Solvent-free lipase-catalyzed synthesis of long-chain starch esters using microwave heating: Optimization by response surface methodology

Starch is an abundant natural polysaccharide that is inexpensive, renewable, and fully biodegradable. Modification of starch O–H groups by esterification to form an appropriate degree of substitution imparts thermoplasticity and water resistance to the starch ester over the unmodified starch. Unlike chemical esterification, enzymatic esterification is an environmentally friendly method which occurs under milder conditions. A non-commercial CaCO3-immobilized lipase from Staphylococcus aureus (SAL3) was used to catalyze the esterification reaction between oleic acid and starch in pure substrate conditions using microwave heating followed by liquid state esterification. Response surface methodology based on three variables (the reaction temperature, the amount of lipase and the molar ratio of starch/oleic acid) at three levels was adopted to optimize the experimental conditions of the starch oleate synthesis. The optimal conditions for achieving 76% conversion with a degree of substitution (DS) of 2.86 are 386 IU of immobilized lipase, a starch/oleicacid molar ratio of 0.18 during 4 h of incubation at 44 °C. The structure of the modified starch was checked by 13C NMR spectroscopy, FT-IR and differential scanning calorimetry (DSC). Results showed that the α-amylolysis of the starch was significantly affected after esterification. The hydrophobic starch fatty acid esters produced may have potential industrial applications such as surface coating materials, flavoring agents in food industry and biomedicals for bone fixation and replacements.