Over the centuries mankind has benefited from the natural materials that occur in plants. In earlier times the whole plant or an extract was used in cooking or as a medicine but nowadays the active constituents of plant extracts provide targets for the synthetic chemist. Nowadays the main uses of herbal extracts are medicinal with an ever-growing interest in nutritional supplements often termed nutriceuticals. In the last century the interest for herbal medicine declined in favour of synthetic drugs but this interest has now been revived due to the discoveries of many new and powerful drugs that are obtained from plants e.g. paclitaxel from yew tree, used in cancer therapy; tamiflu, used to combat flu diseases, has as base material shikimic acid extracted from Star Anise; quinine extracted from the bark of the cinchona tree, used for malaria treatment, artemisin extracted from Artemisia annua another drug to combat malaria, etc.
Aspirin, acetyl salicylate, is one of the most widely used medicines and is prepared by chemical syntheses – but its precursor, salicylic acid, is found in willow tree bark. Some natural remedies such as anti rheumatic ointments are also derivatives of salicylic acid. Thus methyl salicylate which is a powerful compound used to fight rheumatic pain was known and used by Native Americans and was obtained from birch tree bark.
In principle the plant world could provide a renewable resource of raw materials for pharmaceutical, cosmetics, food, fuel and chemical industries. With current interests focussing on green and renewable sources for these industries a new term “biorefinery” has appeared to cover emerging technologies based in this area.
The use of ultrasound can enhance the extraction process by increasing the mass transfer between the solvent and plant material. In addition the collapse of cavitation bubbles leads to better cell disruption via the formation of microjets due to asymmetrical bubble collapse near a solid surface. This allows for improved solvent penetration into the plant body itself and can also break down cell walls. As a consequence, employing ultrasound in the use of plant extraction has benefits in increased mass transfer, better solvent penetration, less dependence on solvent used, extraction at lower temperatures, faster extraction rates and greater yields of product . These features make sonication an attractive proposition for many extractions and for scale-up it should be used in the extraction unit itself where the plant material is in direct contact with the solvent.