Enrique Riera

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

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.

Influence of High-Intensity Ultrasound on Drying Kinetics of Persimmon

Drying persimmon pieces is recognized as a way to preserve and add value to the excess production of the fruit in Spain. To this end, air drying kinetics of persimmon cylinders (30 mm height and 13 mm diameter) were determined under different drying conditions: 8 air drying velocities (0.5, 1, 2, 4, 6, 8, 10, and 12 m/s) with and without application of high-intensity ultrasound (21.8 kHz and 154.3 dB). The drying process was modeled using two diffusion models with and without the influence of external resistance to drying. From the effective diffusivity and the mass transfer coefficient identified from the data it was concluded that high-intensity ultrasound increased the drying rate at the lowest air velocities tested, affecting both external and internal resistances.

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 (De ). 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.

Application of Ultrasound

Publisher Summary Major mechanical effects of ultrasound are provided when the power is sufficiently high to cause cavitation. Like any sound wave, ultrasound is propagated via a series of compression and rarefaction waves induced in the molecules of the medium through which it passes. At sufficiently high power, the rarefaction cycle may exceed the attractive forces of the molecules of the liquid and cavitation bubbles will form. Such bubbles grow by a process known as rectified diffusion, that is, small amounts of vapor (or gas) from the medium enters the bubble during its expansion phase and is not fully expelled during compression. The effectiveness of ultrasound as a food processing tool has been proven in the laboratory and there are a number of examples of scale-up. In most cases, commercially available frequency is used, that is 20 or 40 kHz, and this has proved quite satisfactory. In such cases, the variable parameters are temperature, treatment time, and acoustic power. Little attention has been paid to the use of different frequencies except in a few cases. One such is the use of ultrasound in food preservation using the bactericidal action of sonication combined with other techniques such as heat, ultraviolet light, and the use of a biocide.

Power ultrasonic transducers with extensive radiators for industrial processing

High-power ultrasonics (HPU) is a green emerging technology that offers a great potential for a wide range of industrial processes. Nevertheless such potential have remained restricted during many years to a limited number of applications which reached commercial development. The possible major problem for extending the range of HPU industrial applications has been the lack of power ultrasonic transducers for large-scale application, adapted to the requirements of each specific problem with high efficiency and power capacity. A new family of HPU transducers with extensive radiators have been recently introduced. It comprises a variety of transducer types designed with the radiators adapted to different specific uses in fluids and multi-phase media. Such transducers implement high power capacity, high efficiency and radiation pattern control. In addition, their design incorporate strategies to eliminate or reduce modal interactions produced at high power as a consequence of their nonlinear behaviour. The introduction of such new transducers has significantly contributed to the development at semi-industrial and industrial level of a number of processes in the food and beverage industry, in environment and in manufacturing. This paper deals with the basic structure and main characteristics of such transducers as well as their performance in the developed application processes.

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.

Improvement of Convective Drying of Carrot by Applying Power Ultrasound—Influence of Mass Load Density

Power ultrasound is considered to be a novel and promising technology with which to improve heat and mass transfer phenomena in drying processes. The aim of this work was to contribute to the knowledge of ultrasound application to air drying by addressing the influence of mass load density on the ultrasonically assisted air drying of carrot. Drying kinetics of carrot cubes were carried out (in triplicate) with or without power ultrasound application (75 W, 21.7 kHz) at 40°C, 1 m/s, and several mass load densities: 12, 24, 36, 42, 48, 60, 72, 84, 96, 108, and 120 kg/m3. The experimental results showed a significant (p < 0.05) influence of both factors, mass load density and power ultrasound application, on drying kinetics. As expected, the increase of mass load density did not affect the effective moisture diffusivity (De, m2/s) but produced a reduction of the mass transfer coefficient (k, kg water/m2/s). This was explained by considering perturbations in the air flow through the drying chamber thus creating preferential pathways and, as a consequence, increasing external mass transfer resistance. On the other hand, it was found that the power ultrasound application increased the mass transfer coefficient and the effective moisture diffusivity regardless of the mass load density used. However, the influence of power ultrasound was not significant at the highest mass load densities tested (108 and 120 kg/m3), which may be explained from the high ratio (acoustic energy/sample mass) found under those experimental conditions. Therefore, the application of ultrasound was considered as a useful technology with which to improve the convective drying, although its effects may be reduced at high mass load densities.

Intensification of Low-Temperature Drying by Using Ultrasound

The main aim of this work was to test the feasibility of power ultrasound to intensify low-temperature drying processes. For this purpose, the convective drying kinetics of carrot, eggplant, and apple cubes (side 10 mm) were carried out at atmospheric pressure, 2 m/s, −14°C, and 7% relative humidity with (acoustic power 19.5 kW/m3) and without ultrasound application. Under the same experimental conditions, kinetics studies of ethanol removal from a solid matrix were also performed. Diffusion models were used to describe drying curves and identify kinetic parameters in order to evaluate and quantify the process intensification attained by ultrasound application. The effect of ultrasound application was similar for all products tested; that is, the drying time was shortened between 65 and 70%. In the case of ethanol removal, the time reduction achieved by ultrasound application was 55%. The mass transfer coefficient and effective moisture diffusivity increased by 96 to 170% and by 407 to 428%, respectively, when ultrasound was applied.

High-power ultrasonic system for the enhancement of mass transfer in supercritical CO2 extraction processes

Oil is an important component of almonds and other vegetable substrates that can show an influence on human health. In this work the development and validation of an innovative, robust, stable, reliable and efficient ultrasonic system at pilot scale to assist supercritical CO(2) extraction of oils from different substrates is presented. In the extraction procedure ultrasonic energy represents an efficient way of producing deep agitation enhancing mass transfer processes because of some mechanisms (radiation pressure, streaming, agitation, high amplitude vibrations, etc.). A previous work to this research pointed out the feasibility of integrating an ultrasonic field inside a supercritical extractor without losing a significant volume fraction. This pioneer method enabled to accelerate mass transfer and then, improving supercritical extraction times. To commercially develop the new procedure fulfilling industrial requirements, a new configuration device has been designed, implemented, tested and successfully validated for supercritical fluid extraction of oil from different vegetable substrates.

Influence of material structure on air-borne ultrasonic application in drying.

This work aims to contribute to the understanding of how the properties of the material being dried affect air-borne ultrasonic application. To this end, the experimental drying kinetics (40°C and 1m/s) of cassava (Manihot esculenta) and apple (Malus domestica var. Granny Smith) were carried out applying different ultrasonic powers (0, 6, 12, 19, 25 and 31 kW/m(3)). Furthermore, the power ultrasound-assisted drying kinetics of different fruits and vegetables (potato, eggplant, carrot, orange and lemon peel) already reported in previous studies were also analyzed. The structural, textural and acoustic properties of all these products were assessed, and the drying kinetics modeled by means of the diffusion theory. A significant linear correlation (r>0.95) was established between the identified effective diffusivity (DW) and the applied ultrasonic power for the different products. The slope of this relationship (SDUP) was used as an index of the effectiveness of the ultrasonic application; thus the higher the SDUP, the more effective the ultrasound application. SDUP was well correlated (r ⩾ 0.95) with the porosity and hardness. In addition, SDUP was largely affected by the acoustic impedance of the material being dried, showing a similar pattern with the impedance than the transmission coefficient of the acoustic energy on the interface. Thus, soft and open-porous product structures exhibited a better transmission of acoustic energy and were more prone to the mechanical effects of ultrasound. However, materials with a hard and closed-compact structure were less affected by acoustic energy due to the fact that the significant impedance differences between the product and the air cause high energy losses on the interface.