| The extensive use of non-degradable packaging materials causes serious pollution to the environment.With the enhancement of people’s awareness of environmental protection,the development and research of green,environmentally friendly and renewable biodegradable materials have been paid more and more attention.Chitosan（CS）is the second most abundant biological material on earth.CS not only has a wide range of sources and no toxicity,but also has good biodegradability.CS has been widely used in food industry and other fields.However,the disadvantages of low mechanical strength and poor chemical stability of CS material greatly limited its application.Generally,the properties of CS materials are usually improved by modification.The abundant amino groups and hydroxyl groups of CS provided good reaction sites for its modification.CS and cellulose have attracted much attention due to the biocompatibility and chemical similarity of their polysaccharide structures.Cellulose is the most widely distributed and highest content natural polymer in nature,which has excellent biodegradability and biocompatibility.However,in addition to its excellent properties,cellulose also has the disadvantages of simple performance,poor chemical reactivity and poor dimensional stability.C2,C3 and C6 have highly reactive hydroxyl groups on the cellulose molecular chain,which can be oxidized into aldehyde or carboxyl groups,providing more possibilities for the functional modification of cellulose.2,3-dialdehyde cellulose（DAC）is a derivative of cellulose in which the C2 and C3 hydroxyl groups are oxidized under the action of periodate.And the obtained DAC suspension can be heated at 100°C for 1 h to make it soluble.The aldehyde groups in DAC can be used as a potential crosslinking joints to further react with the amino group through a Schiff base reaction to realize the strong covalent bond between CS and DAC.CS was selected as the matrix material in this study.Based on the Schiff base reaction between CS and DAC,two forms of composites（hydrogels and films）were prepared.The structure and properties of the 2,3-dialdehyde cellulose/chitosan（DAC/CS）composites were systematically evaluated,and the effects of DAC/CS composite coating on the storage quality of tangerine fruits were explored.The main research contents and results are as follows:（1）Preparation and characterization of DAC/CS hydrogels.High degree of oxidation DAC was prepared by sodium periodate oxidation of cellulose（25℃,72 h）,and heated at 100°C for～1 h to obtain water-soluble DAC.Four kinds of DAC/CS hydrogel with different ratios（9）DAC:9)CS=1、2、3、4,respectively)were prepared by adding DAC solution into CS solution drop by drop.The composition,structure and morphology were characterized by elemental analysis,Fourier Transform Infrared Spectroscopy（FT-IR）and Scanning Electron Microscope（SEM）.The grafting rate of CS on DAC and the change and movement of absorption peaks at 1732 cm-1 and 1593 cm-1 in FT-IR confirmed the crosslinking effect between DAC and CS;SEM results showed that the porous network structure of DAC/CS hydrogels was more and more dense with the increase of DAC content.The thermal stability experiment results showed that the addition of DAC changed the thermal stability of CS.The DAC/CS hydrogels had higher degradation initiation temperature and more weight loss stages due to the more complex crosslinking network;The results of water content and swelling property test showed that the water content and swelling rate of DAC/CS hydrogel decreased gradually with the increase of DAC content,and the swelling rate had no significant dependence on p H;The water retention test results showed that the increase in temperature would make the hydrogel release more water,and the higher the DAC content,the better the water retention capacity of the hydrogels;The compression test results showed that the gel strength of DAC/CS hydrogel was gradually strengthened with the increase of DAC content,and the maximum compression stress was increased by 275%;The rheological properties test results showed that the storage modulus（G’）and loss modulus（G"）of DAC/CS hydrogel increased with the increase of DAC content;Compared with DAC-1/CS hydrogel,the G’and G"of DAC-4/CS hydrogel increased by about 906%and 629%,respectively.（2）Preparation and characterization of DAC/CS hydrogel films.DAC was prepared by sodium periodate oxidation of cellulose（45°C,26 h）,and heated at 100°C for～1 h to obtain water-soluble DAC.Four kinds of DAC/CS hydrogel films with different ratios（9）DAC:9)CS=2%、6%、10%、14%,respectively)were prepared by adding DAC solution into CS solution by the solvent evaporation method at 45°C.FT-IR and SEM were used to characterize the structure and morphology.The change of peak shape of 1380 cm-1 and1559 cm-1 in FT-IR spectrum confirmed the interaction between DAC and CS;SEM images showed that the surface of DAC/CS hydrogel films was slightly agglomerated and orderly layered folds appeared on the cross section.The thermal stability test results showed that the thermal stability of DAC/CS hydrogel films decreased gradually with the increase of DAC content;Swelling test results showed that the addition of DAC could significantly enhance the stability of DAC/CS hydrogel films in aqueous solution,especially under acidic conditions,and with the increase of DAC content,the stability of hydrogel films was gradually enhanced;The results of color and light transmittance tests showed that the addition of DAC made the yellow color of the DAC/CS hydrogel films more and more obvious,enhanced the light transmittance and UV blocking ability.The mechanical properties showed that the addition of DAC could increase the tensile strength（TS）of DAC/CS hydrogel films.Compared with pure CS film,the TS of DAC-2%/CS hydrogel film was increased by 58%.The results of contact angle（CA）and water vapor permeability（WVP）showed that DAC/CS hydrogel films had good hydrophobicity,and the hydrophobicity gradually decreased with the increase of DAC content.Oxygen permeability（OP）test results showed that the addition of DAC could reduce the Oxygen permeability of DAC/CS hydrogel,and DAC-6%/CS composite membrane had the best Oxygen barrier effect.（3）Effect of DAC/CS composite coating on the storage quality of tangerine fruits.The composite coating solution of DAC-6%/CS was applied to the coating preservation of tangerine fruits,and compared with bagging treatment,commercial wax coating treatment and CS coating treatment.The weight loss rate,decay rate,hardness,color,nutrient content and breathing intensity of tangerine fruit were measured regularly.The morphology of peel surface was observed regularly.The results showed that CS and DAC/CS coatings were more stable than commercial wax coatings,and pores appeared only on the 18th day of storage;DAC/CS composite coating treatment could significantly reduce the respiration intensity of tangerine fruits during storage;Compared with bagging and CS coating,DAC/CS coating could effectively slow down the natural yellowing time of tangerine fruit during storage;DAC/CS coating could effectively inhibit the softening,weight loss and decay of tangerine fruits during storage;DAC/CS coating had no significant effect on the changes of nutrients during storage of tangerine fruits. |
