| Four cultivars of rice with different amylose content (AC:1.90-26.98%) were used to prepare hydrophobic rice starch particles by octenyl succinic anhydride (OSA) modification. The oil/water (O/W) Pickering emulsions were prepared using the hydrophobic modified rice starch particles with different substitution degree as emulsifier. The effects of the physicochemical properties of starch particles and emulsions preparation conditions on the stability of Pickering emulsion are studied systemicaly. The adsorption behavior of starch particles in the oil/water interfacial film and the rheological properties of the emulsions were researched by methods of optical microscope and interfacial rheometer to reveal the mechanism of stability. The study is important for the design of new functional starch based particle emulsifier, which can be used in the food and related industries. The main research contents and conclusions were as follows:(1) Four cultivars of indica rice (Tenuo 2072, Teyou 2035, II you838 and Gangyou188) were used as raw material to separate rice starch. The physicochemical properties of starch such as particle size distribution, gelatinization characteristics, and amylopectin chain length distribution were analyzed by laser particle analyzer, rapid viscosity analyzer (RVA) and ion chromatography, respectively. Results show that the average particle size (Dav) decreased with the increase of amylose content. RVA results showed that Teyou 2035 has the highest peak viscosity, while the peak viscosity of Tenuo 2072 was lowest. The gelatinization temperature (PT), hot paste viscosity, cold paste viscosity of starches increased with the increase of amylose contents.(2) Hydrophobic modified starches with different substitution degree of substitutent were prepared from four cultivars of rice starch. The effects of hydrophobic modification on gelatinization characteristics, particle size distribution and particle structure were studied. Results showed that PT was significantly lower after modification compared with the native starches. Moreover, the RVA parameters of different cultivars change greatly. AC were significantly positive correlated with HPV (P< 0.05), SBV (P< 0.05) and positive with CPV (P< 0.01). Laser particle size analysis showed that hydrophobic modification has no obvious influence on starch particle size distribution. SEM observation suggested that concave holes were formed in the surface of starch particles, but the starch still remained granule structure.(3) The amylose content and degree of substitution on the stability of Pickering emulsions stabilized by hydrophobic starch particles were studied. The microscopic structures of the emulsions were observed using optical microscope. Results showed that the emulsion had larger cream volume, smaller droplet size and uniform particle size distribution. The starch particles adsorbed on the oil-water interface and enveloped the oil droplets to form physical barrier and three-dimensional network structure, which inhibited oil droplet to coalescence. The droplet size of the emulsion stabilized by starch particles ranged from 32.99 μm to71.82 μm. The emulsion was stable for 100 days and had no obvious change. Emulsion stability were significantly positive with DS (P<0.01), while the correlate with AC was not significant (R=-0.284).(4) Teyou 2035 (DS= 0.0287) was used as particle emulsifier to prepate Pickering emulsion. The effects of different preparation conditions (particle concentration, the volume of oil phase, pH, sucrose concentration, and NaCl concentration) on the stability of emulsion were researched. Results showed that the cream volumes obviously increased with the increase of starch particles concentration, while the droplet sizes of the emulsions were decreased. With the increase of oil phase, the droplet sizes of the emulsions increaded. Cream volume significantly decreased when the oil phase concentration was greater than 50%. Emulsification of starch particle was better at pH 4-8. The stability of the emulsion decreased when pH> 9. Under different concentration of NaCl, Dav of emulsion range from 20.22 μm to 25.08 μm. NaCl concentration had no significant effect on the stability of the emulsion. With the increase of sucrose concentration, the droplet size of the emulsion firstly increased and then decreased. Dav firstly increased from 20.22 μm to 32.08 μm, then decrease to 16.30 μm.(5) Shear experiments showed that emulsion viscosity decreased with the increase of shear rate, which characterized by shear thinning behavior. The fluid index was less than 1, so the Pickering emulsion stabilized by starch particle was a typical pseudoplastic fluid. Dynamic viscoelasticity experiments showed that the emulsion have the highest emulsion elastic modulus and loss tangent when starch particle concentration was 4.0%. With the increase of oil phase concentration, elastic modulus decreased while the loss tangent increased. With the increase of pH value, elastic modulus had no significant change, but the loss tangent decreased. Pickering emulsion stabilized by hydrophobic starch particles has typical gel rheological behavior. The loss tangent decreased with the increase of G’. |
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