Contact ultrasound

Cho, B., Irudayaraj, J., and Bhardwaj, M. C. (2001). Rapid measurement of physical properties of Cheddar cheese using a non-contact ultrasound technique. Am. Soc. Agric. Eng. 44, 1759-1762. 

Fig. 7.6 Contact ultrasound effects on mass transfer at cellular and tissue levels. Effects are shown in relation to the distance from the sonicated surface. Temperature effects and sound pressure amplitude are damped in deeper tissue layers, as depicted in the graph on the left. M, cell membrane W, cell wall L, middle lamella I, intercellular spaces P, pore network. Dotted lines represent membrane damage due to dehydration. 1, Loosening of cell-to-cell...

Ultrasound-assisted drying processes can be based on sound transmission in air (airborne ultrasound), liquids (osmotic solutions) or solids (contact ultrasound). Sound transmission and sound emitter designs must be adapted to the respective applications and have, together vdth the transmission medium, a major influence on process design and performance. 

Although the application of ultrasound in osmotic dehydration is, technically speaking, not a contact ultrasound application, its analysis provides an interesting insight into the principles of ultrasound-assisted drying and the different approaches to minimize mass transfer barriers applying PEF and ultrasound. It is therefore briefly discussed here. 

Fig. 7.9 Assembly of the ultrasonic system used in contact ultrasound-assisted air drying. 1, samples 2, drying screen 3, ring sonotrode RIS 200 4, ultrasound processor UIS250L. Reproduced with permission from Schossler et a/. (2012a).

Further reductions in the energy consumption of contact ultrasound-assisted drying can be realized by applying an intermittent ultrasound treatment. Whereas, the maximum impact of ultrasound is achieved when the sound waves are... 

The application of contact ultrasound-enhanced air drying of the above-mentioned products has led to a positive evaluation of the industrial applicability of this technology. The application of a static pressure in order to further improve sound transmission, as described earlier (Gallego-Juarez, 1998 Riera-Franco de Sarabia et al, 2002), represents an interesting route for future process optimization, although equipment still needs to be improved and updated at the industrial scale. 

Fig. 7.11 Laboratory-scale system for contact ultrasound-assisted freeze-drying. A, Ac7lic lid with ultrasonic system B, screen for control samples C, freeze-d er 1, ultrasound processor UIPIOOO 2, sonotrodes BS2d34 3, vibration-free flange ...

The latest studies analyzing the mechanisms of contact ultrasound-assisted mass transfer have revealed that ultrasound-related cell disruption is limited to the thin layer of tissue that is in direct contact with the vibrating surface. In any tissue layers deeper than 1 mm, the structural changes that occur are attributed to water removal moreover, ultrasound can enhance such water removal as a result of mechanical effects that can loosen cell-boundary layers, and remove moisture from solid-liquid interfaces. The transport of moisture through the pore network can also be increased, and low-pressure cycles vhll enable an improved evaporation in the tissues. 

Schossler, K., Jager, H., Knorr, D., 2012b. Novel contact ultrasound system for the accelerate freeze-drying of vegetables. Innovative Food Sci. Emerg. Technol. 16 113-120. 

Schossler, K., Thomas, T, Knorr, D., 2012c. Modification of cell structure and mass transfer in potato tissue by contact ultrasound. Food Res. Int. 49 ... 

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