Synthesis Of Chitosan-protocatechuic Acid Graft Copolymer And Its Preservation Effect On Pleurotus Eryngii

In this study,protocatechuic acid was graftecd onto chitosan through amide and ester linkages by an EDC/NHS coupling method.The structural characterization,physical property,antioxidant and antibacterial activity,film-forming ability of' protocatechuic acid grafted chitosan(PA-g-CS)and its effect on the postharvest quality of pleurolus eryngii were investigated in details.(1)Three grafted copolymers with different grafting ratios(61.64.190.11 and 279.69 mg/g)were obtained by varying the molar ratios of' reaction substrates,and named as PA-g-CS I,PA-g-CS II and PA-g-CS III,respectively.PA-g-CS showed similar UV absorption peaks as protocatechuic acid at 258 and 292 nm.As compared to chitosan,PA-g-CS exhibited a decreased band at 1596 cm-1 and a new band at 1716 cm-1,suggesting the formation of amide and ester linkages between protocatechuic acid and chitosan.New proton signals at ? 6.77-7.33 ppm were observed on 1H NMR spectrum of PA-g-CS.assigning to the methine protons of protocatechuie acid.Signals at ? 150,8?116.6 ppm on 13C NMR spectrum of PA-g-CS were assigned to the aromatic ring carbon of protocatechuic acid moieties.All the structural information confirmed the successful grafting of protocatechuic acid onto chitosan.XRD revealed the crystallinity of PA-g-CS was lower than chitosan.SEM observation showed chitosan had a smooth surface,while PA-g-CS had a rough surface.TGA also revealed the thermal stability of PA-g-CS was lower than chitosan.The rheological property of PA-g-CS solutions was closely related to the grafting ratio.(2)The antioxidant and antibacterial activity of' PA-g-CS were also investigated.Results showed that PA-g-CS exhibited higher reducing power,lipid peroxidation inhibition effect,O2·-OH,H2O2 and DPPH radicals scavenging activities,as well as higher antibaetri(?)activity on Vibirio Pcarahemolyticus,Staphylococcus aureus,Listeria monocytogenes.Salmonella than native chitosan.(3)Chitosan and PA-g-CS were further prepared into films.THe physicochemical.mechanical and antitoxidant properties of filmsms with different grafting ratios were characterized.The transparent PA-g-CS films exhibited yellowish color with thickness ranging from 44.1 to 48.6 ?m.By contrast with chitosan film,PA-g-CS ? and PA-g-CS ? films showed decreased moisture content,water vapor permeability:however,increased water solubility and tensile strength.The barrier to UV light of chitosan film could he improved by grafting with protocatechuic acid.An increase in the total color difference was observed for PA-g-CS films,indicating PA-g-CS films were more colored.Among all the kinds of films tested.PA-g-CS ?film exhibited the highest tensile strength and elongation at break values(45.7 MPa and 5.05%).According to thermogravimetric analysis,the maximum rates of weight loss for PA-g-CS films appeared at 291.8?294.0?.By contrast with chitosan film,SEM observation showed that PA-g-CS films were much rough.The cross-section morphology of PA-g-CS ? film was much more smooth than that of other films.Antioxidant activity assays showed that PA-g-CS films had both dose-dependent and time-dependent scavenging activity on DPPH radical.(4)The effect of PA-g-CS coating on the postharvest quality of Pleurotus eryngii was investigated by determination of various physico chemical parameters and enzyme activities.At the end of 15 days of storage,serious mushroom browning was observed in the control and chitosan coating groups.By contrast,PA-g-CS coating groups with medium and high grafting degrees maintained better physical appearance.Among all the treatment groups,Pleurotus eryngii in PA-g-CS III coating group exhibited the highest firmness;however,the lowest weight loss,browning degree,respiration rate,malondialdehyde content,electrolyte leakage rate,superoxide anion production rate and hydrogen peroxide content.Moreover,Pleurotus eryngii in PA-g-CS III coating group maintained relatively higher antioxidant enzyme activities but lower polyphenol oxidase activity than other treatment groups.