Preparation Of Cellulose Nanocrystals From Litchi Peels And Study Of Their Properties In Pickering Emulsions

Based on the demands of developing food-grade,highly stable emulsion systems,the alternative of green and sustainable stabilizers is crucial.Nanocellulose derived from agricultural by-products have attracted much attention as biocompatible natural stabilizers,and their application in stabilizing Pickering emulsions has gradually become a research trend in recent years.The total annual production of litchi in China ranks first globally,and litchi peel as a waste resource in food processing,which is rich in cellulose.However,there is still a lack of information about the resource utilization of cellulose in litchi peels.Therefore,it is of great significance to develop novel stabilizers of natural origin to achieve high value-added utilization of litchi peels.In this study,cellulose nanocrystals(CNCs)were isolated from litchi peel,and the properties of litchi peel cellulose nanocrystals(LP-CNCs)were characterized;Furthermore,the Pickering emulsions stabilized by LP-CNCs were constructed.The adsorption of LP-CNCs at the oil-water interfaces was investigated.The impact of p H,temperature and ionic strength on the stability of emulsions were evaluated;Finally,protein nanofibers(PNFs)were introduced to explore the synergistic effect of LP-CNCs and PNFs on the properties of emulsions,and the emulsions were used as encapsulation carriers to evaluate their protective functions on the hydrophobic active substance curcumin.The main research contents and results are as follows:(1)Preparation and characterization of litchi peel cellulose nanocrystals(LP-CNCs):cellulose nanocrystals(LP-CNCs)were isolated from litchi peels by alkali bleaching treatment combined with APS oxidation,and the chemical structure,crystalline structure,microscopic morphologies,zeta potential and thermal properties of the nanocellulose were characterized.The results showed that the LP-CNCs were rod-like or needle-like nanoparticles with the length of about 124 nm and crystallinity(72.34%),which is mainly caused by the degradation of amorphous region in the cellulose molecule during oxidation of APS.The LP-CNCs are negatively charged after oxidation,and their mean zeta potential was about-35 m V.The electrostatic repulsion among the particles contributes to the good stability of LP-CNCs suspensions over a long period of time.(2)The properties and stabilities of Pickering emulsions constructed with LP-CNCs: The effects of different concentrations of LP-CNCs and different contents of oil on the stabilities and the rheological properties of emulsions were investigated;The arrangements of LP-CNCs at the oil/water interfaces were clarified by microscopic interface observations;The impacts of p H,ionic strength and temperature conditions on the stabilities of emulsions were evaluated.The results showed that the Pickering emulsions with good stabilities were obtained when the concentrations of LP-CNCs was ≥ 0.7 wt % or the contents of oil was ≤ 0.5.The emulsions showed a typical shear thinning behavior of pseudoplastic fluids.Their elasticity was dominated and they displayed a gel-like behavior,and their gel strength could be improved by regulating the contents of LP-CNCs or oil.The results of microscopic observations confirmed that the LP-CNCs formed dense and strong interfacial layers on the surface of oil droplets,which effectively prevented the aggregation and flocculation among emulsion droplets.The Pickering emulsions stabilized by LP-CNCs maintained good stabilities over a wide p H range(4～10),very wide ionic strength(≤ 800 m M)and temperature range(≤ 100°C).(3)The protective roles of Pickering emulsions co-stabilized with LP-CNCs/PNFs on curcumin: the PNFs were produced by thermally induced fibrillization of peanut protein isolates under acidic conditions,and their morphologies,sizes and zeta potentials were characterized;the oil-in-water Pickering emulsions were constructed by the synergy of LPCNCs and PNFs,and the impacts of their interaction on the emulsion properties were examined.Finally,the emulsions were used as encapsulation carriers to evaluate their protective roles on the hydrophobic active substance curcumin.The results showed that the addition of PNFs contributed to the improvement of the emulsifying properties of LP-CNCs,the stabilities and rheological properties of the emulsions.The Pickering emulsions synergistically stabilized by LP-CNCs/PNFs demonstrated excellent curcumin retention as carriers of encapsulated hydrophobic functional factors,which was attributed to the stable and thick interfacial layers of LP-CNCs and PNFs on the droplet surface,providing physical barriers against UV irradiation and thermal resistance,which in turn improved the retention of curcumin by about 70%.In conclusion,the microstructures and properties of the resulting Pickering emulsions were studied using LP-CNCs isolated from litchi peel,an agricultural by-product,as stabilizers.Then,PNFs were introduced to evaluate the synergistic effect of both on the properties of emulsions and their protective role as encapsulation carriers for the hydrophobic active substance curcumin.This study can provide a theoretical basis for the high value-added utilization of litchi peels and the development of hydrophobic functional factors in food products.