Gelatin Films With Improved Performance Based On Non-Covalent Combined With Covalent Double Cross-Linked Strategy And Its Mechanism

Gelatin is a kind of natural macromolecular compounds obtained by partial hydrolysis of collagen,which could replace the plastic products and alleviate environmental stress due to its characteristics of biodegradability,biocompatibility,non-toxicity,gelation and film-forming properties.There are many ways to prepare gelatin film.However,the poor mechanical properties,hydrophobicity and thermal stability（high-temperature sterilization）of pure gelatin films could not be solved by these ways,which limits the practical applications of gelatin films.So it is important to investigate and solve these problems to expand the application of gelatin films,and it also provides some research directions for the improvement of gelatin film properties.In this study,gelatin is the main research object.Firstly,gelatin films with different concentration were prepared by dehydration with soaking the different-concentration gelatin gel via absolute ethanol or saturated（NH₄）₂SO₄ solution.The effects of ethanol precipitation or Hofmeister effect on the film thickness,mechanical properties,moisture content,water solubility,surface hydrophobicity,transmittance,color difference,transparency and thermal stability of gelatin films with different concentrations were investigated.The influence mechanism was revealed by Fourier Transform Infrared（FTIR）,differential scanning calorimetry（DSC）and polyacrylamide gel electrophoresis（SDS-PAGE）.Secondly,the effects of transglutaminase cross-linking combined with ethanol precipitation or Hofmeister effect on the properties of gelatin films with different concentrations and its mechanism were clarified.Thirdly,the cross-linked gelatin films with 10% gelatin were prepared by ethanol precipitation effect,and the surface hydrophobicity and sterilization heat stability of the gelatin films were further enhanced by co-blending with montmorillonite and self-assembling with chitosan.The structure of films was characterized by X-ray diffraction（XRD）,FTIR and atomic force microscopy（AFM）,and the films were applied to the preservation and storage of bread.The specific study are as follows:（1）The mechanical properties of gelatin films can be improved by enhancing non-covalent interactions during dehydration processPreparation of gelatin films by soaking gelatin gels in absolute ethanol or saturated（NH₄）₂SO₄ solution was feasible due to the ethanol precipitation or Hofmeister effect.The improvement of gelatin film properties by ethanol precipitation effect was more significant at a low gelatin concentration.Ethanol precipitation effect could induce non-covalent interactions such as hydrogen bonding and hydrophobic interactions,which could keep the triple helix structure stable,thus improving the mechanical properties,surface hydrophobicity,water solubility and thermal stability of the gelatin films.In addition,ethanol precipitation effect could also improve the color difference and transparency due to the decolorization effect of absolute ethanol.However,Hofmeister effect was more significant for improving the properties of films with high-concentration gelatin.This was due to the adsorption of（NH₄）₂SO₄ particles during soaked process especially at low gelatin concentrations,which led to the increase of film thickness and hydrophilicity and the decrease of visual sensory properties and thermal stability.It is worth noting that Hofmeister effect could also improve the mechanical properties of gelatin films through non-covalent interactions such as hydrogen bonding and hydrophobic interactions.（2）The thermal stability of gelatin films can be improved by non-covalent combined with covalent double cross-linked strategiesFirstly,the gelatin films prepared by transglutaminase cross-linking combined with ethanol precipitation or Hofmeister effect.The cross-linked gelatin films retained its integrity after sterilizing at 121 ℃ for 20 min or boiling water bath for 10 min due to the formation of a stable chemical cross-linked network.The synergistic effects of transglutaminase cross-linking and ethanol precipitation or the Hofmeister effect could reduce the thermal shrinkage of the cross-linked gelatin films.The gelatin films prepared by transglutaminase cross-linking combined with ethanol precipitation effect had more hydrogen bonding and triple helix content than the control cross-linked gelatin films especially at a low gelatin concentration（5%,10%）,and the gelatin structure was more order,which further improved the mechanical properties and thermal stability of the cross-linked gelatin films.It is worth noting that the gelatin films prepared by transglutaminase cross-linking combined with the Hofmeister effect had improved mechanical properties but higher water solubility and lower thermal degradation temperature than the control cross-linked gelatin films due to the migration of（NH₄）₂SO₄ particles.In summary,non-covalent combined with covalent double cross-linked strategies could be applied to enhance the properties of gelatin films especially sterilization stability,and the gelatin films prepared by cross-linking combined with ethanol precipitation effect have broader applicability.（3）Self-assembly of the hydrophobic layer based on the electrostatic interaction can improve the surface hydrophobicity of the gelatin filmsOn the basis of double cross-linked preparation,gelatin and chitosan were used to self-assembly through hydrogen bonding and electrostatic interaction to form a hydrophobic layer on the surface of the gelatin film,and its water contact angle value was higher than 110°.The hydrophobic layer remained stable after dehydration by ethanol precipitation effect.After gelatin was blended with negatively charged montmorillonite,it has stronger electrostatic interaction with positively charged chitosan ions,resulting in more deposition of chitosan moleculars.Montmorillonite changed the surface structure of gelatin films and increased its surface roughness.The surface hydrophobicity improved by montmorillonite was lower than that of gelatin-chitosan bilayer films,but its water contact angle value was still greater than100°.At the same time,gelatin blended with montmorillonite could form embedded/exfoliated structures and change the crystallinity of the gelatin films,thus further reducing the sterilization heat shrinkage of the gelatin films.In addition,gelatin+montmorillonite-chitosan composite bilayer films had the lowest water solubility,the highest mechanical properties and barrier properties due to the synergistic effect of montmorillonite and chitosan.The composite bilayer films had certain antibacterial properties and the bread packaged by the composite bilayer films stored（30 ℃,80%RH）for 5 days without growing mold.In conclusion,the gelatin-based bilayer hydrophobic films based on the electrostatic interaction had improved comprehensive performance,which could offer the possibility to expand the application range,and provide reference and theoretical support for its application in the food field.