Preparation Of Agar/Sodium Alginate Composite Films And Application In Beef Preservation

The white pollution caused by traditional plastic packaging has been paid more and more attention.Since the"Plastic Restriction Order"was promulgated in China,finding degradable packaging materials that can replace plastic packaging has become an urgent problem for many scholars at home and abroad.Therefore,in recent years,edible film has become a research hotspot in the field of food packaging at home and abroad.However,the performance of the edible film prepared from a single film-forming substrate often has some drawbacks.Compound different film-forming materials with complementary advantages can prepare a composite edible film with better comprehensive performance.Agar and sodium alginate are hydrophilic colloids extracted from marine plants,which have good film-forming,degradability and safety,and can be used as raw materials for edible film.Therefore,in this study,using agar and sodium alginate as the film-forming materials,the agar/sodium alginate composite membrane was prepared by solution casting method.The comprehensive performance of the composite membrane was improved by introducing cross-linking agent binding layer self-assembly method.Then,the composite membrane was applied to the refrigeration of fresh beef to explore the effect of the composite membrane on the freshness of beef.The results are as follows:（1）Edible films were prepared with agar and sodium alginate as film-forming materials.The effects of four different crosslinking agents on the properties of agar/sodium alginate composite films were compared.Agar/sodium alginate composite films were prepared by solution casting method using glycerol as plasticizer and four edible acids（ferulic acid,tannic acid,citric acid,succinic acid）as crosslinking agents.The effects of different crosslinking agents on the properties of composite films were investigated by measuring the mechanical strength,water solubility,water vapor permeability,opacity,scanning electron microscopy and infrared spectroscopy.The results showed that the compatibility of agar and sodium alginate was good,and the crosslinking agent could make the cross section of the composite films more compact and smooth.The four crosslinking agents could significantly improve the tensile strength,water resistance and moisture resistance of the composite film（P<0.05）,but significantly reduce the transparency of the film（P<0.05）.When the addition amount of ferulic acid and tannic acid was 5%,citric acid and succinic acid was 10%,the crosslinking agent had the best improvement effect on the composite films.Among the four cross-linking agents,the citric acid cross-linked film had the best performance.When the addition amount was 10%,the tensile strength was 46.98 MPa,the elongation at break was17.87%,the water solubility was 24.17%,the swelling rate was 38%,and the water vapor permeability was 0.51 g·mm·（m²·h·KPa）^-1.Infrared spectroscopy analysis showed that citric acid and succinic acid improved the performance of the composite films by esterification with-OH in agar and sodium alginate molecules.Ferulic acid and tannic acid achieved cross-linking by forming intermolecular hydrogen bonds with agar and sodium alginate.（2）In order to further improve the comprehensive performance of agar/sodium alginate composite films,the effects of blending and layer-by-layer self-assembly methods combined with crosslinking agents（citric acid and ferulic acid）on the comprehensive performance of composite film were compared.Agar/sodium alginate blend films and bilayer films were prepared by blending and layer-by-layer self-assembly methods using agar and sodium alginate as film-forming substrates,and cross-linking agents citric acid and ferulic acid were introduced.The mechanical properties,moisture resistance,thermal stability,color difference,opacity,scanning electron microscopy and infrared spectroscopy of the composite films were measured.The effects of different film preparation methods combined with cross-linking agents on the performance of the composite films were investigated.The results showed that the double-layer film had better mechanical properties,water resistance and thermal stability.Compared with the blending film,the tensile strength TS of the double-layer film increased by 22.54%,the elongation at break EB increased by 19.58%,the water solubility decreased by 21.20%,and the swelling ratio decreased by 6.15%.The addition of cross-linking agent further improved the TS,water resistance and thermal stability of the double-layer films.Among them,the citric acid cross-linked double-layer film had the best comprehensive performance,that is,the tensile strength was 51.57 MPa,the elongation at break was26.02%,the water solubility was 24.42%,and the swelling ratio was 101.67%.Infrared spectroscopy analysis showed that citric acid was covalently cross-linked by forming C=O by esterification with seaweed polysaccharides,while ferulic acid was non-covalently cross-linked by forming hydrogen bonds with the film-forming matrix.（3）In order to explore the preservation effect of agar/sodium alginate composite films,the application of composite films in beef preservation was studied.Agar/sodium alginate bilayer film was used to wrap fresh beef.The p H,color,total volatile basic nitrogen,thiobarbituric acid value,sensory score and total number of bacterial colony of beef were measured at 4°C,and the preservation ability of the bilayer film to fresh beef was evaluated by comparing with the commercial PE film.The results showed that compared with commercially available PE film and Bi film,Bi CA and Bi FA film wrapping treatment significantly delayed the increase of p H,TVB-N,TBARS and total number of bacterial colony of beef during 10 days of storage,and the color of beef was more stable and the sensory score was higher.Although the mechanical properties of Bi film meet the requirements of food packaging,the preservation effect is not ideal,which is the same as that of commercial PE film.Beef deteriorates after 4 days due to microbial action and lipid oxidation.Bi FA film had strong antioxidant activity due to the addition of ferulic acid,which significantly delayed the fat oxidation of beef during storage,maintained the color of beef,and had the best preservation effect.Compared with commercial PE film and Bi film,the preservation period of beef was prolonged by 3 days.