Study On Lipase-catalyzed Esterification Of Starch Using Oleic Acid Under Microwave

As a new type of modified starch, oleic acid starch ester (OSE) has a broad development prospect in the field of modified starch. Unlike chemical esterification, enzymatic esterification is an environmentally friendly method which occurs under milderconditions. It is very important to carry out the study of biosynthesis of OSE. However, some key problems limited its further development. e.g. starch granule structure is tight; active hydroxyls is hidden deep within the molecule; the polarity between starch and oleic acid is very different etc. To solve the above problems, the advantages of microwave chemical and biocatalysis in organic macromolecular synthesis were combined. OSE was obtained using microwave-assisted enzymatic transformation of starch. The main contents are as follows:(1) The effect of the ultrasonic treatment, enzymatic hydrolysis, chemical activation on molecular structure, granular morphology of maize starch were investigated respectively using XRD, SEM and starch-iodine compound absorption spectrum. Combined with the properties of pretreatment starch, and compared analysis the effect of pretreatment method on the DS of starch ester and the initial rate of enzymatic reaction, the ultrasonic pretreatment method were chosen as at the subsequent test.(2) By the single factor experiment, the effects of the lipase types, microwave power, microwave time, oleic acid content, lipase loading, solvent loading on the DS were investigated. On the basis of single factor experiment, the perpendicular experiments were designed to optimize the condition:microwave reactive power was84w, microwave time was10.5min, oleic acid mass fraction was22%(in starch dry basis), lipase mass fraction was4.5%.(3) The FTIR showed that a new absorption peak of OSE was not obvious at the wavelength of1724cm-1and the weaking intensity of the peak which was between3146～3631cm-1due to hydroxyls bond replaced. The XRD and SEM showed that esterification occurred mainly in the amorphous area and the hurt of starch particle was slight. Crystal pattern did not changed and was still the type A. Compared with the native starch, the maximum absorption peak of OSE increased obviously and moved to the lower wavelength in starch-iodine compound absorption spectra.The physical-chemical property of OSE synthesized by microwave-assisted enzymatic was studied. Compared with the native starch, the viscosity was significantly decreased, with the shear-thinned phenomenon, belonged to the non Newtonian fluid. With the increase of the substitution of degree, decreased transparency but higher than the native corn starch,weakened retrogradation, improved freeze-thaw stability, increased emulsifying stability, and reduced digestion property of OSE were observed.