Preparation Of Nanocellulose From Ginkgo Seed Shells And Its Application In Pickering Emulsions

Pickering emulsions stabilized by solid particles instead of traditional small molecule surfactants have attracted more and more attention in the fields of food,cosmetics and pharmaceutical industry.These Pickering emulsions have the advantages of less stabilizer dosage,more functionality,and good emulsion stability.Due to the advantages of nanocellulose with low cost,renewability,sustainability,and biodegradability,the Pickering emulsions stabilized by nanocellulsoe have gradually become an interesting research direction.The emulsifying property of nanocellulose is affected by its own morphology and size.In the past few studies,researchers usually prepared nanocellulose with different sizes from different raw materials.However,the differences in raw materials may also cause differences in other properties between target nanocelluloses,such as crystalline structure,surface charge,etc.In contrast to these studies,the present thesis applied agro-industrial wastes(ginkgo seed shells)as cellulose materials to prepare nanocellulose with different lengths based on modulating conditions of the high-energy mechanical post-treatment.Subsequently,the effects of nancellulose with different lengths on particle interfacial adsorption behavior,emulsifying property,and in vitro digestion of emulsions were investigated.Moreover,the possibility of preparation of Pickering high-internal-phase emulsions using unmodified nanocellulose was explored.Additionally,the polyphenol-nanocellulose system was constructed to improve the oxidation stability of high internal phase emulsion.Main research contents and results are as follows:First,the nanocellulose with different lengths ranging from 300 nm～1500 nm could be efficiently prepared by modulating conditions of the high-energy mechanical post-treatment,including changing high-homogenization pressure(10～70 MPa)or adjusting ultrasonic powers(150～300 W)and time(10～60 min).These nanocelluloses with different lengths exhibited significant differences in crystallinity,wettability,and interfacial adsorption behavior.Short nanocellulose exhibits relatively low crystallinity and high hydrophobicity.In terms of interfacial adsorption behavior,long nanocellulose particles(1500 nm)exhibited very limited interfacial activity due to their large size and electrostatic repulsion.The 527 nm nanocellulose formed an interfacial film with cellulose entanglement structure.This entangled structure strengthened the interfacial film strength.Due to its short size and high hydrophobicity,400 nm nanocellulose can be quickly adsorbed on the oil-water interface to form a discontinuous monolayer interfacial film.Secondly,these nanocelluloses with different lengths were used as oil-in-water Pickering emulsion stabilizers to study the differences in the emulsification performance.The results showed that the ability of nanocellulose to stabilize emulsions had a good correlation with properties of interfacial films: the long nanocellulose(1500 nm)cannot effectively stabilize Pickering emulsion due to its low interfacial activity;the interfacial entanglement behavior of527 nm nanocellulose made the emulsion showing high viscosity and fluid gel behavior;the emulsion stabilized by nanocellulose with a length of 400 nm exhibited smaller particle size and higher surface coverage due to its relatively high interfacial activity.Additionally,the emulsion stabilized by the nanocellulose(527 nm)exhibited the best storage stability,which can be attributed to the fact that it can not only adsorb on the surface of oil droplets to form an interfacial film,but also entangle with each other to build a three-dimensional network structure.The Pickering emulsions stabilized by these nanocellulose with different lengths were used to encapsulate β-carotene,and in vitro digestion experiments were further performed.The effects of nanocellulose lengths on the emulsion structure,lipid digestion,and β-carotene bioavailability were mainly studied.The results showed that the low-acid and high-salt environment in the gastric digestion stage caused emulsion structure changes,which is manifested as: the absolute value of the zeta potential of emulsion system dropped rapidly,and emulsion droplets aggregated together to form a lump emulsion with gel properties.Moreover,the gel strength and size of these lumps were positively correlated with the nanocellulose length.These formed emulsion structures reduced the lipid digestion of emulsions due to the limited surface areas for the adsorption of bile salts and lipases.Moreover,the longer the nanocellulose length caused the more obvious decrease in lipid digestion.This decreased lipid digestibility reduced the bioavailability of β-carotene.These results indacted that the lipid digestion of Pickering emulsions and the bioavailability of lipid-soluble actives can be modulated by changing lengths of nanocellulose.Nanocellulose hydrogels were formed by increasing the concentration of cellulose suspensions under the effect of ultrasonic post-treatment.Hydrogels obtained from different ultrasonic durations were used to stabilize HIPEs.The results showed that the formation of hydrogel was attributed to the physical entanglement between the nanocellulose particles,and this physical entanglement behavior is reversible.The benign stability of HIPEs could be fabricated using the hydrogel obtained by a 60 min ultrasonic duration.The stabilization mechanism was clearly exhibited by the three-dimensional image from confocal laser scanning microscopy(CLSM): gaps between oil droplets were fully filled with the continuous network structure which was re-formed by dispersed hydrogels due to its reversible gelation property.This continuous network structure effectively solidified the surrounding oil droplets,preventing oil droplet aggregation and coalescence.Finally,the tannic acid-nanocellulose hydrogel system was constructed by the addition of tannic acid into cellulose suspensions.The effects of tannic acid addition on emulsion properties,interfacial structure and oxidative stability were investigated.The results showed that the tannic acid-nanocellulose hydrogel system significantly improved the particle size distribution,enhanced the gel strength and improved the physical stability of the high internal phase emulsion.However,these improvements did not continuously increase when the tannic acid concentration exceeds 0.3%.The interfacial structure of the emulsion was observed by scanning electron microscopy,and the results showed that the nanocellulose hydrogel system formed a thicker interfacial film with the participation of tannic acid,and the cellulose network structure between the emulsion droplets was denser.The TA-nanocellulose hydrogels could siginificantly reduce the POV value of HIPEs from 80.42 meq/kg to 46.25 meq/kg.