The main aim of various immobilization techniques is to design an efficient lipase — immobilized system for biodiesel production. A number of researches done in this field show that the design of an efficient immobilized lipase is rather a difficult task. Still researchers and organizations worldwide are focused on this task to make an efficient lipase-immobilized system with maximum number of reuses along con­tinuous production of biodiesel. The selection criteria for immobilization technique and carrier are largely dependent on the particular lipase type, the type of reaction system (aqueous, organic solvent, or two-phase system), the process conditions (pH, temperature, and pressure) and the goal of immobilization.

An immobilized enzyme has to execute two essential functions: namely, the non­catalytic functions which are designed to aid separation and catalytic functions that are designed to convert the targeting substrates within a desired time and space. Both the activity and operational stability of immobilized lipase are important. Enzyme activity yield following immobilization does not depend only on losses caused by the immobilization procedure but can be further reduced by mass transfer effects. On the other hand, improved stability of immobilized lipase under process conditions can compensate for such drawbacks, resulting in an overall benefit of lipase immobilization (Cao 2005; Knezevic 2004). Techniques for immobilization have been broadly classified into four categories based on physical and chemical retention, namely, adsorption, covalent binding, entrapment, and encapsulation (Fig. 12.1). A combination of two or more of these techniques has also been inves­tigated (Jegannathan et al. 2008).

Adsorption results in attachment of lipase to the surface of support particles. Attachment of lipase to the support can be by physical adsorption or ionic binding or both. Adsorption technique of immobilization is the easiest and least expensive technique to prepare solid support biocatalyst. The immobilized lipase does not undergo mass transfer limitations and also gives high biodiesel yield in relatively shorter time. The main disadvantage is that the enzyme can be washed off easily from the surface due to weak forces (van der Waals, hydrogen bonds, or hydropho­bic interactions) of attachment.