3.1. Effect of reaction medium
In the present work, the esterification reaction of stearic acid with solketal (Scheme 1) has been studied, aiming at producing monostearin. For this purpose, free CaL B lipase has been encapsulated in microemulsions and used to catalyze the esterification reaction. Fig. 1 shows the reaction profile for the esterification of solketal with stearic acid in AOT microemulsion catalyzed by CaL B. As it can be seen, the reaction takes place with high conversion yield, namely 80% in 30 min. The choice of the microemulsion used for enzyme immobilization is important for achieving the optimum reaction system. For this purpose, two microemulsions based on lecithin or AOT as a surfactant have been used for CaL B immobilization. The conversion yield of the esterification reaction of stearic acid with solketal (100 mM/1:1) at 40 ◦C catalyzed in both microemulsions has been monitored. As can be seen in Fig. 1, the reaction that has taken place in AOT microemulsions showed high conversion yield. More specifically, 80% of conversion yield was observed in 30 min by using only 0.9 mg of lipase. For the reaction catalyzed by CaL B in lecithin microemulsions,the conversion yield was much lower, namely 42% after 2 h of incubation (data not shown). For this reason, the AOT
microemulsion system was selected to be used in further studies. It should be mentioned here that the reactions catalyzed by commercial immobilized lipases demonstrated higher conversion yields, however, longer time was required and the amount of enzyme used was much higher (50 mg instead of 0.9 mg) [9], therefore, the AOT microemulsion system can be considered as an improved system for obtaining monostearin synthesis compared to commercial immobilized lipases.