Application of High-Power Ultrasound for Dehydration of Vegetables: Processes and Devices

Transcription

1 Drying Technology, 25: , 2007 Copyright # 2007 Taylor & Francis Group, LLC ISSN: print/ online DOI: / Application of High-Power Ultrasound for Dehydration of Vegetables: Processes and Devices J. A. Gallego-Juarez, 1 E. Riera, 1 S. de la Fuente Blanco, 1 G. Rodríguez-Corral, 1 V. M. Acosta-Aparicio, 1 and A. Blanco 2 1 Power Ultrasonics Group, Instituto de Acústica, CSIC, Serrano, Madrid, Spain 2 IFA, CSIC, Serrano, Madrid, Spain High-intensity ultrasound is a tool with a great potential for vegetable dehydration. Airborne ultrasonic waves have been used for drying materials in combination with hot air systems to obtain adequate drying rates at lower temperatures. Nevertheless, the extension of this technique has been limited because of practical difficulties in the efficient generation of high-intensity ultrasound in air. The implementation of a new technology of plate-transducer power ultrasonic generators has opened up new possibilities in this area. This article reviews the development and testing of an ultrasonic technology for vegetable dehydration based on the application of the new power ultrasound generators. Two experimental procedures have been carried out by airborne ultrasound and ultrasonic vibration in direct contact with the vegetable. Keywords & INTRODUCTION The applications of ultrasonic waves are generally divided into two groups: low and high intensity. Low-intensity applications are those wherein the objective is to obtain information about the propagation medium without producing any modification in its state. On the contrary, high-intensity applications are those wherein the ultrasonic energy is used to produce permanent changes in the treated medium. High-power ultrasound is the part of ultrasound devoted to high-intensity applications. The limit between low and high intensity is very difficult to fix, but it can be approximately established for intensity values that, depending on the medium, vary between 0.1 W=cm 2 and 1 W=cm 2. The use of high-intensity ultrasonic waves in industrial processing is generally based on the adequate exploitation of a series of mechanisms activated by the ultrasonic energy Correspondence: S. de la Fuente Blanco, Power Ultrasonics Group, Instituto de Acústica, CSIC, Serrano 144, Madrid, Spain; & such as heat, agitation, diffusion, interface instabilities, friction, mechanical rupture, chemical effects, etc. These mechanisms can be employed to produce or to enhance a wide range of processes such as plastic and metal welding, machining, metal forming, etc., in solids or cleaning, atomization, emulsification and dispersion, degassing, extraction, defoaming, particle agglomeration, drying and dewatering, sonochemical reactions, etc., in fluids. The power ultrasonic processes are very much dependent on the irradiated medium. In fact, a typical characteristic of high-intensity ultrasonic waves is their ability to produce different phenomena in different media in such a way that these phenomena seem to be opposite at times. This is, for example, the case of the application of power ultrasound to liquid suspensions for particle dispersion and to gas suspensions for particle agglomeration. Such apparently contradictory behavior is clearly due to the different media where the acoustic energy is applied and, consequently, to the different mechanisms that are activated. Another characteristic of high-intensity ultrasonic waves is their capacity to work synergistically with other forms of energy in order to promote, accelerate, or improve many processes. This is the reason why many practical applications of highpower ultrasound are not exclusively ultrasonic processes but ultrasonically assisted processes. The use of high-power ultrasound in industrial processing is a promising area that, in a great part, remains still closed. A large number of ultrasonic processes have been produced at laboratory stage. Nevertheless, only a few of them have already been introduced in industry. Such a situation is particularly significant in those processes related to the food industry where the application of ultrasonic waves would incorporate many important advantages, such as noncontamination, deep action, efficiency, cleanness, etc. An important process in food industry is dehydration. Dehydration is a method for preserving foods. There exist two basic conventional methods for dehydration: 1893

2 1894 GALLEGO-JUÁREZ ET AL. mechanical and thermal. Mechanical dehydration is based on pressing or centrifuging the material. In thermal dehydration or drying, the addition of energy in the form of heat is used to evaporate the liquid. Mechanical dehydration may be useful for the separation of the moisture weakly attached, whereas thermal drying provides a more complete removal of any kind of moisture from the product. The dehydration method suitable for each specific application is related to the attachment of the liquid to the solid material. In general, three types of attachment can be considered: chemical, mechanical, and physicochemical. For food dehydration, the present conventional systems employ two main procedures: hot air drying and freeze drying. Hot air drying is a widely used method but it can produce deteriorative changes in the food. Instead, in freeze drying, where food pieces are first frozen and then ice sublimates, the product deterioration is negligible but the process is expensive. High-intensity airborne sonic and ultrasonic waves have been used to increase the drying rate of materials. The acoustically assisted hot air drying process permits the use of lower temperatures and may be useful for drying heat-sensitive materials. [1,2] High-intensity airborne ultrasound introduces pressure variations at gas=liquid interfaces and therefore increases the evaporation rate of moisture. Moreover, in a forced-air drying system, the effect of the air velocity is to influence the heat and mass transfer. The acoustic energy produces an oscillating-velocity effect, which can increase the drying rate at stable air velocity. In addition, high-intensity airborne ultrasound causes microstreamings at the interfaces that reduce the diffusion boundary layer, increase mass transfer, and accelerate diffusion. Therefore, the application of airborne acoustic energy can positively contribute to the drying process. Nevertheless, the use of this technique has been very limited, probably because of the insufficient improvements obtained in the drying rates and=or the practical difficulties of the efficient generation of high-intensity ultrasound in air. For many years we have been involved in the study and development of new technologies for high-intensity sonic and ultrasonic applications in fluids and in multiphase media. Such technologies are based on a new type of plate-transducer ultrasonic generator that specifically implements high power capacity, efficiency, and directivity for airborne radiation. This article presents a review of the development of an ultrasonic technology for vegetable dehydration based on the application of the new plate-transducer power generators. Two experimental procedures have been developed: forced-air drying assisted by airborne ultrasound and ultrasonic dehydration by applying ultrasound in direct contact with the material. FORCED-AIR DEHYDRATION ASSISTED BY AIRBORNE ULTRASOUND The application of airborne ultrasonic energy for drying materials has been explored for several decades. Nevertheless, few airborne ultrasonic dryers have been reported in the technical literature and apparently none have been commercially used The main difficulties come from the efficient generation of ultrasonic energy in air and the transfer of such ultrasonic energy from air into the product due to the acoustic impedance mismatch. We have developed a new type of airborne ultrasonic generator that implements high power capacity, efficiency, and directivity. This generator is based on the steppedplate transducer (Fig. 1). [4] The main characteristics of the stepped-plate generators are the following: efficiencies of about 80%, beam width (at 3 db) of 1.5 degrees, power capacities up to 2 kw, and intensity levels reached in air as high as 175 db. We applied the new stepped-plate ultrasonic generator, as airborne radiator, in combination with forced air at different temperatures, in order to quantify the synergistic effect of the ultrasonic energy. The experimental setup designed and constructed for airborne ultrasonic dehydration is shown in Fig. 2. It mainly consists of a hot air generator, a stepped-plate power ultrasonic transducer with the corresponding electronic generator and a flat plate parallel to the ultrasonic radiator, acting as a reflector for the formation of a standing wave and also as sample holder. In addition, complementary sets of equipment for measuring temperature, air flow velocity, and weight were used. The acoustic field was previously measured and then controlled through the values of the current and voltage applied. FIG. 1. Stepped plate transducer.

3 HIGH-POWER ULTRASOUND DEHYDRATION OF VEGETABLES 1895 FIG. 2. Experimental setup for forced-air drying assisted by airborne ultrasound. The experimental tests mainly consisted of measuring the water content of vegetable samples after different times of application of high-intensity ultrasonic fields in combination with forced air at various temperatures and flow velocities. In all experiments, the frequency of the ultrasonic radiation was kept constant at about 20 khz while different acoustic pressure levels were applied. The water content of the samples was measured by weighing them. The vegetable samples to be dried were carrot slices. The slices were either of square (12 2 mm) or circular (14 mm in diameter) shape with 2, 4, and 8 mm in thickness. The samples were placed in the flat support plate with their upper surface and contour free. Such an arrangement was kept throughout the process (Fig. 2). A summary of the representative results is presented in Figs. 3 and 4. Figure 3 shows the results obtained with forced air at 60, 90, and 115 C without and with ultrasound (acoustic pressure level applied 155 db). In all cases, the size of the samples was mm square section with 2 mm in thickness. As can be seen, the effect of the ultrasonic radiation is significant at low air temperature and it diminishes when temperature increases. At the highest temperature (115 C) the ultrasonic effect was negligible. The dehydration curves are expressed in terms of moisture content in dry basis (weight of water=weight of dry solid). In order to see the influence of the acoustic pressure level, some tests were done at higher intensity (acoustic pressure level of 163 db) and lower forced air temperature (50 C instead of 60 C and 83 C instead of 90 C). Figure 4 summarizes the results of these tests. Comparison between Figs. 3 and 4 shows that the effect of increasing the acoustic pressure in about 8 db allows the temperature to be diminished by about 10 C with little increase in the airflow velocity. Other tests were done with the same type of samples by increasing the airflow velocity up to 3 m=s. The results obtained showed that at this forced-air speed the differences with and without ultrasound are less significant (Fig. 5). Similar results were obtained with samples of circular shape and with different thicknesses. In conclusion, it can be stated from the results that the application of airborne ultrasound can be useful in increasing the efficiency of forced-air drying processes. Nevertheless, the improvement seems to be relatively limited and the use of airborne ultrasound could be restricted to specific products and=or operations such as heat-sensitive materials and=or applications where rapid drying at low temperature is required.

4 1896 GALLEGO-JUÁREZ ET AL. FIG. 3. Forced-air dehydration kinetics of carrot slices at 1.3 m=s and various temperatures without and with airborne ultrasound (155 db). ULTRASONIC DEHYDRATION BY DIRECT COUPLING OF THE VIBRATION TO THE VEGETABLE Exploration of the Process The main difficulty in dehydration by airborne ultrasonic radiation is the low penetration of the acoustic energy in the food material due to the mismatch between the acoustic impedance of the air and that of the vegetable. In fact, the effectiveness of energy transfer between two media very much depends on their acoustic impedances, defined as the product of the density by the sound speed in the material. Therefore, in order to increase the ultrasonic effect on dehydration, a new procedure was developed and tested in which the ultrasonic vibration was applied in direct contact with the vegetable samples and together with a static pressure. [5] The good acoustic impedance matching between the vibrating plate of the transducer and the food material FIG. 4. Forced-air dehydration kinetics of carrot slices at 1.6 m=s and various temperatures without and with airborne ultrasound (163 db). FIG. 5. Forced-air dehydration kinetics of carrot slices at 3 m=s and 70 C without and with airborne ultrasound (155 db). favors the deep penetration of acoustic energy and increases the effectiveness of the process. The vegetable is subjected to high ultrasonic stresses that, due to a rapid series of contractions and expansions, produce a kind of sponge effect and the quick migration of moisture through natural channels or other channels created by the wave propagation, which result in moisture release from the product. In addition, the production of ultrasonic cavitation inside the liquid may help to the separation of the moisture strongly attached. The experimental setup developed for the initial study of this process is presented in Fig. 6. The samples were placed on the surface of the transducer radiating plate and they were kept there during treatment by applying a static pressure on them. An airflow at 1 m=s and 22 C was also applied to facilitate the removal of moisture. The drying effect was measured following the previous procedure; i.e., by weighing the samples at different times during treatment. Figure 7 shows the results obtained with carrot slices of circular shape with 2, 4, and 8 mm in thickness and 14 mm in diameter. It can be seen that the drying effect remarkably improves. In fact, the dehydration process is not only quicker and less energy consuming than the forced-air drying (with and without airborne ultrasound) but it is more powerful: the final moisture content could be less than 1%. In addition, due to the processing time and low temperature of the air flow, the product qualities are well preserved. Characterization of the Process The results obtained with the application of ultrasonic vibration in direct contact with vegetable samples validate the potential use of ultrasonic vibrations, in direct contact with food samples, at laboratory scale. [6] Nevertheless, for industrial purposes the development of an ultrasonic drying system capable of controlling the parameters of the process is required.

5 HIGH-POWER ULTRASOUND DEHYDRATION OF VEGETABLES 1897 FIG. 6. Experimental setup for dehydration by direct contact ultrasonic vibration. Looking at this objective, the first step was the characterization of the process. [7] To that purpose, a parametric study of the relative influence of the main physical parameters involved in the process was carried out. A specific experimental setup was designed, developed, and tested and is shown schematically in Fig. 8. It consists of a piezoelectric transducer working at 20 khz with a power capacity of 100 W driven by a power generator system. The generator is composed of an impedance matching unit, a power amplifier, and a resonant control system. This system was specifically developed to keep constant the power applied at the resonance frequency of the transducer during the process. FIG. 7. Dehydration kinetics of carrot slices by direct contact ultrasonic vibration (100 W) and forced-air at 22 C and various flow velocities. Comparison with forced-air dehydration at 22 C and 1.7 m=s without ultrasound. The sample to be treated is placed between the tip of the transducer and a porous layer. The layer is 3.2 mm in thickness and 25 cm in diameter. It is made of high-density polyethylene with a porous size distribution varying from 15 to 25 mm. This layer closes the top of a cylindrical vacuum chamber where a suction pump is applied to remove the moisture extracted from the lower face of the sample. A differential pressure meter allows the measurement of the vacuum (or suction) value during the trials. A static pressure was applied to get a good homogeneous mechanical coupling between the sample, the vibrating tip of the transducer, and the porous layer. The magnitude of the static pressure is controlled by means of a static pressure meter. Two different kinds of vegetables were used for experiments: apples of the Granny Smith variety and potatoes. Prior to the dehydration process, apples and potatoes were washed, peeled, cored, and cut into disks (4 mm in thickness and 24 mm in diameter). In addition, the samples were blanched in boiling water for a certain time, then put it fresh water to decrease their temperature, and finally drained. Pretreatment reduces vitamin and flavor loss, browning, and deterioration during storage. Moisture was measured by weighing the disk samples at different times (0, 10, 20, 30, 40, 50 and 60 min) in experiments with and without ultrasound. Figure 9a shows the results obtained without (US0) and with contact ultrasound, at two different applied powers, 25 W (US25) and 50 W (US50), in combination with forced-air (flow velocity ¼ 1m=s at31 C). The influence of the ultrasonic power applied is clearly observed, particularly in the first part of the process when the moisture is rapidly released.

6 1898 GALLEGO-JUÁREZ ET AL. FIG. 8. Scheme of the monosample ultrasonic dehydration setup for the characterization of the direct contact process. To interpret the experimental study a mass transfer model was applied. It is based on the use of the concept of effective diffusivity (D e ), which allows the description of diffusion of moisture using Fick s second ¼ D er 2 ðwþ ð1þ where W is the moisture content in dry basis, (kg water =kg dry solid), D e the effective diffusivity (m 2 =s), and t the time. Equation (1) can be integrated for different geometries and boundary=initial conditions. The solution of Eq. (1) for a vegetable sample slice of thickness 2l is given by: [9] WðtÞ ¼W e þðw o W e Þ X1 8 n¼0 ð2n þ 1Þ 2 p 2! exp D e ð2nþ1þ 2 p 2 t 4l 2 ð2þ FIG. 9. Dehydration kinetics of apple slices (24 mm in diameter, 4 mm in thickness) by contact ultrasound (þus) and forced air at 1 m=s and 31 C: (a) Experimental results with two different ultrasonic powers applied (US 25, 50 W) and without ultrasound US(0). Static force 70 g and suction 10 mbar. (b) Comparison between experimental and computed values. where the subscripts indicate equilibrium (e) and initial conditions (o). Drying kinetics were modeled by using Eq. (2); the parameter D e was calculated by using a nonlinear regression method. Dehydration curves and effective water diffusivity in apples and potatoes were calculated from Eq. (2). From the diffusional model, the effective water diffusivity coefficient was identified for the experiments carried out in the operational conditions. Identified D e in these experiments ranged in apples from m 2 =s without ultrasound up to m 2 =s with ultrasound application (þus(50w) þ 220 g þ 20 mbar); and in potatoes from m 2 =s without ultrasound up to m 2 =s with ultrasound application (þus(50w) þ

7 HIGH-POWER ULTRASOUND DEHYDRATION OF VEGETABLES g þ 20 mbar). Taking into account the diameter of the transducer, one applied force of g produces a static pressure on the sample of kg=cm 2. In Fig. 9b a good agreement between experimental and calculated values by using Eq. (2) can be observed. Therefore, the experimental results are well predicted by the diffusional model and, as a consequence, we concluded that high-intensity ultrasonic vibrations clearly act as a mass transport agent increasing the moisture release from the product. Extension of the Process For the extension of the new procedure, looking at industrial applications, a multisample dehydration system by direct contact ultrasound capable of controlling the parameters of the process was developed. This system is schematically presented in Fig. 10. It consist of the following parts: (a) treatment chamber, (b) ultrasonic power generator, (c) signals and power conditioner, (d) data acquisition unit, and (e) a PC to monitor and control the parameters of the process. For the generation and direct application of highamplitude ultrasonic vibrations to the samples, a power ultrasonic generator, consisting of a transducer with a rectangular flexural-vibrating plate and an electronic unit for driving the transducer, was designed and developed. The new transducer consists of an extensive rectangular plate driven at its center by a piezoelectrically activated vibrator. The extensional vibrator, constituted by a piezoelectric sandwich and a mechanical amplifier, drives the rectangular plate, which vibrates flexurally in one of its modes. For the correct design of the transducer, a 3D simulation was carried out by means of finite element methods (FEM). A rectangular aluminum plate of 308 mm mm 18.5 mm vibrating at 20 khz with eight nodal lines parallel to the shorter side was selected. The back face of the plate is grooved in a central section, parallel to its longer side, to homogenize the vibration amplitudes on its surface. In the design, the potential influence of the application of a uniform pressure on the surface of the plate was also analyzed by FEM. No variation in the vibration mode of the plate was detected when a static pressure was applied homogeneously to its entire surface. [10] The drying process takes place in the treatment chamber. The ultrasonic transducer is located at the upper part of the chamber. A parallelepipedic vacuum chamber, where the suction is applied, is fixed parallel to the transducer FIG. 10. Scheme of the direct contact multisample ultrasonic dehydration system.

8 1900 GALLEGO-JUÁREZ ET AL. plate. Its upper porous surface acts as a multisample holder and facilitates the removal of the moisture extracted from the samples. A pressure cylinder pneumatically controlled by a regulator is fixed at the bottom of the vacuum chamber and permits the application of a constant force at the interface transducer samples. A forced-air generator with flow rate and temperature controlled increases the removal of the internal moisture, which is expelled to the lateral surfaces of the samples. The air velocity is controlled by PWM (pulse width modulation) and measured with a hot-wire anemometer in order to work at constant velocity. The temperature of the samples measured with thermocouples is recorded during the process and stored in a PC. The electronic generator driving the ultrasonic transducer is composed of an impedance matching unit, a power amplifier, and a resonant frequency control system. This system was specifically developed to keep the power applied constant at the resonant frequency of the transducer during the process, independently of the variations of the acoustic impedance of the load. The ultrasonic generator has a maximum power capacity of about 250 W. The different parameters of the driving signal applied to the transducer (frequency, voltage, current, and phase) were continuously monitored and stored in a PC. All the electromechanical and pneumatic devices installed in the treatment chamber are directly controlled by a PC by using a specific software based on LabView 1 and Mathematica 1 codes. In addition, the ultrasonic drying system is able to carry out the different steps of the drying process automatically by means of special software. The evolution of the different parameters involved in the process can also be analyzed and, if necessary, transferred via Internet to other work stations. In addition, the procedure permits the application, monitoring, and control of the parameters of the process. All these advantages will allow us to carry out a more efficient analysis of the basic mechanisms involved in the ultrasonic dehydration process of foods. The vegetable samples used for drying were carrots of cylindrical shape. The samples were cut into cylinders (24 mm in diameter and 8 mm in thickness). To prepare carrot samples for dehydration some general rules were followed: to choose tender vegetables, to wash them, to remove any damaged areas, and to cut them into uniform pieces. In addition, the samples were blanched for a certain amount of time in boiling water before being dried. This pretreatment avoids enzyme attack and oxidation. First, experimental trials were carried out to study the influence of ultrasonic power (0, 25, 50, 75, and 100 W) on the kinetics of the dehydration process. In all trials the temperature and relative humidity were kept between 24 and 26 C and 30 and 46%, respectively. The applied static pressure was fixed at 0.06 kg=cm 2, the suction at FIG. 11. Influence of ultrasonic power on the kinetics of the direct contact ultrasonic dehydration process of carrot cylinders. 60 mbar, and the airflow velocity and temperature at 2m=s and 30 C, respectively. Moisture content of samples was measured by weighing them at fixed intervals of 15 min. Figure 11 shows the evolution of the weight percentage (%) during the dehydration process of a set of 30 carrot samples with different applied ultrasonic powers. The curves obtained up to a maximum applied power of 100 W reveal a direct increase of the drying effect with the acoustic intensity and no saturation was reached. This result confirms the significant role of the ultrasonic intensity when the other thermomechanical parameters (temperature, flow rate, suction, etc.) are kept constants. Another important characteristic to be considered in the performance of the multisample system is the homogeneity in the dehydration effect on all the samples. To analyze this characteristic a study of dehydration of various series of groups of 20 samples was carried out. It consisted of measuring during periodic time intervals the dewatering effect FIG. 12. Curves of direct contact ultrasonic dehydration of 20 carrot slices by using the multisample system.

9 HIGH-POWER ULTRASOUND DEHYDRATION OF VEGETABLES 1901 in each of the single samples along a process extended up to a point at which 90% dehydration was reached. The results (Fig. 12) showed that the maximum dispersion in the dehydration effect among all the samples was of about 12%. Such results confirm the homogeneity in the dehydration action of the developed system. The new system constitutes the basic model to be scaled up for industrial applications. It has to be taken into account that in industrial conditions, even though ultrasound is out of the hearing range, it could be advisable to acoustically isolate the system or to use headphone protectors if the intensity is very high and=or if the frequency is close to the lower ultrasonic range. CONCLUSIONS This article reviews the studies and developments carried out by the Power Ultrasonic Group of the Institute of Acoustics, CSIC, in the application of the ultrasonic energy for the dehydration of vegetables. As a consequence of this work, a series of processes and devices were developed and tested. As a final step, a new high-intensity ultrasonic process by direct contact has been established and a prototype of a multisample ultrasonic dehydration system for the application of this process at pre-industrial stage has been designed, constructed, and tested. The system is provided with electromechanical and pneumatic elements together with the software and hardware necessary for the automatic control and monitoring of all the variables of the process. It represents a basic tool for testing dehydration treatments of different products as a previous stage to industrial scale. In addition, it will facilitate the advancement in the study of the mechanisms involved in the ultrasonic dehydration process. REFERENCES 1. Fairbank, H.V. Applying ultrasound to continuous drying process. Ultrasonic International 1975 Conference Proceedings, IPC Science and Technology Press Ltd, Guildford, UK, 1975; pp Seya, K. Macrosonic drying. Proceedings of the First International Symposium on High-power Ultrasonics, IPC Science and Technology Press Ltd, Guildford, UK, 1970; pp Borison, Y.Y.; Gynkina, N.M. Acoustic drying. In Physical Principles of Ultrasonic Technology, Vol. 2; Rozenberg, L.D., Ed.; Plenum Press: New York, Gallego-Juarez, J.A.; Rodriguez-Corral, G.; San Emeterio Prieto, J.L.; Montoya Vitini, F. Electroacoustic unit for generating high sonic and ultrasonic intensities in gases and interphases. U.S. Patent #5,299,175, March 29, Gallego-Juarez, J.A.; Yang, T.; Vazquez Martínez, F.; Galvez Moraleda, J.C.; Rodriguez Corral, G. Procédé et dispostif de déshydratation. International Patent #WO , November 14, Gallego-Juarez, J.A.; Rodriguez-Corral, G.; Galvez-Moraleda, J.C.; Yang, T.S. A new high intensity ultrasonic technology for food dehydration. Drying Technology 1999, 17(3), de la Fuente, S.; Riera, E.; Gallego-Juarez, J.A.; Tomez, T.E.; Acosta, V.M.; Vazquez, F. Parametric study of ultrasonic dehydration processes. WCU 2003 Proceedings, pp Sablani, S.; Rahman, S.; Al-Habi, N. Moisture diffusivity in foods. An overview. In Drying Technology in Agriculture and Food Science; Mujumdar, A., Ed.; Science Publisher Inc., 2000; pp Cranck, J. The Mathematics of Diffusion: Oxford University Press, de la Fuente, S.; Riera, E.; Acosta, V.M.; Blanco, A.; Gallego- Juarez, J.A. Food drying process by power ultrasound. Ultrasonics (in press).