| Starch and its hydrolysates?dextrin?usually exist as a mixture containing linear chains and branched chains,and the differences of the molecular chain structure lead to distinct restrictions in their applications.Transformation of branched chains to linear chains could not only effectively eliminate the differentiation between those molecular chains,but also achieve a high-value utilization of starch.At present,two approaches involving debranching,and separation-degradation were used to produce linear dextrin?LD?.However,the high polydispersity and randomly degradation of starch molecules lead to a lower yield and broader molecular weight distribution of LD.The aim of this study was to explore the preparation method for LD with a higher yield and narrowed molecular distribution,and further to investigate their phase behaviors and establish a new fractionation method of LD.Finally,a novel curcumin delivery system was constructed based on the amphiphilic spiral structure of LD,as to expand its application.The effect of reaction solvent?ethanol or water?on the acid hydrolysis behavior of starch granules was systematically investigated including solubility,molecular chain degradation and crystalline structure?short-order and long-order?.The results showed that acid-ethanol treated starch had a higher solubility than its counterpart in acid-water treatment.During a96-hour acid hydrolysis,starch molecular chain exhibited a two-stage degradation pattern,and there were significant differences in the degradation rate at each stage under different solvent systems.For ethanol system,the degradation rate?k1 and k2?of starch chain was8.002×10-7 mol/g·h and 1.975×10-7 mol/g·h,respectively,while the degradation rate?k1 and k2?was 8.778×10-9 mol/g·h and 6.044×10-8 mol/g·h,respectively for starch treated in acid-water system.The composite analysis by small-angle X-ray scattering?SAXS?,X-ray diffraction?XRD?and differential scanning calorimeter?DSC?indicated that acid in ethanol solution simultaneously attacked on the amorphous and crystalline regions of starch,and the degradation extent on crystalline regions caused by ethanol-acid treatment was severer than that by acid-water treatment.Starch granules with different crystalline structures but similar molecular structures were prepared through annealing,in order to further investigate the effects of amylose content and crystalline structure on the degradation behaviors of starch by hydrochloric acid-ethanol hydrolysis.The results showed that annealed starch had a higher degradation rate than that of native starch,and the increased degradation rate was significantly correlated with the amylose content of starch.Under acid-ethanol treatment,the degradation kinetic model of high-amylose maize starch and waxy maize starch fitted to1/Mt-1/Mo=kt,while the degradation kinetic model of normal maize starch fitted to1/Mt-1/Mo=n(1-e-kt).High amylose maize starch?HMS?,normal maize starch?NMS?and waxy maize starch?WMS?were selected as the raw materials to prepare LD through acid-ethanol treatment,followed by debranching.The yields of LD obtained from three starch sources were all above88%.Among them,the polydispersity index?PDI?of LD obtained from HMS and NMS decreased from 8.20 and 9.44 to 3.90 and 3.85,respectively;whereas little changes on the PDI were observed for LD obtained from WMS,and their molecular weight was0.50×1040.62×104 g/mol.XRD and DSC data showed that LD had a strong tendency to retrograde,forming B-type crystalline structure.And proper reduction of LD chain-length and narrowing its molecular weight distribution could improve the crystalline content and enhance its resistance to digestion.The phase behavior of LD with different molecular weights in dimethyl sulfoxide?DMSO?/ethanol binary solution system was studied by cloud point titration.LD showed the molecular-dependent phase-separation behavior under DMSO/ethanol solution system.Four LD fractions were obtained through adjusting the proportion of DMSO to ethanol solution,which were F50,F60,F70 and F80?in molecular weight order?,and their PDIs were between 1.02 and 1.45.Based on its spiral hydrophobic cavity,LD was chosen as curcumin delivery system,and the influence of its molecular weight on the functional stability of curcumin was investigated.The curcumin payload,inclusion rate and the yield of LD-curcumin complexes were affected by LD Mw.LD1 with high molecular weight?6.11×104 g/mol?showed a high curcumin payload?28.76?g/mg?,but low inclusion rate of curcumin?23.81%?,and the complexes yield reached 73.98%;while LD2 with low molecular weight?0.52×104 g/mol?showed similar values for curcumin payload and inclusion rate,but a lower yield of complexes?25.87%?.The results of XRD and FT-IR indicated that hydrogen bonding was the main driving force for complex formation,and curcumin was amorphous in the LD-curcumin complexes with B+V6type crystalline structure.TEM showed that the particles size of LD-curcumin complexes was5080 nm,and SAXS analysis further suggested that the complexes had lamellar structure.The thickness of these complexes was 16.11 nm?LD1-curcumin complexes?and 11.86 nm?LD2-curcumin complexes?,respectively.After complexation,the thermostability and photochemical stability of curcumin were greatly improved.Compared with LD2,LD1provided better photic and thermal protection for curcumin,which could be controllably released in the intestine.The hydrophobic cavity structure of LD was modified through carboxymethylation,and the effect of carboxymethylation on the complex of curcumin was investigated.The introduced carboxymethyl on LD chain was verified by FT-IR,and XRD results showed that the aggregation behavior become weaken with increased carboxymethylated level,accompanying a higher solubility.When the degree of substitution?DS?was greater than0.098,the blue value,representing the inclusion ability decreased exponentially,which indicated that more introduced carboxymethyl group to linear chain could hinder its helices formation.Finally,carboxymethylated LD with a DS of 0.070 was chosen as the delivery vehicle for curcumin,and it was found that the payload and inclusion rate of curcumin were influenced by the weight ratio of curcumin/carboxymethylated linear dextrin.With the weight ratio increasing,the payload increased but the inclusion rate decreased,the highest payload and inclusion rate of curcumin reached 40.8?g/mg and 50.5%,respectively.After complexation,the water solubility of curcumin was improved at least 3700-fold by complexation?compared to that of free curcumin in water?.The carboxymethylated LD-curcumin complexes solution showed a good colloidal stability at room temperature and its average particle size was less than 230 nm.FT-IR and fluorescence spectra analysis suggested that there were strong hydrophobic interaction between curcumin and carboxymethylated LD,and curcumin could trapped in the hydrophobic cavity of carboxymethylated LD.Results of in vitro simulated gastrointestinal release revealed that the bioaccessibility of curcumin could reached 47.8%after complexation. |
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