| Pickering emulsion is a stable emulsion in which solid particles are adsorbed on the oil-water interface without any surfactant molecules.With the development of research on Pickering emulsion,more and more nanoparticles are used in emulsion,but little research focused on the application of nanoparticles in food emulsion.Finding natural,safe and biocompatible nanoparticles for food emulsion is an effective way to deal with the natural,safe and healthy development trend of food industry.The solid granule-bacterial cellulose nanofibers(BCNs)have the advantages of nano-scale structure,gel properties,natural non-toxicity,good biocompatibility and other excellent properties with broad application prospect.Herein,hydrolysis method was applied to obtain BCNs from bacterial cellulose(BC)firstly,then the physical and chemical properties of BC and BCNs were investigated.After that,BCNs was used as stabilizer to prepare Pickering emulsion and then the stability mechanism and factors influencing of emulsions stability were investigated.Finally,Pectin and Ca2+were added to prepare renewable emulsion which could be dried and regenerated.The main results are as follow:(1)Preparation and characterization of BCNs.BCNs obtained from BC by hydrolysis were zonal fibers,characteized with narrow particle size distribution and high strength,as well as with lower average particle size(from 590.9 nm for BC to 221.4 nm for BCNs)and polydisperse index(PDI,from 0.37 of BC to 0.18 of BCNs),indicating that hydrolysis could abtain smaller and more uniform particle size of nanometer fiber,and hydrolysis could also increase potential and light transmittance of suspension.Meanwhile,the uniform filamentous appearance of BCNs was observed with transmission electron microscopy(TEM)and atomic force microscopy(AFM).X-Ray Diffraction(XRD)and thermogravimetry analysis(TGA)showed that BCNs was an excellent semi-crystalline polymer with better thermal stability than BC.Rheological results showed that BCNs suspension had higher apparent viscosity and storage modulus(G')and loss modulus(G")compared with BC,suggesting that BCNs had the properties of weak gel properties and shear thinning behavior.In addition,the water holding capacitity(WHC)and water swelling ability(WSA)of BCNs suspension were highly stable,and water release rate(WRR)increased during storage after hydrolysis.(2)Study on Pickering emulsion stabilized by BCNs.The water and oil contact angle of BCNs were 64.46° and 27.29°,respectively,indicating that BCNs prepared by hydrochloric acid hydrolysis showed good hydrophilic and lipophilic properties with the ability to significantly reduce the surface tension of oil-in-water droplets(from 48.52 mN/m to 34.52 mN/m).Only 0.05%(w/v)BCNs could stabilized the Pickering emulsion containing 15%oil(v/v),and the Pickering emulsion had small average droplet size,narrow particle size distribution and high intensity.Morphological studies have confirmed that the droplets in the emulsion are nanoscale(10-30 nm).Finally,the effects of pH,temperature and storage time on the stability of Pickering emulsion were investigated.Results showed that emulsions displayed relatively lower creaming ability when pH<7,while displayed optimal physical stability and dispersibility when pH? 7.In addition,the Pickering emulsion had excellent stability under wide range of temperature(20-100?)and storage time(0?4 weeks).Due to the sustainability and good bio-compatibility of nanofibers from BC,emulsions stabilized by low concentration of nanofibers can be used as new food-grade Pickering emulsions and have great potential to deliver lipophilic bioactive substances in food industry.(3)Study on drying and regeneration of Pickering emulsion.In this study,the effects of Pectin and Ca2+on the drying conditions and regeneration of Pickering emulsion were investigated.The results showed that the Pickering emulsion stabilized by BCNs alone was not suitable for freeze drying or spray drying.After drying,the dry solids had obvious oil leakage at low-dosage of BCNs,while the dispersibility decreased significantly at high dosage of BCNs.In addition,the stable dispersion system could not be achieved after the regeneration of dry solid by adding water.The average particle size and PDI increased with the increase of the addition amount of BCNs.Pickering emulsion stabilized with BCNs/Pectin blend showed no oil leakage in the solid appearance after freeze-drying.Low addition of Pectin(<0.3%)did not effectively improve the effect of emulsion regeneration,and oil leakage still existed.High addition of Pectin(>0.3%)could perfectly stabilize the emulsion.Notably,the average particle size of regenerated emulsion decreased with the increase of Pectin addition,while PDI first increased and then decreased.The Pickering emulsion stabilized with BCNs/Pectin/Ca2+blend did have oil leakage after freeze-drying.The dry solid showed good dispersion,and the regenerated emulsion showed good stability.The average particle size and PDI increased with the addition of Ca2+.Results from the spray drying showed that the emulsion stabilized by BCNs/Pectin/Ca2+ showed good drying effect,characterized with good dispersion and significantly reduced particle aggregation.(4)Effects of drying on rheology of Pickering emulsion.Rheological analysis showed that the apparent viscosity of emulsions stabilized by BCNs,BCNs/Pectin and BCNs/Pectin/Ca2+ exhibited three stages:shear thinning behavior at low shear rate displaying a pseudoplasticity,then stage of slightly Newtonian fluid and shear thickening behavior at high shear rate.The apparent viscosity was dose-dependent with the addition of BCNs,Pectin and Ca2+.Apparent viscosity of regenerated emulsion was similar to the emulsios without drying,but the apparent viscosity decreased.In addition,the majority of the emulsion presented storage module(G')higher than loss module(G"),the results showed that the emulsion exhibited elastic gelatinous behavior.Freeze drying could slightly reduce G' and G" of emulsion,but did not change the gel-like behavior of emulsion.Notably,high content of Pectin and Ca2+caused G" higher than G',suggesting that a high concentration of Pectin and ca2+ could induce the change emulsion from solid elastic to liquid viscosity.The regenerated emulsion obtained by spray drying showed similar rheological properties with the regenerated emulsion obtained by freeze drying,but spray drying could enhance the apparent viscosity.G'and G" of emulsion,which might be helpful in the stability and renewable of emulsion. |
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