4.2.5. The co-solvent assisted method
This is viewed as a new greener biotechnological method because of minimum waste generation and low energy demand. The co-solvent creates a single phase that gives rise to increased reaction rates within a short time. Among the researched co-solvents such as tetrahydrofuran (THF), 1-4-dioxane, acetone and di-ethyl ether, THF was the first reported co-solvent used for biodiesel production [5].
4.2.6. The ultrasonic-assisted method
The immiscibility of alcohol and vegetable oils hinders mass transfer rates between the phases. This limits the reaction rates and the yield of biodiesel. Widayat and Kiono [48]employed ultrasonic irradiation at a frequency of 40 Hz to produce cavitation under constant temperature of 40 °C during RSO transesterification. Their findings showed that ultrasound can reduce process time and increase biodiesel yield significantly.
4.2.7. The microwave-assisted method
This is an alternative heating method to the conventional method of heating [61]. Microwave is non-ionizing radiations with low energy. The changing electrical field component of microwave radiation interacts with the dipoles of the reactants molecules increasing their rotations. This reflects an increase in reaction rates and higher biodiesel yield in shorter time, as compared to conventional systems [61].