Ultrasonic Intensification of Grape Stalk Convective Drying: Kinetic and Energy Efficiency

Grape stalk is a by-product of the winemaking process with a high antioxidant content. Drying is a necessary stage before antioxidant extraction, which may affect not only kinetic and energy efficiency but also product quality. Coupling non-thermal technologies, such as power ultrasound, to convective drying is considered a strategy for process intensification in order to prevent certain drawbacks of conventional technologies. In this work, the use of power ultrasound in the convective drying of grape stalk was tested in order to estimate its influence on kinetic and energy efficiency. For this purpose, convective drying kinetics of grape stalk were carried out with and without power ultrasound application (21.8 kHz, at two ultrasonic power levels: 45 and 90 W). In addition, the inlet and outlet air temperatures of the drying chamber were monitored. The drying process was modeled considering heat and mass transfer phenomena jointly with the total energy consumption and the energy efficiency of the system. Power ultrasound application involved a shortening of grape stalk drying time, which was dependent on the drying air temperature (40 and 60°C) and the ultrasonic power applied (45 and 90 W). The modeling showed the increase in diffusion and convective heat transport phenomena produced by ultrasound application, despite grape stalk being a low-porosity product and, therefore, only slightly sensitive to ultrasonic effects. In addition, it was also highlighted that ultrasound application increased the energy efficiency during the drying of grape stalk.     

Ultrasonic-Assisted Convective Drying of Apple Slices

A promising approach for the application of ultrasound to assist in convective food drying was developed and tested in this study. The application of ultrasound is based on the transmission of ultrasonic energy as a combination of airborne contacts and through a series of solid contacts between the ultrasound element and the product tray as the ultrasonic vibration transmitting surface. A computer-based ultrasonic drying setup was built to allow continuous recording of the process variables in real time and enabled simulation of dehydration to be accomplished under controlled conditions over a range of drying parameters. Apple slices were dried using the drying setup to study the influence of ultrasound in combination with conventional hot air drying on drying kinetics and product quality.

The results from this work indicate that ultrasound can simultaneously be applied to accelerate the processing time (i.e., reduce energy consumption and increase production throughput) in conventional hot air drying without compromising product quality. It appears that the magnitude of influence of ultrasound to enhance the air-drying process depends on the process variables employed. In particular, the ability of ultrasound to improve the efficiency of the convective drying process appears to be maximized when using low temperature and high ultrasonic power level. This finding maybe very useful when there is a need to effectively dehydrate heat-sensitive products or when shorter drying times are required in order to achieve better retention of the functional and nutritional properties of the product.     

Parametric Study of High-Intensity Ultrasound in the Atmospheric Freeze Drying of Peas

Atmospheric freeze drying (AFD) is a dehydration process that can be used to produce high-end products for the food, pharmaceutical, and biological industries. Evaporation or sublimation at the drying temperatures used for these processes is generally low. Airborne ultrasound can be used to increase drying rates. This parametric study investigates the influence of the drying temperature, drying time, and ultrasonic power for atmospheric freeze drying in the presence of an airborne ultrasonic field. Accelerated effective diffusion of up to 14.8% was obtained for atmospheric freeze drying with a fluid bed. The faster drying in ultrasonic-assisted atmospheric freeze drying is assumed to be due to a higher mass transfer rate at the solid-gas interface, caused by a reduced boundary layer due to a higher turbulent interface. Thus high intensity, airborne ultrasound used with modern drying systems has great potential to accelerate drying, reduce investment and production costs, and improve product quality.     

Numerical Study on Moisture Transfer in Ultrasound-Assisted Convective Drying Process of Sludge

A coupled heat and moisture transfer model for ultrasound-assisted convective drying process of sludge was established. In this model, the permeable flow caused by acoustic pressure gradient in sludge was considered. The pore structure variety in sludge with ultrasonic irradiation was microscopically studied, and the pore size distribution of sludge was described by fractal geometry. Based on the fractal characterization, the physical properties of sludge including permeability, porosity, and tortuosity factor were determined, and the effective moisture diffusion coefficient of sludge under ultrasonic irradiation was also derived considering the effects of ultrasonic excitation energy and thermal effect on migration rate of water molecule. The effects of ultrasonic energy density and convective air temperature on convective drying process of sludge were numerically analyzed. The results showed that the ultrasonic irradiation changes the pore size distribution in sludge, the sludge flocs are dispersed, and the connectivity of pore structure is improved. Ultrasonic treatment is favorable to accelerating the moisture transport in the convective drying process of sludge, and the ultrasonic influence on moisture transport in sludge intensifies gradually with the increase of acoustic energy density from 0.2 to 0.6 W/ml. Furthermore, it can be also found that the enhancement effect of ultrasound on the average drying rate of sludge is more obvious at the connective air temperature of 65°C than that at 40°C under the uniform acoustic energy density and air velocity of 1.5 m/s.     

Microwave-Assisted Atmospheric Freeze Drying of Green Peas: A Case Study

Atmospheric freeze drying (AFD) is based on the sublimation of ice due to a pressure gradient (convective drying), and is a dehydration process for temperature-sensitive products. Since the process is slow in general, microwave radiation (MW) was applied in order to increase the sublimation in fluid and fixed bed conditions at drying temperatures of ?6°C, ?3°C, and 0°C. The modified Weibull model was used to describe the drying behavior for all investigations. With 280 Watt power supplied to the magnetrons, it was possible to reduce drying time by approximately 50%. The drying efficiency was approximately 30%, while the SMER was increased by 0.1 to 0.3 kg_waterkWh^?1, which gives better energy efficiency for the microwave drying system used in this investigation. The product quality (color reduction and particle size/porosity) was well preserved in fixed bed drying at ?6°C and ?3°C, while the product quality was reduced significantly in microwave AFD experiments at 0°C and in a fluid bed. The drying rates of AFD in a fluid bed condition were not as high as those in a fixed bed. MW-AFD in a fixed bed condition at temperatures of ?6°C and ?3°C performed best regarding product quality, drying time, and process control.     

Microwave Drying of Porous Materials

Abstract

Experimental results on microwave drying of the porous particles exposed to air stream at 40°C are presented. The temperature and moisture distribution inside a particle were measured for gypsum spheres of 9, 18, 28, and 38 mm. The mass reduction was monitored during the drying process. The rate of drying and changes in temperature and moisture profiles for different drying conditions were analyzed and compared with the ones for convective drying.     

Heat Pump Drying of Green Sweet Pepper

Thin-layer drying experiments under controlled conditions were conducted for green sweet pepper in heat pump dryer at 30, 35, and 40°C and hot air dryer at 45°C with relative humidities ranging from 19 to 55%. The moisture content of sweet pepper slices reduced exponentially with drying time. As the temperature increased, the drying curve exhibited a steeper slope, thus exhibiting an increase in drying rate. Drying of green sweet pepper took place mainly under the falling-rate period. The Page equation was found to be better than the Lewis equation to describe the thin-layer drying of green sweet pepper with higher coefficient of determination and lower root mean square error. Drying in heat pump dryer at 40°C took less time with higher drying rate and specific moisture extraction rate as compared to hot air drying at 45°C due to lower relative humidity of the drying air in a heat pump dryer though the drying air temperature was less. The retention of total chlorophyll content and ascorbic acid content was observed to be more in heat pump–dried samples with higher rehydration ratios and sensory scores. The quality parameters showed a declining trend with increase in drying air temperature from 30 to 45°C. Keeping in view the energy consumption and quality attributes of dehydrated products, it is proposed to dry green sweet pepper at 35°C in heat pump dryer.     

Management of Surface Drying Temperature to Increase Antioxidant Capacity of Thyme Leaf Extracts (Thymus vulgaris L.)

Thyme leaves are an important source of essential oils with antioxidant activity; these compounds are located in trichomes on the leaf surface. The drying conditions affect not only the drying time but also the antioxidant activity. In the literature, a drying temperature of 70°C appears to be the best for drying thyme leaves according to their antioxidant capacity. Considering drying periods at different temperatures also could be beneficial. With these considerations, the goal of this work was to establish a drying strategy with which to manage a drying temperature on the leaf surface that will enable the drying time to be shortened and improve the antioxidant capacity (AC) of the extract of dried thyme leaves. The drying strategy consisted of two consecutive drying periods in order to manage the drying temperature on the leaf surface. The first drying period was carried out at 80°C (T _a1) until the sample surface reached a temperature of 70°C, and the temperature was then immediately set to 70, 60, 50, and 40°C (second drying period, T _a2) at different air velocities (v; 1 and 2 m s^?1). Compared to constant drying conditions, two consecutive drying periods were found to improve the drying kinetics: the AC increased from 10.5 to 27.4% while reducing the drying time by 14.5 to 39.2%. The use of this drying strategy was found to be an interesting means of intensifying the convective drying of thyme leaves and its application should be considered when drying similar materials with bioactive compounds on the surface.     

Experimental and Numerical Investigation of the Heat and Mass Transfer for Cut Tobacco During Two-Stage Convective Drying

In this article, a two-stage convective drying strategy was presented for dehydration of flue-cured tobacco. In order to develop the multistage drying method of tobacco, two-stage drying as well as traditional single-stage drying of cut tobacco was experimentally evaluated and accurately simulated by proposed heat and mass transfer models. The experiments were performed in a dual fixed bed dryer. Different air temperature combinations of 120°C/90°C, 110°C/80°C, and 100°C/70°C were employed during two-stage drying. The drying rate and temperature variations of cut tobacco were investigated. The results showed that the average drying rates during two-stage drying were nearly 50% higher than those obtained from lower-temperature single-stage drying. On the other hand, the two-stage drying method, which used high air temperature for the early period and low temperature for the late period, could reduce the exposure of tobacco to high temperature due to the low final temperature of the dried sample. The temperature and moisture evolution of cut tobacco at different air temperature combinations were consistent with simulation results by developed heat and mass transfer models. This indicated that the models had a good prediction precision for two-stage drying of cut tobacco. The model predictions can be useful for the design of a feasible two-stage drying process for flue-cured tobacco.     

Infrared,Convective, and Sequential Infrared and Convective Drying of Wine Grape Pomace

In this paper, effects of infrared drying and/or convective drying on drying kinetics of wine grape pomace were examined, and drying characteristics, sterilizing efficacy, and effects on pomace's polyphenols and pro-anthocyanidins content were determined. Infrared drying had the highest drying rate, which reduced the drying time by more than 47.3% compared with other methods. Sequential infrared and convective drying had a faster drying rate than convective drying. Five empirical models were chosen to fit the drying curves and the Midilli et al. model had the highest R ² and lowest RMSE and χ ² . For sterilizing efficacy, infrared drying and convective drying (90°C) performed the best with minimum survival yeasts, molds and bacteria, while higher drying temperatures resulted in better pasteurization efficacy. Sequential infrared and convective drying did not yield a satisfactory sterilizing effect, with efficacy not being enhanced by prolonging the infrared drying. The wine grape pomace dried by infrared drying had the highest content of polyphenols and pro-anthocyanidins, showing that decreasing the drying temperature led to less damage to these two substances.     

Influence of the Applied Acoustic Energy on the Drying of Carrots and Lemon Peel

The application of power ultrasound could constitute a way of improving traditional convective drying systems. The different effects produced by the application of power ultrasound may influence the drying rate without provoking any significant increase in product temperature. Due to the fact that the effect of power ultrasound is product dependent, the aim of this work was to address the influence of the applied acoustic energy on the convective drying of carrot and lemon peel.

Convective drying kinetics of carrot cubes (side 8.5 mm) and lemon peel slabs (thickness 7 mm) were carried out at 40°C and 1 m/s by applying different levels of acoustic power density: 0, 4, 8, 12, 16, 21, 25, 29, 33, and 37 (kW/m³). The application of power ultrasound during drying was carried out using an airborne ultrasonic transducer (21.7 kHz). Drying kinetics were described considering a diffusion model.

In both products, the application of power ultrasound improved the effective moisture diffusivity (D_e ). The improvement was linearly proportional to the applied acoustic power density. In the case of lemon peel, the effects of power ultrasound were found over all the range tested (0–37 kW/m³), whereas in the case of carrot, it was necessary to apply an acoustic power density of over 8–12 kW/m³ to be able to observe the influence. The more intense effect of acoustic energy in lemon peel drying may be explained by the fact that lemon peel is a more porous product than carrot.     

Study on Convective Drying Characteristics of Dredged Sludge from Dian Lake

In this work, the thin layer drying behavior of dredged sludge from Dian Lake by convective drying methods was investigated. The results showed that the Modified Page-I model was more suitable for thin-layer drying of dredged sludge. The values of the diffusion coefficients at each temperature were obtained using Fick’s second law of diffusion, and it was varied from 6.472×10^?9 to 1.143×10^?8 m²/s when the temperature was changed from 100 to 160°C for the dredged sludge of 10 mm. When the thickness was changed from 5 to 20 mm, the diffusion coefficients were varied from 4.036×10^?9 to 2.648×10^?8 m²/s at 140°C. The activation energy of moisture diffusion was 13.1 kJ/mol.     

A Mortality Model for Penicillium bilaiae Subjected to Convective Air Drying

Drying of microbial cells for storage purposes is a widely practised technology. A new statistical model is proposed for survivability of microbial cells during convective air drying, where mortality is predicted by hazard functions and acceleration factors applied to temperature, moisture content and drying rate variables. The model is based on experimental survivability data generated from drying Penicillium bilaiae conidia at temperatures from 20 to 60°C and air relative humidities from 3 to 75%. Experimental data using “one at a time” variable manipulation was used to obtain five model parameters. Despite several simplifications to reduce the number of variable cross‐dependencies, the model was successfully validated using combined stress trials with maximum deviations of ±15%.     

Effect of Ultrasound Pretreatment on Vacuum Drying of Chinese Catalpa Wood

Ultrasound pretreatment of wood prior to drying was examined as a method to increase the effective water diffusivity, reduce drying time, and improve product quality of Chinese Catalpa wood. Pretreatment tests were carried out at three pretreatment durations, three absolute pressure levels, and three ultrasonic intensities. All specimens were then dried at 60°C and the absolute pressure level of 0.02 MPa to determine the effects of pretreatment parameters on vacuum drying characteristics. A microscopic analysis was carried out to visualize the formation of microchannels and view any other changes to wood tissue structure that occurred. Results showed that ultrasound pretreatment prior to vacuum drying enhances the effective water diffusivity; the higher the ultrasound power level, the longer the pretreatment time, and the lower the absolute pressure, the shorter is the drying time. Ultrasound creates micro channels within the tissue of wood during pretreatment. However, the pretreatment time should not be too long when the ultrasound is high.     

Drying Kinetics and Antioxidant Phytochemicals Retention of Salak Fruit under Different Drying and Pretreatment Conditions

Kinetics of hot air drying and heat pump drying were studied by performing various drying trials on salak slices. Isothermal drying trials were conducted in hot air drying and heat pump drying at a temperature range of 40–90°C and 26–37°C, respectively. Intermittent drying trials were carried out in heat pump drying with two different modes: periodic heat air flow supply and step-up air temperature. It was observed that the effects of relative humidity and air velocity on drying rate were significant when moisture content in salak slices was high, whereas the effects of temperature prevailed when the moisture content was low. As such, it was proposed that drying conditions should be manipulated according to the moisture transport mechanisms at different stages of drying in order to optimize the intermittent drying and improve the product quality. Generally, loss of ascorbic acid during drying was attributed to thermal degradation and enzymatic oxidation, whereas the loss of phenolic compounds was mainly due to thermal degradation. Experimental results showed that heat pump drying with low-temperature dehumidified air not only enhanced the drying kinetics but produced a stable final product. Heat pump–dried samples retained a high concentration of ascorbic acid and total phenolic compounds when an appropriate drying mode was selected.     

Drying powdered coal with the aid of ultrasound

The results of introducing ultrasound into three different drying processes has been discussed. The processes included: (1) static drying of a layer of powdered coal with airborne ultrasound, (2) rotary kiln drying with airborne ultrasound, and (3) drying of powdered coal on a sloping, ultrasonically vibrated metal trough. In all three processes, the introduction of ultrasound increased the drying rate.

The frequency of ultrasound used was 20 kHz with sound intensities up to 165 dB. The pulverized coal used had a size range from 400 μm to less than 2 smm, with 65% being less than 150 μm in size. Air temperatures used varied from 65 to 300 °C.

The most energy-efficient drying process with ultrasound was found to be the sloping, ultrasonically vibrated trough. However, this process was limited to the use of an initial coal powder feed containing less than 10% moisture. For moisture contents above 10%, a dual drying system could be used. For example, the rotary kiln drier could be used ahead of the trough drier.

Both the static airborne ultrasound drying process and the ultrasonically vibrated trough drying process work best with a relatively thin layer of powdered coal. However, the residence time for the powdered coal being dried is measured in seconds for the sloping trough process in contrast to minutes for the static drying process. The static drying of a layer of powdered coal is also a batch-drying process while the rotary kiln and sloping trough are continuous processes. The advantages of the static airborne ultrasonic drying process would be: (1) closer control of the drying process, and (2) smaller quantities of powder can be used.     

Drying and denaturation characteristics of three forms of bovine lactoferrin

The drying and denaturation characteristics of native, iron-depleted (apo-), and iron-saturated (holo-)lactoferrin (LF) were studied in convective air-drying (CD) process using single droplet drying. The extent of loss of LF due to denaturation and changes in the secondary structures during CD and isothermal water heating (IHT) were measured. The holo-LF was found to be most stable among these three forms both at 70 and 95°C. The apo-LF was the least stable compared to other two. The secondary structural features of LF were altered to significantly higher extent when subjected to IHT than to CD both at 70 and 95°C. Holo-LF should be preferred when producing LF powder through a convective drying process.     

Drying of Seabuckthorn (Hippophae rhamnoides L.) Berry: Impact of Dehydration Methods on Kinetics and Quality

Convective hot air drying and freeze drying were investigated as potential dehydration processes to obtain powders of seabuckthorn fruit pulp. Halved seabuckthorn fruits were placed in a hot air dryer and dried at 1 m/s and at 50 or 60°C or freeze dried at less than 30 mTorr and at 20 or 50°C shelf plate temperature. An initial characterization of the seabuckthorn pulp (moisture, pH, soluble solid content, vitamins C and E, total phenolics, and carotenoids) was performed. Water loss, total phenolic compounds, total carotenoids, and vitamin C were determined at different processing times. Vitamin E was determined before and at the end of drying.

Freeze-drying kinetics were faster than air drying, probably due to lower moisture diffusion in the compact, sugary, and oily structure of the air-dried tissue. The temperature had an important impact on hot air–drying and freeze-drying kinetics. Drying method and processing times affected the remaining phenolic, carotenoid, and vitamin contents of seabuckthorn berries. Freeze drying was revealed as a superior method to obtain seabuckthorn powders because of the lower residual moisture content, the ease of grinding, as well as the better nutritional retention.     

Kinetics of ultrasound pretreated apple cubes dried in fluidized bed dryer

Abstract

This work evaluated the effect of ultrasonic pretreatment on the production of dehydrated apples (Malus domestica L. var Granny Smith) in a fluidized bed dryer. Cube-shaped apple samples were subjected to ultrasound in an ultrasonic bath and dried in a fluidized bed drier. The experimental design evaluated the effect of ultrasound pretreatment time (0 to 30?min) on the soluble solids loss during pretreatment and on the drying time. The ultrasonic pretreatment was carried out in a bath ultrasound operating at 25?kHz and outputting 55?W/m³ of power density. Distilled water was applied in the pretreatment to produce low-calorie apple cubes. Fluidized bed drying was carried out at 30, 40, and 50?°C. Fick’s law was used to model the drying process and to determine the apparent water diffusivity. The soluble solid loss ranged between 8.7 and 21.2% during the pretreatment, and the apparent water diffusivity during air drying ranged from 1.09?×?10^?6 to 2.81?×?10^?6 m²/min. Ultrasound pretreatment increased the apparent water diffusivity up to 58%. Apple cubes subjected to 20?min of ultrasound pretreatment and dried at 50?°C presented the highest apparent water diffusivity and dried to achieve a water activity of 0.4 in 100?min.     

EVALUATION AND MODELING OF TWO-STAGE OSMO-CONVECTIVE DRYING OF APPLE SLICES

ABSTRACT

Two-stage drying kinetics of cylindrical pieces of apples were evaluated by subjecting test samples first to various osmotic treatments and then to convective air drying to complete the drying process. Osmotic drying was carried out with cut apple cylinders of three different sizes (12, 17 and 20 mm diameter), all with a length to diameter ratio of 1 : 1, in a well agitated large tank containing the osmotic solution at the desired temperature. Solution to fruit volume ratio was kept greater than 60. After the osmotic treatment, apple slices were further dried in a cabinet drier at an average temperature 58°C. A central composite rotatable design (CCRD) with five levels of sucrose concentrations (34–63°Brix) and five temperatures (34–66°C) was used for osmotic treatment. Half-drying time and solids gain time were used as measures of rate of drying and associated diffusion coefficients for moisture loss and solids gain were evaluated. Half-drying time decreased with an increase in temperature or concentration, or a decrease in sample size. Diffusion coefficients were lower for smaller samples, and were higher for migration of moisture as compared to solids. For a given level of moisture removal, air drying times were shorter than osmotic drying times. Composite models were developed to describe the effect of process variables and particle size on the drying behavior of apple slices.