Biomedical Engineering Reference
In-Depth Information
13.2.7 High Intensity Ultrasounds
The term “ultrasound” in its most basic definition refers to pressure waves with a frequency
of 20 kHz to 10 MHz (Butz and Tauscher, 2002). Power ultrasound, also known as high
intensity ultrasound, refers to sound waves with low frequencies (20-100 kHz) and high
sound intensity (10-1000 W/cm 2 ) (Feng et al ., 2008). The use of ultrasonics in industrial
processes has two main requirements: a liquid medium and a source of high energy
vibrations (the ultrasound) (Bates and Patist, 2009). During the sonication process,
longitudinal waves are created when a sonic wave meets a liquid medium, thereby creating
regions of alternating compression and expansion (Sala et al ., 1995 ). These regions of
pressure change cause cavitation to occur and gas bubbles are formed in the medium,
creating shock waves. The pressure changes resulting from these implosions are the main
bactericidal effect in ultrasound, creating micromechanical shocks that disrupt cellular,
structural and functional components up to the point of cell lysis (Butz and Tauscher, 2002).
The combination of ultrasound in conjunction with pressure treatment (200-500 kPa)
(manosonication), heat treatment (thermosonication) or both (manothermosonication)
treatments has been proposed to enhance microbial reduction (Piyasena et al ., 2003 ). The
enhanced mechanical disruption of cells by physical damage to the cell envelope in the form
of wrinkles, ruptures, and perforations is the reason for the enhanced microbial destruction
when ultrasound is combined with heat or pressure (Ugarte et al ., 2006 , 2007 ). In recent
years, studies into the application of multifrequency sonication to enhance cavitation have
received increasing interest. The hypothesis is that multiple frequency sonication will allow
bubbles with a wide range of sizes to generate cavitation, and hence increase the cavitation
intensity (Feng et al ., 2008 ).
Enzyme inactivation caused by ultrasound has been attributed to the ability of sonication
to break large macromolecules or particles by shear and mechanical stress generated by
shock waves derived from bubble implosion; this causes the breakdown of hydrogen
bonds and Van der Waals interactions, leading to the modification of the enzymes
secondary and tertiary structure and loss of biological activity (Sala et al ., 1995 ; Feng
et al ., 2008). In addition, as a result of intense cavitation, water molecules can be
dissociated, generating highly reactive free radicals that can react with enzymes, so
modifying their biological activity (Demirdoven and Baysal, 2009). High intensity
ultrasound can also denature proteins and produce free radicals that can adversely affect
the flavor of fruit based and high fat foods and milk (Sala et al ., 1995 ; Riener et al ., 2009 ).
The sonication process seems not to affect to a large extent anthocyanins, ascorbic acid
and color values from strawberry, blueberry and orange juice (5-15% loss) (Tiwari et al .,
2009a , 2009b ; Valdramidis et al ., 2010). However, further research is needed to better
understand the effects of ultrasound processing on bioactive compounds and sensory
properties of foods.
The application of ultrasound as a laboratory-based technique for assisting extraction
(Ultrasound Assisted Extraction, UAE) of metabolites from plant material has been widely
demonstrated (Knorr 2003 ; Vilkhu et al ., 2008). As examples, commercial extraction systems
using high power ultrasound in the food and beverage industry have been used in the
extraction of phenolic compounds and pigments from grape marc (solid waste of the wine
making process) and blueberries, extraction of lipids and proteins from plant seeds,
improvement of oil extraction (palm fruit, corn germ and citrus fruit), extraction of bioactive
compounds from coffee beans, tea and carrot (
α
- and
β
-carotene), extraction of isoflavones
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