Читать книгу Biodiesel Technology and Applications - Группа авторов - Страница 22

Presence of water is not only required for chemically catalyzed biodiesel production but also very much required for enzymatic biodiesel production. It helps in maintaining enzyme structural confirmation and stability so it directly affects activity of enzyme. Oil-water interface is required for enzyme-substrate complex to proceed and water helps to increase this interfacial area [44]. So, without water, transesterification is not possible and absence of water can lead to permanent or temporal changes in protein (enzyme) structure. If water content is minimal, then increase in water concentration moves the reaction equilibrium toward more hydrolysis. Thus, it enhances reaction rate by providing greater stability to enzyme [45]. Excess of water content also has some negative effects on the reaction as well as on enzyme. Excess water content can be accumulated in the reaction medium and within enzyme active site, that leads to decrease the reaction rate as well as its alkyl ester yield [46]. So, concentration of water should be optimally perfect in order to gain maximum benefit from it. Every enzyme has its specific water content requirement, i.e., optimal water requirement, at which that particular enzyme performs its best [47, 48]. Optimal water content not only provides great support, flexibility, and stability to the enzyme but also maximizes transesterification yield by diluting methanol that has an inhibitory effect on enzyme. Factors that determine optimal water content include feedstock and type of solvent used, enzyme, and its immobilization technique used [48]. Chaudhary et al. [49] studied the effect of water content in lipase catalyzed transesterification. At low water activity (a_w = 0.33), synthetic activity of enzyme was increased and at high water activity (a_w = 0.96) enzyme became more hydrolytically active. They tested various enzymes/lipases at different water activity to check transesterification rates. The lipase from Aspergillus niger was found more prominent to give maximum transesterification rate of 0.341 mmolmin⁻¹ mg⁻¹ at a_w = 0.75. Measuring water content as weight percentage is a better choice and more convenient to use than water activity (a_w), measured by Karl-Fischer method [50]. Maximum methyl ester yield was at water concentration of 10-15% while increasing water content from 0% to 40% to study the effect of water in conversion of salad oil into methyl ester. But after much increased water concentration, methyl ester yield became very low. So, for maximum transesterification yield, optimum water concentration is required.