Enhancement Effects Of Mechanical Activation On Acetylation Of Starch

The acetylated starch is a starch derivant formed by the reaction ofhydroxyl in the starch macromolecule with acetic acid or acetate, which hasbeen widely used in the industries such as food, textile and paper manufacturing.In recent years, more and more attentions have been paid on a highly substitutedbiodegradable acetylated starch, a good substitute for petroleum-based material,because of its hydrophobic property, water and oil resistance andthermoplasticity. However, starch granule usually has a semicrystalline structure,consisting of a tight crystalline outer layer and an amorphous inner core. Theratio of crystalline region is about 25%-50vol%. It is the crystalline region thatprevents acetylating reagents from entering into the inner core of the starchgranule and weakens the acetylation effect of the starch. Therefore beforeacetylating starch, it is necessary to destroy the crystalline structure and activatethe starch by various physical and chemical and biological methods. Physicalmethod normally includes stirring gelatinization with strong shearing force,extrusion pressing and etc, while chemical method includes the reactions ofstarch with chemicals such as pyridine, alkalis, liquid ammonia, amylase andetc. In the present study, starches were first milled in a stirring-type ball mill fordifferent time. Then the mechanically activated starches were used as thestarting materials to fabricate the acetylated starch in the presence of acetylatingreagent of acetic anhydride, catalyst of methanesulfonic and reaction medium ofacetic acid. The degree of substitute (DS) is used as a parameter to evaluate theenhancing effects of the mechanical activation on the acetylating reaction ofstarch. The influences of mechanical activation time, reaction time, reactiontemperature, catalyst dosage and acetic anhydride on the DS and macro kineticsof the acetylating reaction were studied in detail. The structures of the acetylatedstarch were characterized by X-ray diffraction (XRD), different scanningcalorimeter (DSC), scanning electron microscopy (SEM) and Fourier transforminfrared (FTIR) spectroscopy. The solubility and film-forming property werealso studied preliminarily.(1) The experimental results show that the mechanical activationpretreatment could dramatically improve the DS of the acetylated cassava andmaize starch. Under the same experimental conditions, the apparent activationenergies (Ea) of the acetylating reactions of the native cassava starch, activatedcassava starch with activation time of 0.25h and 1.0h are 60.95, 48.03 and 22.98kJ.mol^-1, respectively. While the Ea of the native maize starch, activated maizestarch with activation time of 0.25h and 1.0h are 66.32, 51.92 and 25.06 kJ·mol^-1,respectively. For the native cassava starch and activated cassava starch withactivation time of 0.25h and 1.0h, the catalyst concentration orders of the reactions are 2.61, 1.27, 0.32 and the acetic anhydride concentration orders are2.82, 1.69, 0.86, respectively. While for the native maize starch and activatedmaize starch with activation time of 0.25h and 1.0h, the catalyst concentrationorders of the reactions are 2.65, 1.37, 0.82 and the acetic anhydrideconcentration orders are 2.99, 1.75, 1.31, respectively. It can be seen that themechanical activation could decrease Ea and reaction orders dramatically andtherefore weakens the dependence of the acetylating reaction on the temperature,catalyst and acetic anhydride concentration, which indicates that the mechanicalactivation processing enhances the acetylating reaction obviously.(2) The native starch, activated starch and acetylated starch have differentstructure and the difference increase with the increase of the DS. Thethermoplasticity and thermal stability of the acetylated starch prepared from themechanically activated starch are better than those of the acetylated starchprepared directly from the native starch.(3) The solubility of the acetylated starch in acetone improves with theincrease of DS. The film-forming ability of acetylated starch pretreated from themechanically activated starch is better than those of the acetylated starchprepared directly from the native starch, and the higher DS, the better thefilm-forming ability.