Ultrasound-assisted extraction has been developed in order to overcome main drawbacks and disadvantages of conventional extraction techniques, such as:
• low extraction yield,
• increased time, solvent and energy consumption, and
• low product quality.
Ultrasound technique has been considered as generally affordable, reproducible and the most importantly, effective method used for recovery of various classes of bioactive compounds. Various physical and chemical phenomena including agitation, vibration, pressure, shock waves, shear forces, microjets, compression and rarefaction, acoustic streaming, cavitation and radical formation are responsible for ultrasonic effect. The enhancement in extraction obtained by using ultrasounds is mainly attributed to the effects of acoustic cavitation (Figure 1) produced in the solvent by the passage of an ultrasonic wave. Cavitation comes as a result of creation, growth and implosion of gas bubbles under the ultrasonic treatment. These bubbles collapse on the surface of the solid plant material and release high pressure and temperature, which generate shock waves towards the solid surface. Ultrasound also exerts a mechanical effect, allowing greater penetration of solvent into the sample matrix, increasing the contact surface area between solid and liquid phase. Ultrasonic waves may also cause some chemical effects which are rather undesirable due to the changes in chemical composition, possible degradation of targeted compounds and production of free radicals within the gas bubbles.
Figure 1. Ultrasonic cavitation: creation, growth and implosion of gas bubbles
The mechanical effects of ultrasound induce a greater penetration of solvent into cellular materials and improve mass transfer. Increased mass transfer and significant disruption of cell walls come as a result of these combined effects, leading to significant reduction of extraction time and increase in extraction yield. Ultrasound-assisted extractors are ultrasonic baths or closed extractors fitted with an ultrasonic probe (Figure 2). In our research, we have successfully applied ultrasound-assisted extraction for the recovery of different groups of polyphenolic compounds from various sources (chokeberry, apple fruit dust, sage, bearberry, winter savory, basil, coriander, etc.). Moreover, application of this technology for recovery of bioactive compounds at industrial scale has recently significantly grown.
Figure 2. Ultrasonic bath (a) and extractor fitted with an ultrasonic probe (b)