| Rosin acid starch esters(RAS) with degree of substitution(DS) ranging from 0.031 to 0.106 were prepared by enzymatic esterification reaction, rosin, native cassava starch(NCS), pretreated native cassava starch(PNCS) was used as substrate directly for the synthesis of RAS catalyzed by lipase in dimethyl sulfoxide solvent systems. The main contents and conclusions were as followed:In order to improve the efficiency of the enzymatic reaction, native cassava starch(NCS) was pretreated by heat-moisture, ultrasonic, gelatinized, microwave and alkali treatment before the enzymatic esterification. The activation effect of pretreatment on the degree of substitution(DS) of RAS was as follows: alkali RAS(0.048) > microwave RAS(0.0174) > gelatinized RAS(0.0153) > ultrasonic RAS(0.0099) > heat-moisture RAS(0.0073) > Native RAS(0.0053). The result was indicated that all pretreatment had increased the DS of esterified products, which means pretreatments in the experiment had really improved reactivity of starch.The effects of substrate ratio, reaction temperature, reaction time and lipase dosage on synthesis of rosin starch ester(RAS) were explored. On the basis of single-factor experiment, the reaction conditions were optimized according to the principle of the combination Box-benhnken centered design and response surface methodology analysis. The optimum synthesis conditions were as follows: starch-rosin ratio of 1:3.6(w/w), reaction time 4.2 h the reaction temperature 48.5 ℃ and lipase-starch ratio of 3:20. Under this condition, the degree of substitution(DS) 0.106 was achieved.Rosin starch esters(RSE) with different DS(DS=0.031, DS=0.057, DS=0.075 and DS=0.106) were analyzed and characterized by means of UV(ultraviolet-Visible), FT-IR(Fourier Transform Infrared Spectroscopy), 1H NMR(Nuclear Magnetic Resonance), SEM(Scanning Electron Microscopy), XRD(X-ray Diffraction), DSC(Differential Scanning Calorimetry), Thermo Gravimetric Analyzer(TGA). The data from UV analysis on the starch-iodine complexes indicated that the maximum absorption wavelength moved to longer wavelength when the DS increases. The results of FT-IR and 1H NMR analysis were showed that the product was exhibited a new band at 1728 cm-1, which was attributed to the symmetric deformation vibration of carbonyl C=O, and the product was RAS. The results of SEM and XRD analysis revealed that the morphology and crystallinity of the native cassava starch were largely destroyed, and the esterification took place in both the non-stereotyped area and inside the crystalline regions of starch. TGA and DSC analysis indicated that thermal stability of rosin acid starch decreased compared with pretreated starch.Compared with NCS, Due to the introduction of hydrophobic rosin acid group, the emulsion capacity and emulsion stability of RAS were significantly improved. the transparency decreased as the DS of starch ester increased. Due to the introduction of hydrophobic rosin acid group, the molecular interaction between starch molecule and water was weakened. So that, the physicochemical properties of the relative viscosity, intrinsic viscosity, retrogradation stability and freeze-thaw stability was improved, and these physicochemical properties were changed with the DS of RAS increased.Due to the esterification reaction is reversible, so that the kinetics of hydrolysis in DMSO of rosin acid ester was studied, the hydrolysis reaction activation energy 1006 J/mol of rosin acid starch was achieved, hydrolytic kinetic equation of RAS can be expressed as:r2=k2·CA·CAGU=1.52·e-1006/RT·CA·CAGUThe total rate equation of the esterification can be expressed as: r=r1-r2, in addition, r2<<r1, therefore, the total rate equation of the esterification can be expressed as:mrosin:mstarch=6:1 r=k21·CA·CAGU=2252·e-33380/RT·CA·CAGUmrosin:mstarch=7.2:1 r=k22·CA·CAGU=265.3·e-30802/RT·CA·CAGUmrosin:mstarch=9:1 r=k23·CA·CAGU=26.42·e-22669/RT·CA·CAGUmrosin:mstarch=12:1 r=k24·CA·CAGU=11.36·e-19418/RT·CA·CAGU... |
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