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Biodiesel is fatty acid methyl ester derived from vegetable oils and/or animal fats that reacts with an acyl acceptor. Biodiesel is beneficial in sustaining a healthy environment because of its biodegradable properties, renewabilty, and nontoxicity. Biodiesel is produced by a transesterification process either with or without a catalyst. Transesterification through enzymatic routes gives an advantage in high product purity, easy recovery and, green technology process. However, enzymatic catalysis is a timeconsuming process, where it is the slowest pathway in biodiesel production. Thus, the application of ultrasonics in enzymatic transesterification is helping in reducing the reaction time. This review presents the advantage of ultrasonic assisted enzymatic transesterification over conventional enzymatic transesterification by considering the ultrasonic mechanism in accelerating the transesterification process.

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Nyamplung seed (Calophyllum inophyllum L.) oil is a prospective non-edible vegetable oil as biodiesel feedstock. However, it cannot be directly used in the alkaline catalysed transesterification reaction since it contains high free fatty acid (FFA) of 19.17%. The FFA content above 2% will cause saponification reaction, reducing the biodiesel yield. In this work, FFA removal was performed using sulfuric acid catalysed esterification to meet the maximum FFA amount of 2%. Experimental work and response surface methodology (RSM) analysis were conducted. The reaction was conducted at the fixed molar ratio of nyamplung seed oil and methanol of 1:30 and the reaction times of 120 minutes. The catalyst concentration and the reaction temperature were varied. The highest reaction conversion was 78.18%, and the FFA concentration was decreased to 4.01% at the temperature of 60℃ and reaction time of 120 minutes. The polynomial model analysis on RSM demonstrated that the quadratic model was the most suitable FFA conversion optimisation. The RSM analysis exhibited the optimum FFA conversion of 78.27% and the FFA content of 4%, attained at the reaction temperature, catalyst concentration, and reaction time of 59.09℃, 1.98% g/g nyamplung seed oil, and 119.95 minutes, respectively. Extrapolation using RSM predicted that the targeted FFA content of 2% could be obtained at the temperature, catalyst concentration, and reaction time of 58.97℃, 3%, and 194.9 minutes, respectively, with a fixed molar ratio of oil to methanol of 1:30. The results disclosed that RSM is an appropriate statistical method for optimising the process variable in the esterification reaction to obtain the targeted value of FFA.