Preparation Of Cu₃（BTC）₂/PVC/PET Gas-Regulating Membrane For Fresh-cut Pomegranate Preservation

Pomegranate is high valued for its nutritional properties and health care function,but the pomegranate is easy to burst during maturation,which causes great loss.Making the pomegranate grains as“ready-to-eat”food has a good market prospect,which can greatly reduce the loss of pomegranate and increases its value.In this article,the breathing model of pomegranate grains was established and the modified atmosphere packaging of pomegranate grains was designed based on the characteristics of respiratory metabolism of pomegranate grains.According to the modified atmosphere packaging conditions of pomegranate grains,Cu₃（BTC）₂/PVC blended gas regulating membrane was prepared and the moisture permeability,gas permeability,CO₂/O₂ separation coefficient of blended gas regulating membrane were studied.By compositing the Cu₃（BTC）₂/PVC membranes with PET,the Cu₃（BTC）₂/PVC/PET composite membranes were prepared to establish a modified atmosphere packaging system for pomegranate grains and obtained good effects of modified atmosphere preservation.The main contents and results of this study were as follows:（1）Establishment of pomegranate grain respiration model and modified atmosphere packaging design.The closed system method was used to measure the respiration intensity of pomegranate grains in the packaging environment during storage.The respiration model of pomegranate grains at different temperatures was established.According to the respiration model of pomegranate grains,the parameters of modified atmosphere packaging of pomegranate grains were determined.The results showed that the respiration rate of pomegranate grains increased with the increase of storage temperature,and the respiration intensity of pomegranate grains gradually weakened with the extension of storage time.Among the six respiration models of pomegranate grain,the correlation coefficient of the equation of the competitive enzyme kinetic model was the largest under the storage condition of 4℃.This model was chosen to be the optimal model of pomegranate grains at this temperature.By verified,the relative error between the predicted value and the predicted value of the optimal model was between 1.19%and 5.45%.And the T test results illustrated that there was no significant difference between the measured value and the predicted value.The parameters of the modified atmosphere packaging of pomegranate grains,determined by breathing model and the gas balance equation were as follows:storage temperature:4℃,storage environment gas composition:3%O₂、10%CO₂、87%N₂,20×10×3 cm³ PP packaging box,100 g of pomegranate grains per packaging box,effective diffusion area of packaging membranes:221.82 cm²,thickness packaging membranes:76um,and the permeability coefficient of packaging membranes to CO₂ and O₂:/（cm²?h?0.1MPa）.（2）Preparation and structural characterization of Cu₃（BTC）₂/PVC blend membranes.Cu₃（BTC）₂ was synthesized by hydrothermal method,and used to modify PVC by physical blending.Cu₃（BTC）₂/PVC blend membranes were prepared by volatile solvent method.The influence of Cu₃（BTC）₂ on the structure of blended membranes was studied.The results indicated that the Cu₃（BTC）₂ modifier synthesized by hydrothermal method displayed a smooth surface and uniform morphology.The crystals presented regular octahedrons with high crystallinity and a specific surface area of 982.35 m²/g.The adsorption capacity of CO₂ reached to 56.15 cm³/g.The surface of Cu₃（BTC）₂/PVC blend membranes prepared with volatile solvent was dense,and Cu₃（BTC）₂ could be uniformly dispersed in blend membranes without agglomeration.The addition of Cu₃（BTC）₂ could reduce the crystallinity and increase the thermal stability of the blend membranes.（3）Study on properties of Cu₃（BTC）₂/PVC blended membranes.The addition of Cu3（BTC）2 filler improved the hydrophilicity of the blend membranes and reduced the volume expansion rate after absorbing water;The water vapor transmission coefficients of blend membranes with different Cu₃（BTC）₂ addition were significantly higher than that of pure PVC membranes.When the addition amount of Cu₃（BTC）₂ was 0.5%,the water vapor transmission coefficient of the blend membranes reached to 6.0×10^-14 g.cm/（cm²·s·Pa）,which was 3 times than PVC membranes.The moisture permeability of blend membranes was significantly improved;As the amount of Cu₃（BTC）₂ increasing,the permeability of the blend membranes to CO₂ and O₂ differred greatly.The permeability coefficient of the blend membranes to CO₂ was obviously increased greatly,and the permeability coefficient to O₂was increased gently.By controlling the addition amount of Cu₃（BTC）₂,blend membranes with different gas selection coefficients could be obtained.The selectivity of the blend membranes increased from 1.38 to 3.70;And he selectivity of the gas diffusion coefficient of the blend membranes（CO₂/O₂）was less than that of the pure PVC membrane,the selectivity coefficient remained basically unchanged,and the selectivity of the gas solubility coefficient was significantly higher than that of pure PVC membranes.The higher ideal selectivity of blend membranes was attributed to the increase in solubility coefficient selectivity.（4）Study on the preservation of pomegranate grains with Cu₃（BTC）₂/PVC/PET modified atmosphere composite membranes.Using the PET as the base membranes,the Cu₃（BTC）₂/PVC/PET composite membranes were prepared by solvent evaporation method to establish modified atmosphere packaging system of Cu₃（BTC）₂/PVC/PET composite membranes.And the fresh-keeping effect of pomegranate grains under different packaging membranes was studied.The results showed that under different storage conditions,the pomegranate grains of the composite membrane modified atmosphere preservation group showed the best sensory quality,with better sensory on the 21st day of storage,followed by the PE membranes group,which microbial contamination appeared on the 16th day of storage.The quality of pomegranate grains in the control group decreased significantly on the third day of storage,the weight loss rate reached up to 6%,the outer skin began to shrink and lost its luster gradually.Although the gas permeability coefficient of the modified atmosphere membranes used in the actual experiment was slightly different from that of theoretically designed atmosphere membranes,a new packaging environment with low O₂（～3.5%,v/v）and high CO₂（～13.5%,v/v）was formed.The equilibrium gas concentration was not much different from the theoretically designed equilibrium gas concentration.The new equilibrium gas concentration formed by the composite membrane kept the respiratory intensity of pomegranate grains at a relatively low level（<0.40m L/（kg·h））,which was slightly lower than the theoretical design value of 0.44 m L/（kg·h）).The composite membranes could effectively suppress the increase of weight loss rate during storage of pomegranate grains,the decrease of total phenol content,soluble solids content,Vc content and anthocyanin content.On the 18th day of storage,the total phenol content,Vc content and anthocyanin content of the composite membranes group were 1209.61 ug/m L,66.88 ug/m L,and 70.97 mg/m L,which were 1.20%,14.60%,and 36.01%higher than the PE membranes group respectively.Compared with the control group and the PE membranes group,the storage period of pomegranate grains in the composite membranes group was extended by 12 days and 8 days,respectively.