| Petroleum based polymers are widely used in food packaging market and their non-biodegradability and non-renewability bring increasingly serious environmental pollution problems.Edible films made of natural edible polymers as a new type of food packaging material has attracted more and more attention,among which the most successful commercially application is an edible film made from collagen fibers.However,due to the destruction and reorganization of the original structure of collagen fiber,collagen fiber film no longer has the crosslinking network structure and ideal mechanical properties of the original natural collagen fiber.In order to restore the mechanical properties of natural collagen fiber film,based on the study of structural changes and recombination rules of collagen fibers during processing,this study explored the swelling and depolymerization rules of collagen fibers changed by p H and heat treatment,and studied the mechanism of insoluble micron/nanocellulose,soluble polysaccharide and collagen fiber blending network to improve the mechanical properties of collagen fiber film.At the same time,aiming at the deficiency of ductility of collagen fiber film caused by chemical crosslinking agent glutaraldehyde treatment,the effects of two natural protein crosslinking agents proanthocyanidin and transglutaminase were investigated.Following are the main contents and results:1.pH regulating fiber swelling improves the mechanical properties of collagen fiber film:the swelling degree of collagen fiber was p H-dependent.When p H was between 1.5 and 4.0,the swelling degree of fiber was maximum at p H 3.0 and the formed collagen fiber film also had the highest mechanical strength and thermal stability.Transmission electron microscopy(TEM),atomic force microscopy(AFM)and fluorescence microscopy showed that the degree of stacking interlacing of collagen fibers was affected by the degree of swelling of collagen fibers at different p H values,and the high stacking interlacing caused by high swelling made the film structure more compact,thus obtained excellent mechanical properties.The results of Fourier transform infrared spectroscopy(FT-IR)and X-ray diffraction(XRD)analysis showed that low p H could lead to the depolymerization of collagen triple helix,reduced the content of?-helix and ?-fold in protein secondary structure,and increased the exposure of amino acid side chain.Some collagen fibers were even degraded into small molecules of protein,which reduced the mechanical properties of the collagen fiber film.The properties of the films prepared at different p H conditions were compared before and after neutralization.By comparing the properties of the films prepared at different p H conditions before and after neutralization,it was found that the mechanical properties of the films after neutralization still had the p H dependence similar to that of the films before neutralization.2.Cellulose blending and stacking improves the mechanical properties of collagen fiber film: The addition of micron cellulose(CM)and nano cellulose(CN)could improve the tensile strength of collagen fiber film and the effect of CN was stronger.When the addition amount of CN was 8%,the tensile strength of dry,wet and boiled collagen fiber film could be improved by 20.2%,187.6% and 101.4%.The results of rheological properties,thermodynamic properties and FT-IR showed that the molecular interaction between CN and collagen fiber was stronger,which was attributed to the smaller particle size and larger specific surface area of CN.The observation results of super depth of field microscope and scanning electron microscope(SEM)showed that CM and CN existed in different forms between collagen fibers.CM interspersed between collagen fiber networks and partially stacked on the surface of the film,while CN was fully dispersed and stacked between collagen fiber networks to play a better strengthening role.The changes of film swelling ratio,transparency and water contact angle correspond to the different forms and interaction degree of CM and CN in collagen fiber network.3.Soluble polymers filling improves the mechanical properties of collagen fiber film:Rheological analysis results showed that collagen underwent thermal pre-denaturation and main denaturation at 33.0 °C and 42.8 °C,respectively.The results of sodium dodecyl sulfate-polyacrylamide gel electrophoresis(SDS-PAGE)analysis and film solubility test showed that the preheating resulted in the depolymerization of some collagen fibers to varying degrees,and the production of soluble low molecular weight protein increased with the increase of heat treatment temperature.SEM observation results confirmed that the dissolved protein could fill collagen fiber network and improve the barrier property of collagen fiber film.The tensile test results of collagen fiber film at different temperatures showed that the heat treatment below the main denaturation temperature had no significant effect on the mechanical properties and the heat treatment above the main denaturation temperature(?45 °C)reduced the tensile strength.The fluorescence observation results showed that heat treatment lower than the main denaturation temperature had no effect on the structure of collagen fiber network,while heat treatment higher than the main denaturation temperature resulted in partial depolymerization of the fiber structure and reduced tensile strength of the film.FT-IR and XRD analysis results further showed that heating promoted the depolymerization of collagen triple helix structure,and the degree of depolymerization increased with the increase of pretreatment temperature,and the contents of ?-helix,?-fold and random curl in protein secondary structure decreased,corresponding to the increase of dissolved protein content and the change of mechanical properties of the film.In view of the limited improvement of film properties of the depolymerized collagen,acid-soluble polymer chitosan(CHS)was introduced into the collagen fiber network to enhance the film network structure.The results showed that CHS significantly improved the mechanical properties of collagen fiber films and the tensile strength of dry,wet and boiled collagen fiber film increased by 30.0%,398.8% and 433.5%,respectively(p < 0.05).Fluorescence observation showed that CHS molecules distributed and adhered along the collagen fiber network,and the network formed by CHS itself and the collagen fiber network interpenetrating and co-existing to form a stable dual network structure.The results of super depth of field microscope and SEM showed that CHS and collagen fibers mixed well and promoted the expansion of collagen fiber bundles,which was conducive to the filling of CHS and the formation of interpenetrating network.Thermodynamic properties change,FT-IR and XRD results showed that there were strong hydrogen bond and electrostatic interaction between CHS and collagen fibers and CHS promoted the increase of ?-helix and ?-turn content in the film,thus improving the mechanical properties of the film.4.Protein crosslinking improves the mechanical properties of collagen fiber film: The natural protein crosslinking agent procyanidins(PA),glutamine transaminase(TG)and chemical crosslinking agent glutaraldehyde(GA)constructed the connection between collagen molecules in the form of hydrogen bond and hydrophobic interaction,isopeptide bond and Schiff base,respectively.The differences in the cross-linking mechanisms of the three bring different mechanical properties of collagen fiber film.PA and TG could improve the tensile strength of the film in dry and wet states,which is similar to the effect of GA,and maintain the high elongation at break of the film,avoiding the reduction of the ductility of the film caused by GA crosslinking,due to the lower bond energy of the crosslinking bonds formed by PA and TG.GA had the best effect on the tensile strength of the film after boiled,but significantly reduced the elongation at break of the film,followed by TG,and PA had no significant effect.The rheological and fluorescence observation results showed that the protein molecular interaction induced by PA at 6% was the strongest and excessive protein aggregation was induced by PA at high additive amount,corresponding to the change of mechanical properties.The results of cross-linking degree test showed that the increase of TG and GA concentration led to the increase of protein cross-linking degree,which was related to the improvement of film mechanical properties.XRD analysis results showed that PA,TG and GA reduced the molecular spacing of collagen to 10.93 A,10.93 A and 10.53 A,respectively,indicating the generation of crosslinking and partial destruction of the triple helix structure of collagen,corresponding to the decrease of mechanical properties of the film at high crosslinking degrees.Thermodynamic properties and FI-IR analysis showed that the three crosslinking agents all promoted the enhancement of intermolecular interactions and different secondary structure changes of proteins.SEM observation results showed that the new chemical bonds generated by the crosslinking had no significant effect on the microstructure of the film network except for PA at 6% and 8%. |
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