exhaustive in nature and are not eco friendly. Several fresh food products and other items are dried for preservation and utilization for longer period of time. In recent times, with the development in this technology, the solar dryers are becoming well equipped with efficiency increasing add-ons which give higher performance. Several experimental studies have been carried out to mitigate the time consumption and increasing the productivity of solar dryers. Optimization of solar dryers is done to increase their workability increasing accuracy and reducing drying time and cost investment. In order to optimize the complete setup several techniques could be used to get the optimum results. In the present work Taguchi optimization technique was applied for experimental analysis of solar dryer which was used to dry potatoes. The results thus obtained showed the best configuration comprising air velocity, air flow and humidity required to dry potatoes.

Effect of contact ultrasonic power on moisture migration during far‐infrared radiation drying of kiwifruit

AbstractIn order to explore the effect of direct contact ultrasound (DCU) on moisture migration during far‐infrared radiation (FIR) drying of kiwifruit, the drying characteristics, microstructure, water status, and moisture change of kiwifruit slices during drying were investigated based on scanning electron microscopy (SEM), low‐field nuclear magnetic resonance (LF‐NMR), and magnetic resonance imaging (MRI). The results showed that the drying time at ultrasonic power of 54 W was 40% shorter than that without DCU application. SEM observation verified that higher ultrasonic power could produce more and larger microcapillaries in the microstructure of kiwifruit slices, which was beneficial for moisture migration. LF‐NMR detection indicated that water mobility and distribution in kiwifruits changed during drying. The relative proportion of free water in kiwifruits decreased from 70 to 0% with the prolongation drying time at different ultrasonic powers, the removal time for free water at ultrasonic power of 54 W was 57% shorter than that without DCU application during FIR drying, and higher ultrasonic power could accelerate the shifting of water in different states. MRI results illustrated that the DCU application at higher ultrasonic power could promote moisture migration and water removal during FIR drying, and then increase drying rate.Practical applicationsDrying is a commonly utilized process method in food and agricultural industry. Far‐infrared radiation (FIR) drying, as one of the modern drying methods, has desirable advantages including high‐energy efficiency and uniform heating. However, it has unsatisfied promoting effect on internal mass transfer. Ultrasound has significant positive effect on moisture diffusion and water removal. In this study, direct contact ultrasound (DCU) was applied in FIR drying of kiwifruit slices. The study verified that the DCU application in FIR drying could promote drying rate, produce loose microstructure, and shorten drying time significantly. The increase in ultrasonic power could accelerate moisture migration and mass transfer to facilitate water removal. Therefore, DCU application with higher ultrasonic power is beneficial to increasing drying rate and reducing drying time of FIR drying, and FIR drying technology combined with DCU is promising in food industry and can be developed for other food products.

Investigation of drying kinetics and physicochemical properties of mulberry leather (pestil) dried with different methods

The effects of microwave (90 and 180 W), hot air (60 and 70°C) and vacuum (60 and 70°C with 250 mbar) drying techniques on drying characteristics, total phenolic content (TPC), antioxidant capacity (AC), color and texture of mulberry pestils were investigated. L*, b*, C*, and h° decreased while a* value was generally increased in pestils. Hardness, springiness, chewiness, and gumminess of the samples were found to be significantly different (p < 0.05). Between the applied seven thin layer drying models, Page and Modified Page were best fitted with the highest R2 (0.9997) and the lowest values of RMSE (0.000927) and χ2 (0.000011). Effective moisture diffusivity (Deff) of pestils ranged from 4.42 × 10−8 to 8.47 × 10−7 m2/s. Drying treatments caused an increment in TPC (1.41%–57.13%) and AC (0.37%–72.79%). These highest results were both obtained from microwave 180 W (TPC: 209.14 mg GAE/100 g dw and AC: 181.37 μmol TE/100 g dw). Practical applications Mulberry is a perishable fruit with several health benefits. It can be consumed raw or in the form of fruit leather (pestil), jam, jelly, marmalade. By pestil production, in which fruit pulp is dehydrated into leathery sheets, mulberry can be processed into a nonseasonal shelf life prolonged product for markets without the additon of preservatives. In this study, drying kinetics of mulberry pestil by microwave, hot air, and vacuum methods were evaluated via mathematical modeling. Page and Modified Page were the best thin layer drying models for pestil production. Drying characteristics were assessed for the development of food specific system in industrial usage and the simulation and optimization of the drying process. Drying treatmens affected texture and color of pestils significantly while exerting higher total phenolic content and antioxidant capacity when compared to non-dried formulation. Microwave was determined as a promising method to yield high-quality pestils with greater nutritional properties and the advantage of reduced drying times.

Intensification of the convective drying process of Arthrospira (Spirulina) platensis by capillary draining: effect of the draining support

The aim of this work is to study the intensification of the convective drying process of Arthrospira (Spirulina) by capillary drainage. Capillary drainage effect was performed by the integration of different draining supports, namely cotton, microfibers, 30 μm fabric, and 80 μm fabric, between the biomass and the drying tray. Drying tests were carried out in a pilot-scale drying tunnel at a free-stream temperature between 39 and 42 °C, at air velocity of 0.5 m s−1 and at air relative humidity around 20%. The Arthrospira biomass was set at two different initial moisture contents on dry basis: 7 kg kg−1 and 5 kg kg−1. For each of these initial moisture contents, the fresh paste was spread over the different draining support, either in a thin layer (dimensions 80 × 80 × 3 mm3) or in cylindrical extrudates (3-mm diameter and 120-mm length) with two different spacings (10 and 20 mm). Drying experiments of biomass in thin layer after the integration of the different draining supports, particularly the 80-μm mesh fabric, lead to a significant drying time reduction compared to that performed without any draining support. For this most effective support, the percentage time reduction was around 30%. However, the percentage time reduction was around 35% for drying tests of biomass in cylindrical extrusions using 80 μm mesh fabric as a draining medium. For all drying tests, the drying kinetic coefficient kx was identified by fitting the falling rate drying period data by a first-order kinetic equation (Lewis model). This coefficient was significantly increased by the presence of the porous material. Its value was equal to 5.604 × 10−4 g dry matter s−1 for drying test without any support and reached a value of 11.556 × 10−4 g dry matter s−1 for drying test with 80-μm mesh fabric.

Microwave drying of wet clay with intermittent heating

In this article, we study an alternative process for the conservative drying of wet clay based on combined microwave heating and convective drying. The study is founded on an experimental analysis of the processes that control drying of porous matter in presence of temperature gradients. We show and discuss that process control cannot be reliably based on temperature alone because mass transfer phenomena associated with microwave heating dictate drying kinetics. Essentially, the occurrence of pressure build up and liquid expulsion, dominating mass transfer if microwave heating is uncontrolled or with high power, vitally depend on the moisture content of the clay. As a consequence, the deterioration of clay is problematic if internal evaporation rates are high, thus at the start of drying when also the moisture content is high. Following this, we propose intermittent microwave heating in combination with convective drying periods with the aim to reduce overall drying time (compared to conventional convective drying) and to improve product quality by reduced material shrinkage. Drying experiments indicate that the overall drying time reduction and product quality depend on the frequency and duration of the two subsequent periods of intermittent heating and convective drying. The observations can be explained with the evolution of the pore-scale distribution of liquid and vapor during drying.

Upotreba modernih predtretmana prilikom sušenja šljive

The purpose of this study is to determine the influence of abrasive pre-treatments on plum drying time compared to the conventional ones. The mechanical pre-treatment examined included the abrasion at three different cylinder rotation times: 5, 10 and 15 min. The chemical pre-treatment under consideration included the immersion of plums in alkaline solution (KOH) of different concentrations (0.5, 1 and 1.5% (v / v)) at 60 °C for one minute. The thermal treatment performed included the immersion of fruits in distilled water at 60 °C for one minute. As a control (untreated) sample, fruits with waxy lawyers were used. According to the results obtained, the drying time of plums was significantly reduced compared to the control sample (97 h): in the samples treated with distilled water (thermal treatment) drying time was reduced by 21 h, whereas the drying time reduction of up to 68 h was reorded using a KOH solution of 1.5%. In the samples pre-treated by abrasion for 15 min, the drying time recorded was also reduced by 68 h compared to the control samples, thus proving the exceptional efficiency of abrasive pre-treatment in plum drying.

Deep bed rough rice air-drying assisted with airborne ultrasound set at 21 kHz frequency: A physicochemical investigation and optimization

This study investigates the feasibility of achieving a modified rough rice air-drying with ultrasound intervention (US) set at 21 kHz frequency. The process was carried out using an ultrasound assisted convective dryer and analyzed in terms of drying time, energy consumption (EC), evaporation rate (ER), broken kernel (BK), water activity (aw), vitamin B3 (niacin), and total phenolic content (TPC). The lab-scale dryer was set to transmit energy at power levels of 30, 60, 90, 120, and 150 W in the air temperatures of 35, 40, 45, 50, and 55 °C and velocities of 0.2, 0.5, 0.8, 1.1, and 1.4 m/s for a thickness of 20 cm. The results showed that in-range drying parameters played a major role in response analysis, suggesting that ultrasound efficiency during convective drying heavily depends on the power, air temperature, and velocity levels. Ultrasound assistance significantly increased bed evaporation rate up to 38.93%. Subsequently, it caused a reasonable reduction in total drying time, EC, and BK percentage up to 27.92%, 25.98%, and 34.22%, respectively in the case of 35 °C, v = 0.8 m/s, and P = 90 W. Remarkable advances were attained in the ultrasound assisted drying domain in grain quality via acceptable decrease in niacin and TPC losses within shorter drying time. There was a significant linear trend in BK, niacin, and TPC values across the bed thickness, which all was increased by the ultrasound application. The highest nutrient retention and physical quality preservation belonged to the top layer, which was influenced by the ultrasound radiation by the highest degree.

Radio Frequency-Vacuum Drying of Kiwifruits: Kinetics, Uniformity, and Product Quality

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.