KHADIJAH HANIM ABDUL RAHMAN DEPARTMENT OF CHEMICAL ENGINEERING

* KHADIJAH HANIM ABDUL RAHMAN DEPARTMENT OF CHEMICAL ENGINEERING TECHNOLOGY, FETech, UNIMAP

*Conventional extraction of plant materials comprises solid–liquid techniques depending usually upon organic solvents which present various shortcomings such as toxic residues, chemical transformation of extracts, and toxic wastes. *As a result, an increasing demand from industries for natural molecules produced from a clean extraction with safer solvents is observed. *Extractions based on sonication have been employed for the isolation of weakly-bound organic compounds from solid samples (e. g. animal tissue, plants) and are comparable to methods involving more intensive treatments (e. g. Soxhlet , accelerated solvent extraction). * Ultrasound irradiation from high-intensity ultrasonic processors opens the door to new perspectives, mainly concerning those analytes that are strongly-bound to the matrix. *Ultrasound assisted extraction can be performed at ambient temperature , normal pressure and mild chemical conditions compared to most of extraction techniques which requires solvents at high temperature (i. e. at boiling point) or pressure. *

* Ultrasound is a mechanical wave that necessitates an elastic medium to spread over and it differs from audible sounds by the wave frequency. * The audible frequencies to humans are comprised between 16 Hz and 20 k. Hz, while ultrasound frequencies range from 20 k. Hz to 10 MHz. *

Ultrasound frequency Diagnostic (high frequency ultrasound) 2 MHz-10 MHz Power ultrasound (conventional ultrasound/low frequency ultrasound) 20 -100 KHz Medical imaging Detection in bond inspections for plastic able to produce physical and/or chemical effects into the medium in order to facilitate or accelerate chemical reactions *

*Ultrasound ranging from 20 to 100 k. Hz is used in chemically important systems, in which chemical and physical changes are desired as it has the ability to cause cavitation of bubbles. *Cavitation phenomena- physical processes that: *create *enlarge *implode micro bubbles of gases dissolved in the liquid. *

* * When applied on liquid, ultrasound waves consist of a cyclic succession of rarefaction and compression phases imparted by mechanical vibration. Compression cycles exert a positive pressure and push the liquid molecules together, while rarefaction cycles exert a negative pressure and pull the molecules apart. *The extent of the negative pressure depends on the nature and purity of the liquid. At a sufficiently high power, the attraction forces between them might be exceeded, generating a void in the liquid. The voids created into the medium are the cavitation bubbles which are formed from dissolved gases.

* *Phenomenon of cavitation consists of the repetition of three distinct steps: * Formation (nucleation) * rapid growth (expansion) during the cycles until it reaches a critical size * violent collapse in the liquid

* When sonicating liquids at high intensities, the sound waves that propagate into the liquid media result in alternating high-pressure (compression) and low-pressure (rarefaction) cycles, with rates depending on the frequency. * During the low-pressure cycle, high-intensity ultrasonic waves create small vacuum bubbles or voids in the liquid. When the bubbles attain a volume at which they can no longer absorb energy, they collapse violently during a high-pressure cycle (compression cycle). * During the implosion very high temperatures (approx. 5, 000 K) and pressures (approx. 2, 000 atm) are reached locally. The implosion of the cavitation bubble also results in liquid jets of up to 280 m/s velocity. * The collapse of cavitation bubbles near cell walls produces cell disruption. * As a result, there is an enhanced solvent penetration into the cells and an intensification of the mass transfer. *

* These fast changes in pressure and temperature (cavitation), which cause shear disruption and thinning of cell membranes, are the phenomena that make ultrasound applicable to alter the medium state by the sonochemistry. * The cavitation and consequently the mass transfer and the extraction rate, which are influenced by temperature, hydrostatic pressure, irradiation frequency, acoustic power, and ultrasonic intensity, are as important as the choice of solvent and sample preparation. * Another effect of this type of waves on the solid structure is that the ultrasound can facilitate swelling and hydration, causing an enlargement in the pores of the cell wall. * This effect will improve the diffusion process and increase mass transfer. *

* *Vegetal tissue consist of surrounded by walls (figure 3. 3). cell *The cavitation bubble generated close to the plant material surface (a) *collapses during a compression cycle (b) *a microjet directed toward the surface is created (b and c) * The high pressure and temperature involved in this process will destroy the cell walls of the plant matrix and its contents will be released into the medium (d).

* For example, in the extraction of basil essential * oil, it is possible to notice the intact cells and essential oil glands (Figure 3. 4 A) in comparison with the empty essential oil gland after conventional maceration (Figure 3. 4 B). * However, due to cavitation, the basil cells are completely destroyed after ultrasound-assisted extraction, allowing the total recovery of the essential oil (Figure 3. 4 C).

* Both probe (Figure 3. 7 A) and bath systems (Figure 3. 7 B) are used industrially, depending on the application, and several types of ultrasonic devices have been developed for industrial uses or scale-up laboratory experiments * The disposition of ultrasound transducers varies upon the device and sometimes an agitation system is also used. Some continuous flow devices have also been developed for both probe and batch systems. * REUS has developed reactors from 30 L to 1000 L to which pump systems are coupled in order to fill the ultrasonic bath, to stir the mixture, and to empty the system at the end of the procedure. * Hielscher has devices of a wide range of power, from 50 W to 400 W for analytical scales and from 500 W to 16000 W in industrial scales. *

*Highest efficiency of UAE, in terms of yield and composition of the extracts, can be achieved by increasing the ultrasound power. *However, several studies show that high ultrasonic power causes major alterations in materials by inducing greater shear forces however, in the natural product industry this parameter is usually optimized in order to use the minimum power to achieve the best results. * Intensity can be expressed as energy transmitted per second and per square meter of medium. *This parameter is directly correlated with the amplitude of the sound wave; with increase in the amplitude, bubble collapse will be more violent. *

*Ultrasonic Bath * the shape of the reaction vessel is critical- best choice would be a flat bottom vessel such as a conical flask in order to attain a minimum reflection of waves. * The thickness of the vessel should also be kept to the minimum to reduce attenuation. *Probe sonicator * in the case of ultrasonic probes a rapid decrease of intensity is observed both radially and axially. For this reason a minimal space between the ultrasonic probe and the wall of the container must be respected, while ensuring that the probe does not touch the container to avoid damages on the material. * the shape and diameter of the probe may have an influence on the extraction. *

* The stepped probe gives the highest amplitude magnification (i. e. power, amplitude gain (D/d)2) of the shapes shown. * the exponential probe shape offers small diameters at its working end, which makes it particularly suited to micro applications. *

* Solvent choice is dictated by : * the solubility of the target analytes in the solvent * physical parameters such as : viscosity, surface tension, and vapor pressure of the medium. * High viscosity solvent requires high intensity ultrasound in order to obtain necessary mechanical vibrations that will result in cavitation. the * For cavitation bubbles to be effective, the negative pressure during the expansion cycle has to overcome the natural cohesive forces in the medium. The rise of viscosity increases these molecular interactions hence the cavitation threshold rises significantly. *Vapor pressure is also directly correlated with the temperature factor, which influences cavitation as well. Therefore, the solvent of choice for UAE should ideally have a very low vapor pressure and the ability to solubilize the molecules of interest. *

* The temperature increase generates the rise of the vapor pressure and the decrease of the viscosity and surface tension, inducing more solvent vapors to enter the bubble cavity, reducing the pressure difference between the inside and outside of the bubble, which will collapse less violently and reduce sonication effects. * As a consequence, at higher temperatures, cavitation can be achieved at lower amplitudes. However, the sonochemical effects of such bubbles may be reduced and the use of temperatures above a certain threshold might generate cavitation bubbles that grow very quickly, diminishing its efficacy. * When the temperature is near the solvent’s boiling point, the extraction efficiency may reduced due to the increasing of vapor pressure, since the bubble’s implosion might not induce sufficient energy shear forces to disrupt cell tissue- thus reducing the cavitation process. * It is important to choose an extraction temperature according to the target compound of extraction. Hence, a temperature control is necessary to prevent the degradation of thermo labile compounds *

* Matrix used can be either fresh or dry. Ultrasound needs an extractive medium to propagate and in order to obtain a correct diffusion of the solvent into the plant cell. * The pre-treatment of the matrix is important and can impact extraction efficiency. *The extraction yields may vary also due to plant material’s structure, compositional differences which will result in different degrees of impacts from cavitation effects. *

* Microwave AE Ultrasound AE MECHANISM: -internal temperature of the plant cells increases due to molecular movement and rupture the cell walls -solvent penetration into the cells and an intensification of the mass transfer. MECHANISM: -formation of cavitation phenomenon to rupture cell walls -solvent penetration into the cells and an intensification of the mass transfer. APPARATUS: microwave oven APPARATUS: not required microwave oven FREQUENCIES: 3 x 1011 to 3 x 108 Hz FREQUENCIES: 20 x 103 Hz Expensive and difficult in operation economic and easy in operation reduction of extraction yield: as it may Promotes higher yield of extraction modify the chemical structures of the target compounds

*Ultrasound-assisted extraction makes use of physical and chemical phenomena that are fundamentally different compared with those applied in conventional extraction techniques. *Ultrasound extraction process can produce green extracts in concentrate form, free from any residual solvents or contaminants. *The new ultrasound systems developed to date offer net advantages in term of yield and selectivity, with better extraction time, extract quality and safety, easily integrated in industry, and are environmentally friendly *