Enzymatic Cross-linking And Film-forming Properties Of Collagen

As an important biological material,collagen has good biocompatibility,excellent biodegradability,and low antigenicity.Accordingly,collagen has also been used as a dietary supplement,carrier,additive in food field,with an increasing focus on its application in edible film and coating.However,the film based on collagen and its hydrolysate(gelatin)has some shortcomings,such as insufficient mechanical strength and some unacceptable barrier properties(water resistance,etc.).Recently,modification of proteins with different cross-linking mechanisms to obtain better substrate materials has become an important approach for research and development of edible films.In this study,transglutaminase(TGase)was used as cross-linking agent to cross-link collagen with different molecular morphology and structure,and the cross-linking mechanism and strength were investigated.At the same time,the edible films with collagen and its hydrolysate(gelatin)as substrate were cross-linked with TGase to form into composite film,and the change of film properties before and after cross-linking was observed in order toobtain better film preparation conditions,with the aim of exploring the application of TGase in collagen films and its industrialized production.Firstly,type I collagen,with and without terminal telopeptide,was extracted from bovine skin by acid and enzyme methods,respectively.TGase was added and cross-linked with collagen at different temperatures.The physicochemical properties of the protein samples were then characterized.The results showed that the amino acid composition and isoelectric point of TRC and TPC changed.After 37? treatment of the sample,the hydrogen bonds to maintain the molecular triple helix structure were destroyed,collagen began to solve the spiral,and subsequently TGase enhanced the thermal stability of protein molecules through crosslinking with collagen molecules.The molecular structure of the samples treated at low temperature was not changed,and the cross-linking effect was not obvious.Secondly,the cross-linking degree between TGase and collagen fibers treated under different pyrolysis temperatures,and the properties of related films were investigated.With the increase of pyrolysis temperature,the degree of hydrolysis(DH)of film-forming solutions increased gradually and particle size distribution tended to be smaller,indicating that the triple helix structure of the collagen fibers was destroyed and partially degraded to short chain proteins.Accordingly,the thermal stability and mechanical properties of films have declined significantly.However,as seen from SEM,the film-forming properties improved and film became dense and smooth.After TGase addition,the related properties of films were enhanced,suggesting that TGase promoted the formation of tighter and more stable physical and chemical network structures within the film.Besides,no matter what TGase presence,no significant diffienrece in the undenatured collagen fiber(treated at 4?,25?),which further proved that TGase is difficult to cross-link with collagen when its molecular structure remains intact.Finally,the gelatin-collagen composite films were prepared with TGase as cross-linking agent,and the properties of the film were tested.The thickness of the composite film varied little.With the increase of gelatin ratio,the light transmittance of films and water soluble(WS)increased,while water vapor permeability(WVP)and,the oxygen permeability decreased.The film surface tended to smoother and internal structure was contact,indicating the modification of the film forming property of the material.Also,the thermal stability of the composite film increased after cross-linking.The mechanical performance of the film was best when the gelatin addition was 10%:tensile strength reached 36.10±3.21 MPa and the elongation rate was 6.51±0.23%.The result indicated that the degradation of gelatin is more complete,more intramolecular enzymatic sites can be exposed outside and more isopeptide bonds(?-(?-glutamine)lysine)form,beneifting the polymerization of these biomolecules.In summary,it is difficult to cross-link with intact collagen by TGase addition,indicating that the cross-linking site exists mainly in the helix structure.Gelatin-collagen composite film can form a more stable and tight network structure after cross-linking,showing excellent mechanical properties and barrier properties,preliminarily providing the potential applications in the edible packaging industry.