Extraction Technique Using Ultrasound? Lets Explore the Concept and Mechanism Behind it!

     Ultrasound-assisted extraction (UAE) uses ultrasound energy and solvents to extract target compounds from various plant matrices. Ultrasound are the mechanical waves having frequency (>20 kHz) higher than audible frequency range of human hearing (20 Hz to 20 kHz). These waves consist of a series of compression and rarefaction cycles that can be propagated through solid, liquid or gas medium inducing displacement and dislodgement of the molecules from their original positions. At high intensity sound wave, the negative pressure during rarefaction exceeds the attractive force joining the molecules together pulling them apart and creating cavitation bubbles. These bubbles grow through coalescence and later collapse during compression phase creating hot spot and extreme local condition. The temperature may reach up to 5000 K and pressure increase may be up to 1000 atm.

                                         

    Acoustic cavitation is the main mechanism involved in the ultrasound assisted extraction. The collapsing cavitation bubbles and the sound waves may induce either one or combination of the phenomena such as fragmentation, localised erosion, pore formation, shear force, increased absorption and swelling index in the cellular matrix of the plant. The collapsing cavitations bubbles generate shockwaves and accelerated inter-particle collision causes the fragmentation in cellular structure. The rapid fragmentation leads to solubilisation of the bioactive component in the solvent due to decrease in particle size, increased surface area and high mass transfer rates in the boundary layer of solid matrix.

     Ultrasound leads to the localized damage to the plant tissues termed as erosion. This erosion may also be attributed to the implosion of the cavitation bubbles on the surface of plant tissues. The eroded part facilitates the contact of solvent, increasing the yield of extraction. The formation of pores during cavitation, a phenomena knows as ‘sonoporation’, in the cell membranes results in the release of the bioactive compounds present in cell. 

    In addition to that, the generation and collapse of cavitation bubbles induces shear force and turbulence within the fluid which results in breakdown the cell walls contributing to releasing the bioactive compound. Ultrasound increases the water absorption of the pomace thereby enhancing the accessibility of the water as solvent to the bioactive compounds to be extracted along with increased diffusivity of the bioactive compounds itself. 

    Ultrasound also increases the swelling index of plant tissue matrix which helps in both desorption and diffusion of solutes resulting in increased extraction. The increase in the extraction yield by UAE can’t be attributed to a single mechanism but due to the combined effect of all the mechanism. There are types of ultrasound assisted extraction techniques and for the better understanding, below given image is shown.