| China produced approximately 180 million tons paddy every year. The old rice and the broken rice produced during rice milling are good sources of starch. However, the use of native rice starch is limited by its high pasting temperature, low pasting transparency, low solubility and easily retrograde commonly applied to processed foods. Mechanical activation is one of the safe, effective and environment-friendly modifications which can be utilized to modified starch. Mechanical activation could not only increase the reactive activation but also sharply change the characteristics of starch. Recently, the study of octenyl succinic starch (OSA starch) is one of the main topics of modified starch. Especially, the low-viscosity OSA starch, which has good film forming and emulsifying properties, can be used as wall materials of lipid microcapsule.In the present study, firstly, the MARS (mechanically activated rice starch) was prepared by dry ball-milling with different moisture content (about 1.00%,3.50%,6.00%, 8.50% and 11.00%). The effects of moisture content on the granules agglomeration, structure characterization and physicochemical properties of MAIS during mechanical activation were studied and suitable moisture content for mechanical activation was obtained. In addition, the mechanism of mechanically activated starch at molecular level was investigated. Then the properties of OSA rice starch were studied which prepared by different modified methods, including modified by OSA after mechanical activation (OSA-MARS), mechanical activation after modified by OSA (mechanically activated OSA-NRS (native rice starch)) and modified by OSA during mechanical activation. Lastly, lipid microcapsule was successfully produced by low-viscosity OSA rice starch prepared by mechanical activation and the properties were studied. The objectives of the study were to supply theoretical basis and experimental data of preparation and application of OSA starch. The main results were as follows:1. The effects of ball-milling conditions and chemical compositions on the mechanical activation of rice starchAccording to the effects of ball-feed ratio and the rotational speed on the cold-water solubility, reducing power, blue value (BV) and kinematic viscosity dissolved in boiling water bath, in the experimental range, the higher the ball-feed ratio and the rotational speed, the stronger the mechanical force to starch, which resulted in higher cold-water solubility and reducing power, and lower BV and kinematic viscosity dissolved in boiling water bath. When rotational speed was below 350 r/min, the cold-water solubility, reducing power and kinematic viscosity dissolved in boiling water bath changed slowly. In addition, the protein and fat in starch made negative effects on the mechanical activation.2. Structure characterization and physicochemical properties of MARSThe effects of mechanical activation on the morphology, structure characterization and physicochemical properties of IRS with different moisture contents (about 1.00%, 3.50%,6.00%,8.50% and 11.00%) were studied by SEM, X-ray diffractometry, GPC, HPLC, FT-IR,’H-NMR, 13C-NMR, RVA, Rotating Visco Analyser and capillary viscometer. The results were as follows:(1) The optimum moisture content of MARS It was found that the moisture content made significant effects on the structure and macroscopical properties of MARS. When rice starch was treated by mechanical force with the moisture content of 6.00%0-8.50%, the structure and physicochemical properties changed most sharply and the activation efficiency was the highest. Below or above 6.00%-8.50% made lower activation efficiency, which was the optimum moisture content for mechanical activation. However, the reason and the mechanism needed further research.(2) The effects of mechanical activation time and moisture content on the granules agglomeration of MARS The shape of rice starch granule changed from native polyhedron to anomalistic state when treated by mechanical force. The change of granules size was related to the moisture content of starch before mechanically activated. When mechanically activated at moisture content of IRS (Indica rice starch) was 1.05%, Rosin-Rammler mean granules size decreased from 4.53μm of native starch to 1.94μm of 50 h, but had a little increase of 100 h (2.47μm). When the moisture content of IRS was 6.02%, Rosin-Rammler mean granules size decreased from 4.69μm of native starch to 3.52μm of 25 h, but increase to 4.07μm of 50 h and 7.02μm (100 h). However, when the moisture content of IRS was 11.05%, Rosin-Rammler mean granules size gradually increased from 4.73μm (0 h) to 11.87μm of 100 h as mechanical activation time prolonged. It was concluded that mechanical activation time and moisture content made effect on the agglomeration of starch granules in varying degrees.(3) The effects of mechanical activation time and moisture content on the structure of MARS Mechanical activation resulted in destroying of starch crystal structure which disappeared basically after activated for 10 h. Mechanical activation also decomposed both amylopectin and amylose of rice starch and decreased the molecular weight. When IRS was mechanically activated for 100 h at 1.05%,6.02% and 11.05% of moisture content, the weight-average molecular weight of intermediate molecular weight fraction and smaller molecular weight fraction decreased to 11.42×104 g/mol,6.63×104 g/mol and 9.89×104 g/mol, respectively. The moisture content made significant effects on the structure of MAIS (mechanically activated Indica rice starch). When moisture content was 6.00%-8.50%, the rice starch was easily affected by mechanical force and the decreasing rate of crystallinity and molecular weight was the highest. FT-IR,1H-NMR and 13C-NMR spectra curve of native and MARS were almost the same, which indicated that mechanical activation decomposed the molecular chain of starch, but had no effect on the glucose unit of starch molecule.(4) The effects of mechanical activation time and moisture content on the physicochemical properties of MARS When mechanical activation time prolonged, the crystal structure was destroyed and the molecular chain of rice starch was decomposed, so the gelatinization degree, cold-water solubility, transparency, reducing power and storing stabilization of rice starch increased; the BV and the viscosity decreased. When moisture content was 6.00%-8.50%, rice starch was easily affected by mechanical force, which resulted in a higher change rate of gelatinization degree, cold-water solubility and transparency. In the experimental range,10 h of mechanical activation time was the dividing point of changes of crystal structure and molecular weight. Before 10 h, the crystal structure of starch damaged sharply and the kinematic viscosity of rice starch dissolved in cold-water and pasting temperature increased linearly. However, after 10 h, the crystal structures disappeared basically and the molecular weight decreased sharply, which resulted in a logarithmic decrease of kinematic viscosity of rice starch dissolved in cold-water and gelatinization at room temperature.3. The study of synthesis conditions of OSA-MAISThe optimum conditions for the synthesis of OSA and MAIS prepared at the optimum moisture content 6.02% were obtained by single factor and orthogonal-rotational combination experiment. For NIS (native Indica rice starch), temperature, pH and starch slurry concentration were 34.22℃,8.26 and 36.47%, respectively, and the actual DS (degree of substitution) was 0.1706 (where OSA/starch ratio was 3%, reaction time was 4 h). For MAIS 10 (mechanically activated Indica rice starch for 10 h), temperature, pH and starch slurry concentration were 32.82℃,8.27 and 11.94%, respectively, and the actual DS was 0.02062 (where OSA/starch ratio was 3%, reaction time was 3 h). Compared with the esterification of OSA and NIS, the esterification of OSA and MAIS 10 had low sensitivity to pH, short time of reaction. 4. Structure characterization and physicochemical properties of OSA-IRS prepared by different modified methodsThe structure characterization and physicochemical properties of OSA-IRS were studied, which were prepared by different modified methods (including modified by OSA after mechanical activation, mechanical activation after modified by OSA and modified by OSA during mechanical activation) at the optimum synthesis conditions, respectively.(1) Modified by OSA after mechanical activation (OSA-MAIS) Mechanical activation time made obvious effects on the structure of OSA-MAIS. When the mechanical activation time was beyond 5 h, the diffraction peak of X-ray diffraction at 29 20°increased and an new diffractive peak appeared at 2θ13°. As the mechanical activation time prolonged, the crystallinity of OSA-MAIS increased from 3.63% of 5 h to 8.43% of 100 h. When MAIS was modified by OSA, the molecular weight increased. When MAIS 100 was modified by OSA, the weight-average molecular weight of intermediate molecular weight fraction and smaller molecular weight fraction increased from 6.63×104g/mol (before modified) to 15.50×104 g/mol. When MAIS10 was modified by OSA, the morphological structural characteristics did not show any detectable change. However, the OSA group was introduced into molecule of starch by modification, which changed the molecular conformation of starch in solution and increased the apparent viscosity, kinematic viscosity dissolved in boiling water bath, intrinsic viscosity and freeze-thaw stabilization.(2) Mechanical activation after modified by OSA (mechanically activated OSA-IRS) Mechanical activation had no significant effects on DS of OSA-NIS and the change trend of physicochemical properties was similar to the MAIS, but the change rates were higher, indicating that the OSA-NIS was mechanically activated more easily than unmodified native starch. Mechanical activation decreased the molecular weight and viscosity and could be use as a preparation method of low-viscosity OSA starch.(3) Modified by OSA during mechanical activation The esterification between OSA and starch occurred during mechanical activation. The DS increased as the mechanical activation time prolonged and increased to 0.00968 of 100 h. Adding Na2CO3 increased the esterified speed and DS compared with not addition of Na2CO3 at the same mechanical activation time. When the mechanical activation time was 5 h, the DS of OSA-IRS reached 0.01376, and could further increase to 0.01864 at 100 h.(4) Structure characterization of OSA-IRS When IRS was modified by OSA, the spectrum of FT-IR showed a new peak at 1724 cm-1 and 1572 cm-1. The new peak located at 1724 cm-1 was corresponding to the stretch of ester carbonyl groups, which confirmed the esterification between OSA and starch. When IRS was modified by OSA, the spectrum of 1H-NMR and 13C-NMR showed some new peaks atδ0.86-2.31 andδ13.87-31.82, respectively. The relative intensity of these peaks increased with an increase in DS. According to the spectrum of 1H-NMR of OSA-MAIS10 with different DS, the relative intensity of the chemical shift peak of OH2 decreased as the DS increased and the relative intensity of others chemical shift peaks did not change, which indicated that the esterification of MAIS 10 and OSA could mainly occur at OH2, but needed further confirmation.5. The film forming and emulsifying properties of OSA-IRS and preparation of lipid microcapsuleThe film forming and emulsifying properties of OSA-IRS were studied, which were prepared with different modified methods, and then preparation and properties of lipid microcapsule made by mechanically activated OSA-NIS as wall material were studied. Rice starches modified by OSA both before and after mechanical activation had good film forming and emulsifying properties. When IRS was modified by OSA during mechanical activation, adding Na2CO3 could increase the emulsifying properties, because of the increase of the DS. The lipid microcapsule produced by mechanically activated OSA-NIS as wall material had high encapsulation efficiency (70.69%-87.13%) when the apparent viscosity of starch slurry was at moderate levels 100-500 mPa-s (12 r/min). When mechanical activation time and starch concentration increased, the satiation of microcapsule increased and the microcapsule dispersed in water easily. The microcapsule had the highest thermal and mechanical stability which was produced by mechanically activated OSA-NIS 100 h with starch concentration at 35% as wall material. |
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