Construction Of O/W Pickering Emulsion Stabilized By Bacterial Cellulose Nanofibrils And Soy Protein Isolate And Its Effect On Lipid Digestion

The Pickering emulsion stabilized by food-grade colloidal particles is a hot topic in the food industry,which has an important impact on modulating the appearance,physical properties,flavor,nutrition and stability of foods.Bacterial cellulose nanofibrils as green and sustainable colloidal particles,can be irreversibly adsorbed at the oil-water interface,but its poor dispersion stability,amphiphilicity and single functional group structure limit its application for Pickering emulsion stabilization.In this work,the O/W Pickering emulsions synergistically stabilized by bacterial cellulose nanofibrils（BCNFs）and soy protein isolate（SPI）were intensively studied.The particle structural properties,interfacial adsorption behaviors and emulsification performance of complex particles were systematically discussed and deeply analyzed.Then,the effects of pH,cellulose/protein ratios and contents,and oil/water ratios on the microstructure,particle size and rheological properties of stabilized emulsions were discussed,as well as exploring the storage stability,oxidative stability and digestion characteristics.Moreover,combining spray drying technique with Pickering emulsions for oil microencapsulation,investigating the influence of modifying emulsion properties on the physical properties,encapsulation efficiency and oil digestion properties of spray-dried oil powders.The main research results were presented as follows:（1）Effects of the interaction between bacterial cellulose and soy protein isolate on the oil-water interface on the digestion of the Pickering emulsionsThe insoluble BCNFs/SPI complexes were fabricated at pH 3.0,as well as their interaction and interfacial adsorption properties were investigated.Subsequently,the stability and digestion properties of medium chain triglyceride（MCT）oil-in-water（O/W）Pickering stabilized by BCNFs/SPI complexes were explored.The interfacial adsorption characteristics of SPI particles were improved in the presence of BCNFs,resulting in the enhanced interfacial pressure and dilatational viscoelastic moduli,due to the increased rigid and compact layers.O/W Pickering emulsions stabilized by BCNFs/SPI complexes were prepared,and the prepared emulsion presented improved stability by tuning the BCNFs/SPI ratios.Moreover,the emulsions stabilized by BCNFs/SPI complexes displayed lower amount of free fatty acid（FFA）released than that of SPI stabilized emulsion after in vitro simulated digestion,with the minimum amount of FFA released of 58.0%at the BCNFs/SPI ratio of 1:5.Main mechanisms were proposed for the results:the compact interfacial layers formed by complexes partially restricted interfacial displacement by bile salts or lipases;bridging structure and interconnected networks formed by BCNFs reduced the surface area available for binding with lipases and bile salts.This work would be suggestive for the development of cellulose-protein Pickering interface to modulate the lipid digestion.（2）Impact of oxidized bacterial cellulose on the conformation and interfacial adsorption characteristics of soybean protein isolateThe influences of pH（2-9）on the conformational states,interfacial adsorption characteristics and emulsifying properties of SPI and SPI/2,2,6,6-Tetramethylpiperidine 1-oxyl（TEMPO）oxidized bacterial cellulose electrostatic complexes were investigated.The spectroscopy analysis methods,including far-UV circular dichroism and intrinsic tryptophan fluorescence spectroscopy,were applied to evaluate protein conformational transitions.The interfacial adsorption behaviors were determined by adsorption kinetics and dilatational viscoelasticity modulus analysis.According to spectroscopy analysis,the addition of TEMPO-oxidized bacterial cellulose（TOBC）reduced surface hydrophobicity of SPI at pH 2-9,due to the added TOBC partially hindered SPI unfolding to decrease the formation of unordered structure.Moreover,the interfacial adsorption property of SPI around isoelectric point（p I,pH 4.5）was improved in the presence of TOBC with increased balanced interfacial surface pressure（π）and diffusion rate constant(K_diff)values,while the viscoelastic moduli had no significant difference.In general,the SPI-stabilized emulsions presented better emulsifying performance at a pH deviating from p I than at a pH around p I.However,the emulsions stabilized by SPI/TOBC electrostatic complexes exhibited more uniform droplets distributions and smaller sizes（<20μm）at pH 4-5,and had excellent storage stability and enhanced viscoelasticity properties.（3）Research on the preparation and performance of O/W medium and high internal phase Pickering emulsionsThe effects of SPI/TOBC ratios and contents,and pH values on the morphology,droplet size and distribution,stability and rheological behaviors of the fabricated emulsions were investigated.Then the physical and microstructure characteristics of medium/high internal phase Pickering emulsions（MIPEs/HIPEs）stabilized by SPI/TOBC complexes were evaluated.It was indicated that the emulsion droplets had uniform particle size distribution when the ratio of SPI/TOBC was controlled to 12.5:1（w/w）.The emulsification performance could be improved with the increase of solid content from 1.0 wt%to 3.0 wt%.Moreover,the pH played an important role in regulating the structure and properties of the emulsions.By microscopic observation,the emulsions at pH 7.0 had smaller droplet size（10-15μm）than that（15-40μm）of the emulsions at pH 3.0.While the emulsions at pH 3.0maintained a low creaming index（CI）value than those at pH 7.0.The MIPEs/HIPEs with different oil fraction（50-74.05%）were successfully prepared by SPI/TOBC complexes at content of 3.0 wt%.Rheological analysis showed that the apparent viscosity and viscoelastic moduli of obtained emulsions were increased with the increase of the complex contents,and the elastic storage modulus（G’）was always higher than that of the loss modulus（G’’）,indicating the elastic gel-like structure of emulsions.The emulsion had a certain flow ability at a low oil volume（50-60%）.While the oil volume was increased（70-74.05%）,the gel-structured emulsions could not flow and emulsion droplets presented hexagonal accumulation.（4）Study on oxidation stability and digestion behaviors of Pickering emulsionsThe oxidative stability and digestion properties of canola oil-in-water（O/W）Pickering emulsions stabilized by SPI/TOBC electrostatic complexes（pH 3.0）were investigated.The results showed that the droplets size was 37.1-25.5μm and the zeta potentials was 29.9-32.5 m V.With the increase of SPI/TOBC complexes contents,the droplets size was decreased and became uniform gradually,and had no phase separation during long-term storage at the solid content of 3.0 wt%.The rheological experiments demonstrated that the emulsions showed shear-thinning and gel-like behaviors,and the apparent viscosity and viscoelastic moduli were increased with the increasing SPI/TOBC complexes contents.Under the condition of accelerated oxidation（40°C）,both peroxide values（POVs）and thiobarbituric acid reactive substances（TBARS）products of emulsions were significantly lower that of pure canola oil.A simulated gastrointestinal tract（GIT）model was used to evaluate the influence of complexes contents on the lipid digestion properties of emulsions.Eventually,the rate and extent of lipid hydrolysis were decreased with the increase of complexes contents.These results confirmed that the SPI/TOBC complexes stabilized Pickering emulsions had improved oxidative stability and inhibited digestibility,which be beneficial to further application in functional foods.（5）Preparation and regulation of oil powder microcapsules based on spray-dried Pickering emulsionStable O/W Pickering emulsions synergistically stabilized by heat-induced soy protein isolate（HSPI）and BCNFs were fabricated,and spray-dried to obtain edible oil powders.The effects of BCNFs contents and oil-to-water ratios on the morphology and particle size of the obtained emulsion were investigated.Then,the microstructure,size distribution,moisture content,fluidity and bulk density of the spray dried oil powders were discussed,and the encapsulation efficiency and digestion properties of the oil powders were further explored.The emulsions with 0.2 wt%BCNFs and 2 wt%HSPI displayed homogeneous droplets size（<1μm）and narrow size distributions,and had no obvious droplet aggregates at different oil phase ratios（10-20%）with 2 wt%beeswax.After spray drying,the obtained stable oil powders（35.9-56.8%）presented hollow smooth spherical structure with small and uniform size.The moisture content,flowability,bulk density and encapsulation efficiency of oil powders were improved by tuning the BCNFs contents and oil-to-water ratios of emulsions.Furthermore,the presence of BCNFs partially inhibited lipid hydrolysis,and oil powders with 0.1 wt%BCNFs performed optimum anti-digestibility with the minimum released FFA values of 72.5%.This work combined spray drying technique with Pickering emulsions for oil encapsulation,and optimized the emulsions properties to address the limitations of spray drying for liquid oil microencapsulation,which would provide a reference for the development and utilization of oil powders.