Dehydration Assisted by Ultrasound

Power ultrasonic plate-transducers introduced on the market by PUSONICS represent a sustainable technological novelty which is capable of transmitting sonic and ultrasonic energy through air. Ultrasound is an appealing green method for drying heat-sensitive materials, where micro-vibrations could prevent spoilage due to damaging high temperature levels. Ultrasonic dehydration performed via air-borne radiation or in direct contact with the materials has proven to be an effective procedure, alone or in combination with other drying methods.

New Ultrasonic Technology To Assist Drying Processes:

With the introduction of the novel plate-transducer technology for air-borne applications by PUSONICS SL, new possibilities for enhancing drying have been opened up. These devices are characterized by higher electro-acoustic efficiencies compared with former sound sources, and have already shown promising results in several drying operations.

• Convective drying assisted by air-borne ultrasound
• Use of an air-borne ultrasonic transducer as a fluidized bed dryer
• Ultrasonic drying by applying ultrasound in direct contact with samples
• Lyophilization at atmospheric pressure, assisted by ultrasound
• Ultrasonic drying combined with other processes such as microwave or infrarred radiations

High-intensity airborne ultrasound introduces pressure variations at gas-liquid interfaces that increase the evaporation rate of material moisture. Moreover, in forced-air drying systems the ultrasonic wave produces an oscillating-velocity effect, which can increase the drying rate at stable air velocity. In addition, high-intensity airborne ultrasound causes micro-streamings at the air/material inter- faces that reduce the diffusion boundary layer, increasing mass transfer, and accelerating diffusion. Also ultrasonic vibrations produce a kind of “micro-sponge” effect that favours the quick migration and the release of the moisture.

• Convective air drying assisted by air-borne ultrasound

The ultrasonically  assisted forced-air drying could be applied in two different ways. One way is convective drying with hot air using moderate temperature and another way is low-temperature drying at atmospheric pressure. In both cases the application of ultrasound intensifies and accelerates the process.

Example of convective utrasound dehydration with almonds and other vegetable products.

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The graphic below shows dehydration kinetics of almonds obtained experimentally with different configuration settings (air temperature, and applied power to transducer).

 

It is noticeable that acceleration of process reaches 50% for a drying at 60ºC , by applying 200W electric power to the transducer. 

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Also other vegetable products have been efficiently dried with forced-air at moderate temperature. The figure below shows dehydration kinectic for carrot slices (12x2mm) with forced-air at 1.3 m/s and 60, 90 and 115ºC with and without ultrasound (155dB).

As can be seen, the effect of the ultrasonic radiation is remarkable at low air temperature while it diminishes when temperature increases [1]

 

 

• Use of an air-borne ultrasonic transducer as a fluidized bed dryer

Another approach to the application of ultrasound in drying and dehydration has been carried out by using a transducer with a cylindrical radiator inside which a fluidized bed is created. The product to be dried is kept in suspension within such bed which is sonicated in the whole volume by the cylindrical radiator of the transducer. Such process has been experimented positively in close cooperation between the CSIC and the Food Technology Department (ASPA Group) of the Polytechnic University of Valencia (UPV). [2]

 

Product in suspension inside the cylindrical radiator

• Ultrasonic dehydration by direct contact

The good acoustic impedance matching between the vibrating plate of the transducer and the material, favours the deep penetration of acoustic energy and increases the effectiveness of the process. This results in quicker and deeper moisture release from the product. Dehydration kinetics of carrot slices by direct contact ultrasonic vibration (100W) and forced air at 1.7 m/s and environmental temperature (22ºC), is shown in the figure below in comparison with the dehydration kinetics by only aire at 22ºC and 1.7 m/s. [1] 

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• Lyophilization at atmospheric pressure (freeze drying)

Drying at low temperature enables high qualilty products. However at atmospheric pressure it is a slow process.

The use of airborne power ultrasound in freeze drying processes (or lyophilization at atmospheric pressure) can reduce the processing time by more than 40%. The time reduction and the low electric energy required by the transducer allow a fast economic return and the possibility of processing higher amount of food while preserving the maximum quality of the final product. [4]

•  Ultrasonic drying combined with other drying processes

 The simultaneous combination of ultrasound and other drying methods such as microwaves, infrarred, etc, has shown promising results to enhance drying.

 Hybrid dryer (PromisTech, Poland), equipped with a PUSONICS Airborne Ultrasonic System (AUS) [3]

Effects associated to acoustic energy

 The ultrasonic drying process is low energy consuming,and the product qualities are very well preserved.

Ultrasound does not heat the product  to significant temperature. As a consequence, the use of ultrasonic waves either to dry heat sensitive materials or to perform drying processes at low temperature constitutes a sustainable and energy efficient method.
 

References 

1. “Application of high- power ultrasound for dehydration of vegetables: Processes and devices “J.A. Gallego- Juárez, E. Riera, S.de la Fuente, G Rodriguez-Corral ,V. M. Acosta and A Blanco. Drying Technology 2007, 25, pp. 1983-1901. https://doi.org/10.1080/07373930701677371

2. “Ultrasonic drying for food preservation” J.V. García-Pérez, J.A. Carcel, A. Mulet, E. Riera and J.A. Gallego-Juárez. Chapter 29 pp. 875-910 in “Power Ultrasonics” J.A. Gallego-Juárez and K. Graff (eds), WP-Elsevier, Cambridge UK, 2015. https://doi.org/10.1016/B978-1-78242-028-6.00029-6

3. "High power airborne ultrasound assist in combined drying of raspberries," S. J. Kowalski, A. Pawłowski, J. Szadzińska, J. Łechtańska and M. Stasiak, Innovative Food Science & Emerging Technologies, 34, pp. 225-233 (2016). https://doi.org/10.1016/j.ifset.2016.02.006

4. "Airborne power ultrasound for drying process intensification at low temperatures: Use of a stepped-grooved plate transducer,"R. R. Andrés, E. Riera, J. A. Gallego-Juárez, A. Mulet, J. V. García-Pérez, and J. A. Cárcel, Drying Technology, pp. 1-14, 2019. https://doi.org/10.1080/07373937.2019.1677704.

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