Production of triacylglycerols rich in palmitic acid at sn-2 position by lipase-catalyzed acidolysis

Abstract

This paper studies the synthesis of triacylglycerols (TAGs) rich in palmitic acid (PA) at sn-2 position from palm stearin (PS), a vegetable oil highly rich in PA (60%, but only 12.8% of this is located at sn-2 position). These PA rich TAGs were obtained by lipase-catalyzed acidolysis of this oil with free fatty acids (FFAs) highly rich in PA, such as commercial PA (98% PA) and a FFA extract obtained by saponification of PS (60% PA). PA has a melting point of 63 ºC and during the acidolysis reaction the substrates, highly rich in this acid, must remain liquid; therefore high temperatures or solvents must be used. An important objective of this work was to operate without solvent and at the lowest possible temperature. In this acidolysis reaction four factors were firstly studied: type of lipase, temperature, solvent amount and the intensity of treatment (IOT = lipase amount × reaction time/PS amount). The influence of these variables was studied in a stirred tank reactor (STR). The lipases tested were Novozym 435 from Candida antarctica (immobilized on a macroporous acrylic resin), and lipases QLC (immobilized on diatomaceous earth), and QLM (non immobilized), both from Alcaligenes sp., and the one selected was lipase QLC. According to the manufacturer the optimum temperature for this lipase is 65-70 ºC, which allows it to operate without solvent. The best results with lipase QLC (TAGs with 80% PA, both total and at sn-2 position) were obtained with commercial PA, at 65 ºC, a 3:1 FFA/PS molar ratio (1:1 w/w), without solvent and an IOT = 7 g lipase × h/g PS (for example 2.5 g PS, 2.5 g commercial PA, 0.75 g lipase and 24 h). These results were the basis for establishing the operational conditions to obtain PA rich TAGs with the lipase immobilized in a packed bed reactor (PBR), operating by recirculation of the reaction mixture through the lipase bed. In this system TAGs with 75% PA were obtained at an IOT = 8 g lipase × h/g PS. This result and the apparent kinetic constants obtained in both reactors show that the reaction rate is lower in the PBR than in the STR. Subsequently, PA enriched TAGs were separated from FFAs by two procedures: the first one at room temperature and in presence of hexane and the second one at 65 ºC and without hexane. Using the first procedure, 95% of TAGs in the acidolysis reaction mixture were recovered with a purity of 99%. Using the second one, 98% pure TAGs were obtained with a recovery yield of 80%. Therefore, these highly rich PA TAGs can be obtained by acidolysis of PS and PA rich FFAs in solvent-free media, and then these TAGs also can be purified to 98% in absence of hexane, using only a hydroethanolic KOH solution.