High-Value Utilization Of Polysaccharide Substances In Citrus Residues

Citrus residues are the main by-products from the citrus processing industry,which are rich in polysaccharide substance,mainly including soluble pectin and insoluble cellulose.Among them,citrus pectin is a natural emulsifier,and cellulose nanocrystals in cellulose are considered to be highly potential biomaterials.These ingredients are widely used in food,medicine,composite materials and other fields.However,the emulsification process of citrus pectin has not been clarified,and the regulating mechanism on its emulsifing properties has been yet systematically elucidated,and there were few studies about the cellulose nanocrystals in citrus residues,limiting the innovative and high-value industrial application of citrus residues.In this study,citrus pectin was extracted from citrus residues,then the citrus pectin structure was investigated at molecular and mesoscopic levels.The internal relationship among citrus pectin structure,interfacial property and emulsifying ability was explored,and the key parameters of pectin structure were determined.After that,the emulsifying mechanism of citrus pectin was revealed.Cationic cellulose nanocrystals were successfully prepared from citrus residue by acid hydrolysis and surface chemical modification.The assembly between cellulose nanocrystals and pectin at the interface of aqueous two phases system was studied.Combining their assembly and electrospray,all aqueous microcapsules with a core-shell structure were prepared by one-step.The main findings are as follows:(1)Preparation of citrus pectin-stabilized emulsion and the emulsifying mechanism of citrus pectin——The extraction rate of citrus pectin was as high as 21.28% at p H 2.Different species of citrus pectin(orange,mandarin,grapefruit)was extracted at pH 2.It was found that there was a significant difference in their molecular structures and emulsifying performances,and mandarin pectin showed the best emulsifying ability.The aggregation behavior of citrus pectins in solution was found for the first time with the critical aggregation concentration of 0.04 g/L,so pectin exists as aggregate during the emulsification.The difference lying in citrus pectin aggregate structures caused by species(orange,mandarin,grapefruit),molecular structure and solution environment was studied,and the intrinsic correlation among citrus pectin aggregate structure,interfacial properties and emulsifying properties was investigated,then the emulsifying mechanism of citrus pectin were clarified.Regulation mechanism of solution environment on emulsifying properties: acidic pH and NaCl ionic strength could reduce the surface charge of citrus pectin through protonation and electrostatic shielding,weaken the electrostatic repulsion between pectin molecules,and make the citrus pectin aggregates smaller and more compact.This kind of citrus pectin aggregate could form a dense interfacial layer on the interface,improving the interfacial activity,inhibiting the coalescence of emulsion droplets during the emulsification,and improving the emulsifying properties of pectin.Influence mechanism of molecular weight on emulsifying properties: reduction of citrus pectin molecular weight decreased its aggregate size without changing compactness,then improved its interfacial property,but cannot inhibit the coalescence of emulsion droplets during the emulsification.As a result,the emulsifying property became poor when the molecular weight of citrus pectin was reduced.Therefore,the effect of citrus pectin aggregates compactness on its emulsification performance is greater than the effect of aggregate size.The results deepen the scientific understanding of the pectin emulsification process,and provide scientific basis and theoretical guidance for the formulation design of related emulsion products.(2)Preparation,characterization and application of cationic citrus cellulose nanocrystals——Cationic cellulose nanocrystals were prepared from citrus residues by concentrated sulfuric acid hydrolysis and chemical modification.The structure of cellulose nanocrystals was characterized using transmission electron microscopy and Fourier near-infrared spectroscopy,showing that the surface potential and size of cationic cellulose nanocrystals were 65 mV and 100~150 nm,respectively.The effects of pH and system composition on the assembly of cationic cellulose nanocrystals and pectin at the interface of aqueous two phases system were investigated.At pH 2~3,the quaternary ammoniums of cationic cellulose nanocrystals were protonated with a higher charge.The interaction force between citrus pectin and cationic cellulose nanocrystals was high producing intact interfacial membrane.However,the membrane formed at pH 4~7 was fragile.The combination of low methoxy pectin and cationic cellulose nanocrystals is higher than that of high methoxy pectin.Combining the assembly and electrospray,all aqueous microcapsules with a core-shell structure were prepared in one step.The microcapsule structure was determined and the influence of system pH and composition on the preparation of microcapsules were investigated.According to the results from confocal and Cryo-SEM microscopy,the microcapsules have a core-shell structure,and the capsule shell possesses a double-layer structure composed of cationic cellulose nanocrystals and pectin.Only at pH 2,the microcapsules with stable structure could be formed from the assembly between low methoxy pectin and cationic cellulose nanocrystals.By adjusting the system composition,the microcapsule shell structure can be regulated,improving the microcapsules stability.The microcapsules prepared with 1% cellulose nanocrystals and 0.5% pectin can be stably stored for more than 12 hours after being transferred into aqueous solution.The all-aqueous microcapsules in this study are completely permeable to small molecules,showing a good encapsulating ability for biological macromolecules and stimulus-responsibility to ionic strength.This research expands the application of citrus cellulose nanocrystals in biomedicine,biomaterials and other fields.The results of this thesis deepened the scientific understanding of the emulsification process of citrus pectin and expanded the application of citrus cellulose nanocrystals in high-tech biomaterials fields,providing theoretical guidance for the scientific and high-value utilization of citrus residues.