| The post-harvest loss rate of fresh fruits and vegetables in China is over 30%,which is much higher than the average level in developed countries,making the storage,transportation and preservation technology of fresh fruits and vegetables become the key technical problem to be solved urgently in the processing,storage and transportation of agricultural products.Equilibrium modified atmosphere packaging?EMAP?is able to adjust the gas composition in the storage environment to keep it at a suitable level for a long time,thus can prolong the storage period of fruits and vegetables.The key of EMAP technology lies in the membrane used.The excellent EMAP membrane can automatically establish and maintain ideal atmosphere conditions under a certain storage environment mainly depending on its own gas permeability,and ultimately achieve the goal of extending the storage period of fruits and vegetables.In this paper,a novel polymer material,polyether ether ketone?PEEK?,was used as the organic matrix,and sulfonated polyether ether ketone?SPEEK?was prepared by sulfonation modification with concentrated sulfuric acid.AGO/SPEEK EMAP membrane with sulfonate-based?-SO3H?network was prepared by solvent volatilization with the amino-functionalized graphene oxide?AGO?as the inorganic filler.The interaction between SPEEK organic polymer chain and AGO inorganic filler and its regulation mechanism to improve the performance of EMAP membrane,especially the CO2 permeability,CO2/O2permselectivity and water vapor permeability were clarified.Results obtained could provide theoretical basis and technical support for the development and application of novel spontaneous EMAP membrane,and has a great significance for the storage and transportation of fresh fruits and vegetables in China.The main research contents and results are as follows:?1?SPEEK EMAP membrane was prepared by sulfonation modification of PEEK,and the effect of sulfonation time on the membrane performance was studied.Results indicated that the sulfonation degree of PEEK after 5-25 h sulfonation was between 51.6 and 92.5%.After sulfonation of PEEK,the sulfonic acid group was successfully introduced into the molecular structure,resulting in significant increase of solubility and continuous decrease of crystallinity.Compared with the one-stage thermal degradation behavior of PEEK,the mass loss of SPEEK could be divided into three stages,SPEEK membrane with low or moderate sulfonation degree had higher thermal stability.In addition,with the increase of sulfonation time,the sulfonation degree of SPEEK increased significantly,the tensile strength and elastic modulus of SPEEK membranes decreased significantly,the elongation at break had a increasing trend,the hydrophilicity,water uptake rate,area swelling rate and thickness change rate also increased significantly,the CO2 and O2 gas permeability increased gradually,the ideal gas separation factor??CO2/O2?increased from 2.50 to 8.37.The water vapor permeability of SPEEK membranes with different sulfonation degrees was between 1.48-3.1210-12 g.cm/?cm2·s·Pa?,which was much higher than that of PE membrane,making SPEEK membranes not conducive to the preservation of fresh fruits and vegetables.?2?GO was first synthesized by improved Hummers method and AGO was then prepared by modification of GO with ethylenediamine.The structure and properties of AGO were characterized.Results indicated that the carboxyl and epoxy groups in GO reacted with ethylenediamine,demonstrating the successful modification of GO.After functionalization,the 2?angle of AGO decreased continuously,the diffraction peaks became wider,and the thermal stability increased.XPS results revealed that a new absorption peak?N1s?at the binding energy of 400.0 eV appeared,and the intensity of C-OH,C-O-C and O-C=O groups significantly reduced or disappeared,the emergence of a new absorption peak under the binding energy of 285.8 eV corresponded to the C-N group.All these results could further verify that the functionalization of GO mainly achieved by the reaction of ethylenediamine with oxo groups to form C-N bond?amide and imine?.SEM and TEM observation showed that GO had a flaky structure with irregular folds,and presented in the form of transparent yarn thin slice with clear edge contour,while AGO had loose and flat flake layer,shallow surface wrinkles and more fuzzy surface and edge.?3?AGO was used as an inorganic filler to embed in SPEEK membrane to prepare AGO/SPEEK EMAP membrane.The interaction between SPEEK organic polymer chain and AGO inorganic filler and its regulation mechanism to improve the performance of AGO/SPEEK EMAP membrane were clarified.Results indicated that there was strong acid-base interaction between-SO3H groups and NH2 groups in AGO/SPEEK EMAP membrane.The hydrogen bonds and static electricity formed between AGO and SPEEK?polar groups such as-OH,-O-and C=O in AGO and O-H group in SPEEK?greatly destroyed the orderly arrangement of SPEEK polymer chain.XRD spectra showed that the diffraction peak at 15-30°was further weakened in AGO.SEM observation showed that AGO particles were uniformly embedded into SPEEK membrane,and the cross section of the film was rougher than that of the unmodified one due to the interaction between AGO inorganic filler and SPEEK organic molecular matrix.Compared with GO/SPEEK membrane,AGO/SPEEK membrane had higher thermal stability and mechanical properties.Compared with SPEEK membrane,the water absorption rate and area swelling rate of GO/SPEEK and AGO/SPEEK membrane decreased with the increase of GO or AGO content.After the addition of AGO,the water vapor permeability showed a decline trend.With the increase of AGO content,the CO2 permeability,and both??CO2/O2?and??CO2/N2?of AGO/SPEEK EMAP membrane increased.Improved CO2 permeability and??CO2/O2?would be more conducive to the storage and freshness preservation of fruits and vegetables.?4?AGO/SPEEK EMAP membranes were adopted for the storage of cherry tomato and lettuce,and their sensory quality and physical and chemical changes in the storage process were analyzed.Results showed that AGO/SPEEK EMAP membranes had higher CO2permeability and??CO2/O2?permeability.The sensory scores and hardness of cherry tomatoes packaged with AGO/SPEEK EMAP membranes decreased more slowly with the storage time than other groups,the changes of color difference in the same storage period was lower and the acceptable storage period was 25-30 days.The pH values of cherry tomatoes presented a gradual increasing trend during the early storage and decreased during the late storage for all groups.No significant differences of weight losses were observed in cherry tomatoes packaged with AGO/SPEEK EMAP membranes,but were all significant lower than those of control group and PVDC group.Soluble solids content of cherry tomatoes packaged with AGO/SPEEK EMAP membranes only decreased in the range of 10-20%in the late storage,which were significant better than the control group and other packaging group.The vitamin C content of cherry tomatoes increased first and then decreased,but remained at about 30mg/100 g at the end of storage?30 days?.The sensory score,color difference,pH value,weight loss rate and other quality changes of lettuce packaged with AGO/SPEEK EMAP membranes were similar to those of cherry tomatoes.Vitamin C content of lettuce packaged with AGO/SPEEK EMAP membrane showed a overall decline trend during storage,and chlorophyll content gradually decreased from 58 mg/kg to 12-15 mg/kg at the end of storage,significantly better than the control group.Therefore,AGO/SPEEK EMAP membrane could maintain the sensory quality and physical and chemical indexes of cherry tomato and lettuce to a certain extent,and played a certain role in extending the freshness preservation period of fruits and vegetables.However,AGO/SPEEK EMAP membrane was obviously far from satisfying the storage of leafy vegetables with high respiratory intensity.?5?A robust and sensitive ultra high performance liquid chromatography tandem mass spectrometry?UHPLC-MS/MS?method was developed for the analysis of seven mycotoxins in fruits and vegetables.The variations of seven mycotoxins concentrations in fruits and vegetables during storage with AGO/SPEEK EMAP membranes were also determined for the first time.Results showed that the validated method provided sufficient selectivity,sensitivity and accuracy to screen for seven mycotoxins at trace levels,without using standard addition or matrix-matched calibration to compensate for matrix effects.Limit of quantification?LOQ?of the method could reach 0.2?g/kg,and the average recoveries were between 81.1 and 116%with intraday-and interday-precision being in the ranges of 3.0-6.2%and 4.2-6.1%,respectively.The method was successfully applied for mycotoxins analysis in fruits and vegetables,and mycotoxins concentrations were all below their limits of detection?LODs?.The low temperature and EMAP membrane were beneficial for extending storage period of leafy vegetables.Alternariol?AOH?and alternariol monamethyl ether?AME?were detected in strawberry,and tenuazonic acid?TeA?was detected in tomato fruits during long-term storage.Their concentrations significantly increased along with storage time.EMAP membrane could significantly slow down the occurrence time of mycotoxins in strawberry and tomato and to a certain extent to reduce their contents. |
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