Improving fatty acid methyl ester production yield in a lipase-catalyzed process using waste frying oils as feedstock

The application of waste frying oil (WFO) mixed with rapeseed oil as a feedstock for the effective production of fatty acid methyl esters (FAME) in a lipase-catalyzed process was investigated. The response surface methodology (RSM) was used to optimize the interaction of four variables: the percentage of WFO in the mixed feedstock, the methanol-to-oil ratio, the dosage of Novozym 435 as a catalyst and the temperature. Furthermore, the addition of methanol to the reaction mixture in a second step after 8 h was shown to effectively diminish enzyme inhibition. Using this technique, the model predicted the optimal conditions that would reach 100% FAME, including a methanol-to-oil molar ratio of 3.8:1, 100% (wt) WFO, 15% (wt) Novozym 435 and incubation at 44.5 °C for 12 h with agitation at 200 rpm, and verification experiments confirmed the validity of the model. According to the model, the addition of WFO increased FAME production yield, which is largely due to its higher contents of monoacylglycerols, diacylglycerols and free fatty acids (in comparison to rapeseed oil), which are more available substrates for the enzymatic catalysis. Therefore, the replacement of rapeseed oil with WFO in Novozym 435-catalyzed processes could diminish biodiesel production costs since it is a less expensive feedstock that increases the production yield and could be a potential alternative for FAME production on an industrial scale.