Starch Nanocrystal: Preparation, Dispersion, Modification And Its Emulsifying Property

Transformation of native starch form micro grade to nano grade through degradation could combine the advantages of starches and nanoparticle. This will not only expand the application areas of starch but also enrich the species of modified starches, which had important theoretical and practical values. Till now the methods for preparing starch nanocrystal(SNC) were quite limited and mainly conducted through sulfuric acid hydrolysis. However, there were disadvantages for the acid hydrolysis method, such as long duration time and low yields. This paper starting with the theory of starch structure and exploring the preparing method for starch nanocrystal with short time and high yields, and aiming to improve their dispersity and hydrophilicity through chemical modification. This could provide foundations for the application of starch nanocrystals and basic theory for the starch granule structure.The yields and preparation time of SNC were improved by high pressure homogenization(HPH) combined with acid hydrolysis. The acid hydrolysis efficiency for the waxy maize starch(WMS) increased by 10% as a result of the pretreatment of defatting. Replacing the defatting by HPH, this could reach the purpose of defatting and reduce the starch granule size at the same time. The results showed that the weight-average molar mass(Mw) of starches in granular state linearly decreased with increasing pressures/cycles, whereas the polydispersity index and the molecular density(ρ) decreased as the HPH pressures increased. The diameter of the disrupted starch granules decreased with the increase of pressures/cycles, and was adequately fitted by an empirical equation. Furthermore, a “cone-like” inside-out disruption mechanism of WMS was observed during HPH. This demonstrated that increase of HPH pressures was more significant in disrupting starch granule than the increase of HPH cycles. Finally, the hydrolysis time for preparing SNC shorten to 4 d and the yields increased by 5.4% through HPH combined acid hydrolysis method.Surface chemical compositions of SNC prepared using H2SO4 and HCl hydrolysis were analyzed by X-ray photoelectron spectroscopy(XPS) and FT-IR. The results showed that carboxyl groups and sulfate esters were presented in SNC after hydrolysis with H2SO4, while no sulfate esters were detected in SNC during HCl-hydrolysis. TEM results showed that, compared to H2SO4-hydrolyzed sample, a wider size distribution of SNC prepared by HCl-hydrolysis were observed. The higher zeta-potential and relative smaller particle distribution of SNC(hydrolyzed by H2SO4) caused more stable suspensions compared to HCl-hydrolyzed sample.The effect of different dispersion p Hs on zeta potentials, size distribution, and aggregation behavior of starch nanocrystals(SNC) prepared by sulfuric acid hydrolysis was examined. The results showed that zeta potential of starch nanocrystals decreased from-6.7 m V to-34.5 m V as the dispersion p H increased from 2.07 to 11.96. Smaller starch nanocrystals and wider distribution peaks were observed with the increase of dispersion p H. The data obtained from field emission scanning electron microscopy(FE-SEM) also indicated that the aggregated parallelepiped nanoplatelets(1.5 μm) changed to monodispersed spherical-like nanoparticles(50 nm) with increasing of dispersion p H from 2.07 to 11.96. Considering these findings, the stable SNC suspension could be obtained by adjusting the dispersion p H to the range of 7.44-9.45.An aqueous homogenous suspension of starch nanocrystal, used as fillers in polymer composites, is important for the high strength of composites. The re-dispersity of starch nanocrystal powder was significantly improved by hypochlorite(Na Cl O) oxidation, with more than 2% active chlorine(w/w). Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analyses verified the oxidation process of Na Cl O. The surface charge density increased from 0.095 to 0.131 mmol/g, while the applied active chlorine increased from 1% to 4% as determined by conductimetric titration. X-ray diffraction showed that the surface oxidation induced little degradation to starch nanocrystal. Due to the introduction of carboxyl groups to starch nanocrystal(2% active chlorine), the zeta potential of its aqueous suspension(re-dispersed from oven-dried powder) decreased to –23 m V. Consequently, a high degree of individualized starch nanocrystal was prepared and the suspension maintained the stability for at least 20 days.The hydrophobicity of SNC was improved by silicone coupling through hexadecyltrimethoxysilane(HDS). The contact angle of modified SNC increased with the increase of added HDS. The dispersion behavior of SNC in aqueous or organic solution were significantly improved after the introducing of long hydrocarbon chain. It can be homogeneously dispersed in acetone and n-hexane. When the added HDS reached to 0.3%(v/v), the particle size of SNC enlarged due to the multilayer adsorption of HDS. The relative crystallinity of modified SNC increased with the increase of added HDS due to the crystallizing of long hydrocarbon chain on its surface. Also, the modified SNC floated on the surface of water phase after HDS modification(0.3%, v/v) as a result of the reduction of its density.SNC can adsorb to the interface between oil and water forming stable Pickering emulsion due to its different wettability. The drop size of the emulsion reduced with the increase of the added SNC(≥0.2%, w/v). SNC with different size distribution were derived by controlling the duration time of the acid hydrolysis, and its effect on the stability of emulsion was explored. The stability of the emulsion stabilized by SNC increased with the reduction of the SNC size distribution, meanwhile, the drop size of the emulsion decreased. Oxidation of SNC show two opposite effects on the stability of the emulsion based on the oxidation levels. When the applied active chlorine below 1%(w/v), oxidation facilitated the stability of the emulsion due to the improvement in the dispersity of SNC. On the opposite, the emulsion become unstable as a result of the reinforcement of the repulsive forces among oxidized SNC when the applied active chlorine exceeding 2%(w/v). When the applied active chlorine was 3%(w/v), the emulsion show high stability in spite of the much larger emulsion drop size(≈200μm). The Pickering emulsion type(O/W or W/O) was controlled through adjusting the hydrophobicity of SNC, and the O/W or W/O emulsion show excellent stability as the contact angle close to 90°.