Lipase-Catalyzed Synthesis Of Starch Stearate In Ionic Liquids And Its Characteristics

Starch stearate is a typical kind of modified starch, in traditional ways, the modification of starch is usually conducted in organic solvents. But the traditional organic solvents are poor in stability and operation, low in safety. Room temperature ionic liquids can overcome the shortcomings above as a green solvent for its unique physicochemical properties, and reflect advantages in the aspects of starch dissolution and its modification. At the same time ionic liquids can provide a different environment for chemical reactions from traditional solvents, which can further improve the catalyst activity, conversion rate by changing the reaction mechanism. The purpose of this paper is to realize the homogeneous lipase-catalyzed synthesis of starch stearate in ionic liquids, identify its structure and study its properties, and explore the influnce of lipase activity regulation on esterification under this system.Homogeneous lipase-catalyzed synthesis of starch stearate were conducted in ionic liquids mixtures consisting of 1-butyl-3-methyl-imidazoliumacetic([BMIM]Ac) and 1-butyl-3-imidazolium tetraflouroborate([BMIM][BF4]), the influnce of reaction conditions on the degree of substitution of starch stearate under this reaction system such as the mass ratio of two ionic liquids, enzyme amount, the dosage of methyl stearate, reaction temperature and reaction time were studied. Identified the structures by means of XRD、FTIR、1H NMR. Optimum modification conditions for maximum degree of substitute (DS 0.096) of starch stearate were shown as following:rice starch 1.2g, Candida rugosa lipase (CRL) dosage 0.1 g, reaction temperature 60 ℃, reaction duration 3 h, total mass of ILs 24 g, mass ratio of [BMIM][BF4]/[BMIM]Ac 19:5 and molar ratio of methyl stearate/anhydroglucose units(AGU) 3:1.A series of physicochemical properties of starch stearate such as transparency, solubility, emulsification, anti-precipitability, freeze-thaw stability, RVA pasting characteristics, rheology properties, thermal stability and film forming characteristics of different degrees of substitution of starch stearate were determined. The results showed that the transparency and solubility of the starch increased after esterification. The emulsification, anti-precipitability and freeze-thaw stability of starch stearate got enhanced, whearas its thermal stability reduced; RVA pasting characteristics showed that viscosity decreased with the degree of substitute increased; dynamic rheology studies showed that the storage modulus and the loss modulus of starch significantly dropped after esterification; static rheology studies showed that starch and starch stearate were both non-Newtonian fluids, which exhibited shear thinning phenomenon. The water vapor permeability of starch stearate-polyvinyl alcohol gradually decreased with the increasing degree of substitution of starch stearate, while tensile strength and elongation at break first increased and then decreased.The influence of lipase activity regulation on the synthesis of starch stearate in ionic liquids were researched from three aspects:water activity, pH value and freeze-drying protective agent. The results showed that degree of substitution of starch stearate increased with increasing water activity. When water activity reached 0.61, the degree of substitution reached maximum value of 0.104. The amount of hydrated salts(Na2HPO47/2 Aw0.61) had significant effects on catalytic efficiency of the reactions; pH values had certain effect on the catalytic ability of enzyme, and the highest degree of substitution could be obtained-when the lipase was freeze-dried in a buffer solution with pH 7.1; The lipase that lyophilized with organic compounds such as glycine or polyethylene glycol had higher catalytic efficiency in ionic liquids, the protective effect of polyethylene glycol is the best.