| In this study,rice starch was hydrophobically modified by octenyl succinic anhydride(OSA)to obtain esterified starch.Oil in water(O/W)Pickering emulsions were prepared by using rice esterified starch as particle emulsifier and four different types of oils(coconut oil,corn oil,olive oil and sunflower oil)as oil phases.The effects of formulation parameters(oil phase type,starch concentration,and oil phase volume fraction)on the emulsification stability of Pickering emulsion were studied by laser particle size analyzer,rheometer and confocal laser scanning microscope(CLSM).In addition,an in vitro simulated oral-gastric-intestinal three-stage digestion model was established,and the effects of different oil types and starch concentrations on the gastrointestinal digestion characteristics of starch-based emulsions were discussed.On this basis,curcumin was selected as a fat-soluble bioactive substance to construct a curcumin-loaded Pickering emulsion delivery system.The physical stability of curcumin Pickering emulsion and the protective effect of emulsion on curcumin were evaluated.The effects of different oil types and starch concentrations on the bioavailability of curcumin were investigated by simulating in vitro digestion.The main research contents and conclusions are as follows:(1)Rice starch was modified by OSA to obtain hydrophobically modified rice starch.The physicochemical properties of rice modified starch were studied by Fourier transform infrared,scanning electron microscope and rapid visco analysis.The physicochemical properties of the four raw oils were characterized by gas chromatography.Pickering emulsion systems were constructed by using rice modified starch as particle stabilizer and four different types of oil as oil phases,respectively.The effects of different oils on the stability of emulsion were studied from the aspects of particle size distribution,microstructure and rheological properties of emulsion.The results showed that there were obvious holes on the surface of starch granules after OSA modification of rice native starch.The hydroxyl groups of starch molecules were replaced by the carbonyl groups of OSA,but the morphological structure remained granular,which could be used as a particle emulsifier to prepare Pickering emulsion.The particle size of starches increased from 2.77-8.66 μm to 3.53-9.95 μm.Hydrophobic modification significantly increased the peak viscosity of native starch and reduced the gelatinization temperature.The fatty acid composition of the four oils showed that the saturations of the four oils from high to low were: coconut oil(91.41%)> olive oil(16.58%)> corn oil(14.63%)> sunflower seed oil(10.85%).The results of physical stability of Pickering emulsion showed that the four kinds of oil could form stable emulsion when rice modified starch was used as particle stabilizer.With the extension of storage time,the cream indexs of different emulsions decreased and the droplet sizes increased.Among them,corn oil emulsion had the highest emulsification stability,and its cream index was the highest(91.73%)after 80 days.Followed by olive oil emulsion(83.53%),there was no significant difference between the cream index of coconut oil and sunflower oil emulsion.After 15 days of storage,the droplet size of the corn oil emulsion was the smallest,and the droplet size of the coconut oil emulsion was the largest.The shear rheological results showed that the viscosity of emulsions with different oil phases decreased rapidly with the increase of shear rate,and then gradually stabilized.The viscosity of emulsion with coconut oil was the lowest,and the viscosity of emulsion with corn oil was the highest.The storage modulus(G ′)of all emulsions was higher than the loss modulus(G "),and the loss tangent(Tanδ)was less than 1,indicating that the emulsions had gel rheological behavior.The results of CLSM showed that starch particles were uniformly wrapped on the surface of oil droplets at the oil-water interface of the emulsion,which effectively blocked the aggregation between oil droplets and improved the stability of the emulsion.Compared with coconut oil and sunflower seed oil emulsions,the emulsions of corn oil and olive oil had smaller and uniform droplets,and the physical stability of both were higher(p < 0.05).(2)O/W Pickering emulsions with different starch concentrations and oil phase volume fractions were prepared by using rice modified starch as particle stabilizer and four oils with different fatty acid compositions as oil phases.The effects of formulation parameters on the physical stability of the emulsion were evaluated by particle size distribution,CLSM and Cryo-scanning electron microscope(Cyro-SEM).An in vitro oral-gastric-intestinal three-stage digestion model was established to investigate the effects of different oil types and starch concentrations on the gastrointestinal digestion characteristics of starch-based emulsions.The results showed that when the starch concentration was in the range of 0.5% to 4.0%,the higher the starch concentration was,the smaller the droplet size was and the more stable the emulsion was.When the oil phase volume fraction increased in the range of 25%-70%,the droplet size of the emulsion increased and the cream index increased.CLSM results showed that all oil droplets were evenly distributed in the emulsion except for coconut oil emulsion(1.0% starch concentration),and starch granules tightly surrounded the oil droplets to prevent them from aggregating.In addition,the continuous phase dispersed starch granules increased the viscosity of the system and enhanced the stability of the emulsion.The results of Cyro-SEM showed that the starch particles were still tightly adsorbed on the surface of the droplets at lower starch concentration,but the droplet size was larger and there was no particle bridging between the droplets.When the starch concentration increased to 2.0%,the particle bridge and network structure could be observed between the oil droplets,and the droplet size decreased with the increase of starch concentration.In addition,starch granules were embedded at the oil-water interface.The emulsion after oral digestion,flocculation and aggregation occurred,and the droplet size increased.After gastric digestion,the amount of starch granules wrapped around the droplets were reduced,but the structure of the emulsion was still maintained.After intestinal digestion,the structure of the emulsion was completely destroyed,most of the oil was hydrolyzed,and the size of the oil droplets was significantly reduced.It showed that the starch-based Pickering emulsion system has good stability under strong acid gastric digestion conditions.The starch-based emulsions with different oil phases were studied by simulated in vitro gastrointestinal digestion.The results showed that the order of free fatty acids(FFA)release rate was as follows: coconut oil(25.71%)> olive oil(12.64%)> corn oil(11.16%)>sunflower seed oil(8.99%).Under the same emulsification conditions,the difference in lipid digestibility of emulsions with different oils was mainly due to the difference in saturation: coconut oil(91.41%)> olive oil(16.58%)> corn oil(14.63%)>sunflower seed oil(10.85%)and initial droplet size.When the concentration of starch granules increased from 0.5% to 4.0%,the FFA% of the four oils increased significantly(p < 0.05),indicating that the increase of starch concentration had a positive effect on the release of FFA.When the oil phase volume fraction was in the range of 25%-70%,FFA% decreased with the increased of oil phase volume fraction,so oil phase volume fraction was negatively correlated with FFA%.(3)The Pickering emulsion delivery system loaded with curcumin was constructed by using rice modified starch as food-grade particle stabilizer,coconut oil,corn oil,olive oil and sunflower oil as oil phase,and curcumin as fat-soluble bioactive substance.The physical stability of curcumin Pickering emulsion delivery system,the protective effect of emulsion on curcumin and the bioavailability of curcumin were evaluated by analyzing the cream index,curcumin retention rate and bioavailability of curcumin-loaded Pickering emulsion.The results showed that the four kinds of emulsions with rice modified starches had good physical stability.After stored at25℃ for one month,the emulsions with olive oil and corn oil had the best physical stability,followed by the emulsions with sunflower seed oil and coconut oil.The retention rates of curcumin in Pickering emulsions of four oils decreased continuously within 30 days.Overall,the retention rate of curcumin in corn oil emulsion was the highest,followed by olive oil emulsion.In the late storage period,the retention rates of curcumin in sunflower oil and coconut oil emulsions decreased significantly(p <0.05).The results of UV irradiation stability of curcumin showed that the content of curcumin in the emulsion decreased with the increase of UV irradiation time.After108 h of UV irradiation,the retention rate of curcumin in the emulsion with corn oil was relatively the highest,followed by olive oil and sunflower seed oil emulsions,while the degradation of curcumin in coconut oil emulsion was the most significant,and its protective effect on curcumin was also the worst.In the simulated intestinal digestion stage,FFAs in the oils of four emulsions were released rapidly in the first 30 min,and the release rate gradually slowed down in the range of 30-120 min.The FFA% trends of Pickering emulsions of four oils were: coconut oil(30.74%)>sunflower seed oil(15.06%)> corn oil(12.67%)> olive oil(12.38%).The bioavailability of curcumin in different emulsions was: sunflower seed oil(78.01%)>coconut oil(64.56%)> corn oil(54.58%)> olive oil(52.51%).Except for coconut oil,FFA% rates in the emulsions were positively correlated with the bioavailability of curcumin.For the same oil,with the increase of starch concentration,the initial particle sizes of emulsion droplets decreased,and the bioavailability of curcumin in emulsion increased significantly(p < 0.05). |
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