Preparation And Application Of Bi-Mn Bimetallic Oxide Catalyst For Degradation Of Ethylene In Banana Preservation By Modified Atmosphere Packaging

Bananas（Musa nana Lour.）are rich in nutrients and have high health value,but they are often wasted after harvest due to their rapid respiration rate and production of plant hormones such as ethylene.Catalytic degradation of ethylene and inhibition of banana respiration rate can effectively prolong the shelf life of bananas,which has a broad development prospect.According to the characteristics of physiological metabolism of banana,the respiration intensity model of banana was established at different temperatures and the conditions of banana freshness preservation were designed and determined from the perspective of low temperature storage.In addition,Bi-Mn bimetallic oxide catalyst for ethylene degradation was prepared according to the characteristics of ethylene generation in bananas at different temperatures.Futhermore,the effects of the molar ratio of Bi-Mn,the mesh of catalyst,temperature and the initial concentration of ethylene on the catalytic performance were investigated,and the better preparation parameters and catalytic degradation conditions were obtained.Bi-Mn bimetallic oxide catalyst was applied to preservation of bananas by modified atmosphere packaging,and good results were obtained.The main contents and results of this study were as follows:（1）Research on respiratory metabolism,model-building and packaging design of banana’s preseavation.The amount of ethylene produced in the package varied greatly from the temperture.The higher the temperature was,the greater the amount of ethylene produced.The concentration of ethylene reached 500 ppm at 25℃at the end of storage,higher than 339 ppm at 20℃,65.5 ppm at 15℃and 19 ppm at 10℃,indicating that the catalytic degradation of ethylene catalyst performance level is ppm.The respiration intensity of banana was positively correlated with temperature and negatively correlated with storage time by the sealed system method.The results showed that the respiratory rate of bananas was positively correlated with temperature and negatively correlated with storage time.The empirical model with the highest fitting degree of banana respiration rate was established at 15℃.There was no significant difference between the measured value and the predicted value verified by T-test（P>0.05）.According to the empirical model and gas balance equation,the theoretical parameters of package for the banana’s preservation were obtained:The theoretical permeability coefficients of O₂ and CO₂ were 4.923×10^-13and 5.507×10^-13 cm³·cm/（cm²·s·Pa）.Based on the predicted respiration rate of O₂ and CO₂,the modified atomosphere menbrane with the CO₂ permeability coefficient slightly higher than the theoretical value was selected.The permeability coefficients of the modified PE membrane for O₂ and CO₂ were 2.059×10^-13 and 7.586×10^-13 cm³·cm/（cm²·s·Pa）,which were close to theoretical values.Therefore,the most suitable packaging for banana storage under 15℃is modified PE.（2）Preparation and characterization of Bi-Mn bimetallic oxide catalysts.The Bi-Mn bimetallic oxide catalysts were prepared by the method of hydrothermal synthesis step-by-step.SEM showed that nano-scale spherical Mn O₂ interlaced with micron-scale branch-shaped Bi₂O₃ to make the whole appearance of broccoli,with the particle size of about 10μm.EDS further indicated the coexistence of Mn,Bi and O elements in the catalyst.The elements of Mn and O were distributed evenly,while Bi element was more concentrated.Moreover,Mn O₂ and Bi₂O₃ were the main compound components,which is consistent with XRD results.The size of BET showed a first increase and then a downward trend depending on the content of Bi,which was up to 71.66 m²/g.The thermal stability of the Bi-Mn catalyst was improved with the residual mass increased from 83.28%to 88.48%.The XPS results showed that the element valence of the Bi-Mn bimetallic oxide catalyst had not changed significantly,but the ratio of Mn³⁺/Mn⁴⁺and the content of surface adsorption oxygen had increased,thus enhancing the catalytic degradation of ethylene.In summary,Bi element had been successfully doped.（3）Study on the factors influencing the performance of Bi-Mn bimetallic oxide catalysts and the mechanism of catalytic degradation of ethylene.A series of single-factor experiments exploring the catalyst performance were carried out.The best preparation conditions of Bi-Mn bimetallic oxide catalysts were Bi-Mn（0.1:10）,4 m L HNO₃,and the original mesh;the optimal catalytic conditions in a closed environment of 6L for the degradation of C₂H₄ were catalyst dosage of 1 g,catalytic ambient temperature of35℃and initial ethylene concentration of 50 ppm.The Bi-Mn bimetallic oxide catalysts achieved the catalytic degradation of ethylene at room temperature with the highest capability of completely degradation rate of 40%of 100 ppm within 6 h.Combined with the results of characterization and catalytic degradation experiments,the catalytic degradation mechanism of ethylene over Bi-Mn bimetallic oxide catalyst was explored.It was prelimarily believed that ethylene was degraded into CO₂ and H₂O under the synergistic action of Bi-Mn bimetallic oxide.Therefore,Bi-Mn bimetallic oxides effectively achieved the catalytic degradation of trace ethylene at room temperature.（4）Application of Bi-Mn bimetallic oxide catalyst to degrade ethylene in banana’s preservation by modified atmosphere packaging.According to the theoretical parameters of modified atmosphere packaging for banana’s preservation,the modified membrane of PE matched with banana respiration rate was selected.The experiment was carried out by using Bi-Mn bimetallic catalyst to degrade ethylene at the optimum temperature for banana’s preservation and the temperature at which the catalyst acted well.The results showed that Bi-Mn bimetallic oxide catalyst could effectively prolong the shelf life of bananas for 3days at 25℃and 5 days at 15℃,respectively.Futhermore,the catalyst could effectively inhibit the production of ethylene at room temperature and the production of ethylene at the end of storage was 274 ppm,which was much lower than that of the control group（500ppm）,effectively reducing by 45.2%.Under the condition of 15℃,ethylene production was about 48 ppm,which was lower than that of the control group（65.5 ppm）,effectively reducing by 26.7%.Bi-Mn bimetallic oxide catalyst was used to degrade ethylene in the modified atmosphere packaging,forming a relatively low O₂ and high CO₂ gas atmosphere.The O₂ and CO₂ concentrations were maintained at 10%and 12%at 25℃,while the corresponding concentrations were maintained at 12%and 7%at 15℃,and the respiration intensity of banana was always lower than that of the control group.After storage,the weight loss was reduced,the hardness of fruit flesh and the permeability of cell membrane were well maintained and the change rate of soluble solids or p H was slowed down.Therefore,Bi-Mn bimetallic oxides maintained the overall sensory quality and physicochemical indexes at an ideal level by degrade ethylene,effectively prolonging the shelf life of bananas.