The transesterification of vegetable oil with short-chain alcohols, in the presence of base-catalyst, by means of low frequency ultrasound (28 and 40 kHz) in order to obtain biodiesel fuel was studied. By using ultrasounds the reaction time is much shorter (10-40 min) than for mechanical stirring. The quantity of required catalyst is 2 or 3 times lower. The molar ratio of alcohol/oil used is only 6:1. Normal chain alcohols react fast, while secondary and tertiary alcohols show some or no conversion after 60 min of reaction. Surprisingly, 40 kHz ultrasounds are much more effective in the reduction of the reaction time (10-20 min). Twenty eight kilohertz give slightly better yields (98-99%), but longer reaction time, while higher frequencies are not useful at all for the transesterification of fatty acids.

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. In this review we will explore the advantages that accrue from the incorporation of either ultrasound or microwaves in the extraction process. The two techniques offer different approaches in that ultrasound is generally used to improve conventional solvent extraction whereas microwaves are known for their ability to remove constituents via heating without solvents.