Influence Of Pretreatment On Structure And Chemical Reaction Activity Of Starch

Starch is an abundant natural polysaccharide that is cheap, and fully biodegradable. It is usually modified physically or chemically to achieve a particular property and cater to the requirements for tailor-made. However, the intact starch granules inhibit chemical reagents from making contact with the molecules in the crystalline region, thus the chemical reactivity and reaction efficiency of native starch is usually low. Therefore, there are many methods destined to modify the structure in the crystalline region, or decrease the size of crystalline regions to increase reactivity of starch.In this thesis, native corn starch (NCS) was pretreated with NaOH/urea/H2O solution at cool temperature, the effects of pretreatment on crystal structure, granular morphology and functional groups were investigated, respectively, by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared (FT-IR) spectroscopy. During the pretreatment, the crystal structure of starch was damaged and the crystallinity decreased. X-ray diffraction revealed that PCS had the VH type crystalline pattern, which was different from NCS. It was found by scanning electron microscopy (SEM) that the PCS was spherical and the size of most PCS was about 500 nm. FT-IR showed that there were no new functional groups produced during the pretreatment process. The pretreatment effects on physicochemical properties of corn starch were investigated by analyzing the influence of pretreatment on cold-water solubility, transparency, viscosity and thermal stability. The pretreatment was evidenced to cause changes of physicochemical properties of corn starch: the degrading of the crystalline regions allowed greater entry of water into the interior of the granule thus led to the greater cold-water solubility and transparency of PCS, the viscosity of PCS decreased. In addition, the PCS also exhibited lower thermal stability.Using long-chain fatty acid, methyl laurate and octenyl succinic anhydride as esterification reagents, NCS and PCS were esterified under solvent-free conditions. Moreover, using the degree of substitution (DS) as an evaluating parameter, the NaOH/urea pretreatment effects on chemical reaction activity of starch were investigated by analyzing the influence of pretreatment on DS. The optimal reaction conditions were confirmed through orthogonal test, effects of molar ratio of the raw material, reaction temperature and reaction time on the degree of substitution (DS) of starch esters were studied. The products are characterized by FT-IR, 13CNMR, SEM, X-ray diffraction and TG. It shown that the smaller size and more amorphous domain of PCS improved the degree of substitution (DS) and reaction efficiency (RE) of PCS. The esterification occurred in the amorphous domain, thus the crystalline pattern did not changed but the crystallinity increased. The introduction of the long alkyl chains led to the better thermal stability of the starch ester. Lauroyl groups in the starch ester led to the better water resistance and emulsifying properties, and also resulted in the drop in viscosity. While the octyl alkenyl succinic acids in the starch ester enhanced the hydrophilicity and thus the viscosity increased.