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Suitable biocatalysts are chosen for a reaction, depending on the properties they exhibit. In the acylation of cellulose, a suitable biocatalyst will require access to the macromolecular structure of the substrate cellulose molecule, to create ester bonding between the cellulose molecule and a chosen fatty acid. This can prove difficult due to the extensive intra- and extracellular hydrogen bonding holding the crystalline structure together (Wu et al., 2004; Gremos et al., 2011). Ionic liquids have been used in a pretreatment step of the cellulose, found to make the substrate more penetrable by the biocatalyst for reaction activity (Gremos et al., 2011).One of the first analyses into the enzymatic acylation of cellulose was by Sereti et al. (1998). In this study, an immobilised lipase was used, acquired from Candida antarctica, a basidiomyceteous yeast. The enzyme was first isolated from Antarctica, with hopes that it would exhibit extreme properties. When immobilised, the B-lipase is highly stable and has been found to tolerate a high variation of reaction conditions without loss in activity for production (Kirk and Christensen, 2002). This was a positive feature due to the reaction system containing ionic liquids. The biocatalysts should also be able to withstand these conditions, along with a mid-high range of temperatures, where a reaction can be carried out at 80oC (Barthel and Heinze, 2005).A further study by Gremos et al. (2011), found two other successful immobilised enzymes. This reaction also contained a pretreatment step, which in this instance was using an ionic liquid known as 1-n­-Butyl-3-methylimidazolium chloride (BMIMCI). These immobilised biocatalysts of esterase from hog liver and cutinase from Fusarium solani were found to capable of catalysing the enzymatic reaction of acylation of the Avicel cellulose in the presence of the fatty acids vinyl propionate, vinyl laurate or vinyl stearate. These enzymes were obtained and became successful choices of enzyme, judged on the assumption that the esterase and cutinase enzymes are more hydrophilic, thus function better in the reaction mixture after the pre-treatment step (Gremos et al., 2011).  Enzyme immobilisation is the procedure where an enzyme is confined onto a solid matrix or support, of a phase that is distinct from that of the substrate or product. This application of immobilisation is advantageous as it can generate a higher yield of product from the development of automation processes from their high reusability (Datta, Christena and Rajaram, 2012). The choice of immobilisation method is a crucial part of the process, determining the overall enzyme activity (Figure 2). Figure 2. Enzyme immobilisation techniques showing (a) adsorption, (b) covalent bonding, (c) cross-linking and (d) entrapment.In the Gremos et al. (2011) study, several immobilised enzymes were investigated. The lipase from C. antarctica was immobilised on acrylic resin by adsorption. This method provides high enzyme loading, through interactions such as van der Waals forces that are created between the support and the enzyme, while maintaining a thermostable state for use in the synthesis of esters. The esterase that was investigated from pig liver was immobilised on Eupergit, which supports the enzyme through covalent attachment. This type of immobilisation has the alleged advantage of having an irreversible system in the binding of the enzyme (Trevan, 1988). The last enzyme was cutinase from Fusarium solani pisi, immobilised on Accurel EP100, a macroporous polypropylene support (Sereti, Stamatic and Kolisis, 1997). Enzymes when immobilised perform better in non-aqueous solvents, in turn presenting more efficient recovery and separation of the reaction product.For furthering processes into large-scale applications, immobilisation choice becomes an important factor. A wide variety of binding processes have been investigated, with enzymes binding to almost any polymer, including cellulose. Each of the procedures should be reproducible and the enzyme stability is required to be robust, otherwise considerable loss in enzyme activity can be possible (Trevan, 1988).

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