Preparation And Physicochemical Properties Of Chitosan-Chicken Skin Collagen Composite Films

In recent years,with the increasing consumption of livestock and poultry,a large number of by-products have been made,but the conversion rate for processing these by-products is not high and they are not fully utilized.Livestock and poultry organisms are rich in collagen(Co),whose typical three-stranded helix structure gives it many excellent physical and chemical properties and special physiological effects.Therefore,the extraction of collagen from livestock and poultry by-products can provide a new way for its high-value utilization,thus reducing environmental pollution and creating huge economic benefits.In this study,collagen was extracted by ultrasound-assisted method using chicken skin as raw material.The effects of ultrasound on the physicochemical and functional properties of collagen were examined,and the feasibility of ultrasound-assisted method as a collagen extraction method was investigated.Secondly,the edible composite film was prepared by using chicken skin collagen blended with chitosan(CS)to solve the problems of poor steaming resistance and thermal stability of collagen film.Finally,the effect of adding TGase as a cross-linking agent on the structure and properties of CS/Co films was investigated to further enhance the mechanical properties,cooking resistance and thermal stability of CS/Co composite films.The main research results are as follows:(1)Optimization of the collagen extraction process.In this study,the process of extracting collagen from chicken skin by ultrasound-assisted enzymatic extraction was studied.Taking solid-liquid ratio,pepsin dosage,ultrasonic output power and ultrasonic treatment time as experimental factors,the technology of ultrasonic assisted enzymatic extraction of collagen was explored,and the prediction model was established to optimize the extraction process.Firstly,the influence of 4 factors on collagen extraction and their optimal range were determined by single-factor experiments.Then,the response surface was optimized.The results showed that ultrasonic-assisted enzymatic preparation significantly improved the extraction rate of collagen,and ultrasonic power and ultrasonic time had significant(P﹤0.05)effect on extraction rate.In addition,the optimal extraction condition was obtained based on regression equation and prediction model: the solid-liquid ratio of 1:73 g/m L,enzyme dosage of 1.58%,ultrasonic power of 260 W,and ultrasonic time of 26 min(2 s On-2 s Off).Among the four independent variables,ultrasonic treatment time played the most important role on extraction rate,followed by ultrasonic power,solid-liquid ratio,and enzyme content.The extraction rate reached 83.42% ± 0.65% under the optimal conditions,which was consistent with the predicted value.(2)Acid-soluble collagen(ASC),enzyme-soluble collagen(PSC),ultrasound-acidsoluble collagen(UASC)and ultrasound-enzyme-soluble collagen(UPSC)were prepared by different extraction methods,and their structures and physicochemical properties were compared.The results showed that all four collagens were type I collagens with intact triple helix structure and high purity.The denaturation temperature of ASC was higher compared to PSC,but the denaturation temperature of enzyme-soluble collagen was higher after the ultrasound treatment,which might be related to the interaction between ultrasound and enzymatic extraction methods.The collagen extracted by all four methods had high relative solubility at p H 1-4 and Na Cl concentration less than 3%.The microstructures were all characterized by fibrous,lamellar,loose,porous and multilayer aggregates,and the structures of UASC and UPSC were more loose than those of ASC and PSC.In conclusion,the ultrasound-assisted enzymatic method has no negative effect on the triple helix structure of collagen,has less effect on the physicochemical properties of collagen,and improves the solubility and enhances the thermal stability of collagen to a certain extent,which is a feasible method for collagen extraction.(3)Chitosan was blended with collagen,and the preparation process of chitosan-collagen composite film was optimized by using orthogonal experimental design.The optimal process conditions were: CS/Co ratio of 1:4,glycerol content of 60%,cross-linking temperature of60 °C and cross-linking time of 2 hours.The thermal expansion and water absorption under these conditions were 22.80 ± 1.41% and 30.80 ± 1.54%,respectively.By comparison,it was found that the steaming resistance,mechanical properties and thermal stability of the CS/Co composite film were improved compared to the Co film without the addition of chitosan modification.This was attributed to the strong intermolecular interactions between the two and the stable network structure formed together,while the CS molecules themselves formed a certain network structure with the interspersed coexistence of CS and Co networks.The microstructure showed that the CS was well mixed with collagen.the XRD results showed that the addition of CS also had no significant effect on the crystal structure changes of the films.(4)The cross-linking of TGase was used to improve the physicochemical properties of CS/Co films,and the mechanism of action of TGase was investigated.The results show that the cross-linking effect of TGase can significantly improve the tensile strength and elongation at break of the composite film system,but the mechanical properties can be reduced by excessive addition.The grease barrier properties showed the same result.The steaming resistance shows a continuous increase.The results of the infrared spectroscopy showed that the α-helix content of the collagen increased and the β-fold and β-turn angle content decreased after modification by TGase,which helped to maintain the structural stability of the CS/Co composite film.x-ray diffraction results showed that TGase caused changes in the spatial structure of the molecular chains of the CS/Co composite film.the addition of TGase caused changes in the thermal shrinkage rate and thermal shrinkage temperature of the film system.The microstructural results show that the new chemical bonding generated by the cross-linking action resulted in a flatter film.