| The development of green bio-based materials to replace traditional petroleum-based fresh-keeping films is a research hotspot in the field of food packaging.Casein contains a large number of random crimped structures,which has the great advantages of forming three-dimensional network structures,and its high structural controllability allows modification by various means to optimize mechanical properties and improve its shortcomings in practical applications.Shellac,as a natural resin,has the parental property similar to protein,but has the different structural properties from casein.The interaction between large and small molecules can be established through structural co-constructed technology,which can overcome the inherent brittleness of individual materials and improve the ductility to meet the needs of practical applications.In this project,casein-shellac composite preservation films(CSFs)were developed by using structural co-constructed technology to explored the co-construction mechanism between them,and to improve the water retention and mechanical properties of the films,in order to provide research ideas and theoretical basis for the application of CSFs in food preservation.The main results were as follows:Firstly,casein and shellac were co-constructed by p H-cycle,and CSFs were prepared by flow deposition.The circular dichroism spectra showed that the co-constructed process did not involve the secondary structure change of protein,but the three-dimensional conformation remodeling was driven by structural folding.The fluorescence spectra showed that there was a non-covalent binding between casein and shellac.Zeta-potential and hydrophobicity data showed that the protein structure gradually folded from unfolding during the neutralization process,and casein showed anti-folding behavior with the increase of shellac content.The hydrophobic interaction increased the exposure of charged groups on the surface of the co-constructed structure and enhanced the electrostatic repulsion on the surface,which was conducive to the dispersion and stability of proteins.The co-constructed structure significantly improved the defects of high hardness and brittleness of single-component casein films.When the ratio of casein to shellac was 10:1(w/w),the toughness of CSFs could reach 1.177 MJ/m~3.Secondly,the effects of glycerol(Gly)and eugenol(Eug)on the water retention,mechanical,antioxidant,and bacteriostatic properties of CSFs were studied,and hydrophilic composite films with excellent properties were prepared.The results showed that the addition of Gly,a hydrophilic plasticizer,significantly improved the water retention property of the Gly-CSFs,and only 1%(w/v)of water loss was found after storage for 5 d,and the mechanical properties were further improved.When the Gly content was 20%(w/w),the toughness of Gly-CSFs reached 2.049 MJ/m~3.Appropriate addition of Eug can further improve the mechanical properties of the films.When Eug content was 1%(w/v),the toughness of Eug-Gly-CSFs reached 3.107 MJ/m~3.In addition,Eug endowed the films with antioxidant capacity and antibacterial properties.Eug-Gly-CSFs showed significant inhibition effect on Escherichia coli and Staphylococcus aureus.At the same time,it showed excellent free radical scavenging of DPPH or ABTS and reducing ability.Finally,Eug-Gly-CSFs could prolong the shelf life of rice noodles from less than 10 d to 40 d.Finally,the effects of the addition of transglutaminase(TGase)on the hydrophobic and mechanical properties of Eug-Gly-CSFs were investigated,and modified composite films with different hydrophobicity were prepared.The experiment results showed that appropriate TGase induced crosslinking can not only make the film structure more compact,significantly improve the hydrophobicity of the films(contact angle increased from 60.4°to 92.6°),but also enhance the network structure of the films,and further improve the mechanical properties of the films.When the TGase content was 30 U/g,reaction time was 3 h,the toughness of TG-Eug-Gly-CSFs could reach 4.654 MJ/m~3.When applied to fresh-cut lemon,TG-Eug-Gly-CSFs can maintain the complete structure and fit deformation on the surface of lemon,and prolong the shelf life of lemons from less than 7 d to 12 d.Above all,casein and shellac formed a complementary co-constructed structure after p H-cycle treatment,which was a good substrate for preparing bio-based composite fresh-keeping films.The addition of Gly and Eug made the composite films have a more compact microstructure,excellent mechanical properties,and significant antioxidant and bacteriostatic effects.The cross-linking effect of TGase realized the controllable hydrophilicity and hydrophobicity of the modified composite films,and broadened the application scope of protein films in food preservation. |
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