Research On Adsorption And Diffusion Of Characteristic Flavor Substances From Liquid Foods Into Flexible Plastic Packaging Film

The flavor of food is an important quality characteristic of food,which determines the quality and safety of food.Polyethylene and polypropylene film are commonly used for flexible plastic packaging in the market.After a long time of packaging food,they will absorb the flavor of food.The change of their quantity will lead to the deterioration of food quality,thus shortening the shelf life of the packaged food.The adsorption behavior of flavor substances also changes the performance of packaging materials,thus affecting the migration of residual compounds or oligomers in packaging materials,thus endangering the quality of food and human health.The research on the mechanism of adsorption and diffusion in flexible plastic film packaging for liquid food is still limited and insufficient,which restricts the complex influence factors of flavor substances to a certain extent.The main flavor components and contents of milk,orange juice and yellow rice wine were determined by solid phase microextraction（SPME）and gas chromatography-mass spectrometry（GC-MS）.The flavor substances of the alcohol,ester,ketone and terpene in the adsorption study were determined.A two-sided and one-sided adsorption model were constructed to analyze and compare the adsorption amounts of flavor substances from packaging materials with different thicknesses and types,influenced by film thicknesses,flavor structure,initial concentration,food simulated species and storage temperature.Molecular dynamics（MD）simulation method was used to analyze the effects of degree of polymerization,single diffusion,co-diffusion,simulation temperature,polymer species,flavor molecule species and their mass fraction on the diffusion behavior of flavor molecules in polymer cells.The diffusion behavior of O₂ and H₂O before and after polyethylene’s（PE）adsorption was further analyzed by combining the binding energy between molecules and the free volume of the polymer.Based on the results of experiments,MD simulation and prediction models,the diffusion coefficient of flavor substances in packaging materials and the partition coefficient of flavor substances in packaging materials/liquid phase were compared and analyzed,so as to better guide the correct use of food packaging materials.This paper mainly carries out the following research:（1）Study on adsorption detection method of characteristic flavor substances in liquid foodAiming at eight kinds of pure milk,six kinds of orange juice and 4 kinds of yellow rice wine with different brands and packaging materials on the market,solid phase microextraction（SPME）was established for the extraction and analysis of flavor substances in three kinds of liquid food,and the chromatographic conditions for the detection of flavor substances in food by GC-MS were explored.A fast sample pretreatment method for solid phase extraction was determined.After experimental comparison,a 50/30μm DVB/CAR/PDMS solid phase extraction head was used to extract samples at the optimum temperature of 70℃for 30 minutes.The volatile flavor compounds in food were retrieved by NIST05 mass spectrometry database,and the relative content of each volatile flavor substance was calculated by area normalization method.By comparing the experimental results of flavor substances in three kinds of food,nine volatile flavor substances of alcohol,ester,ketone and terpene were determined.（2）Determination of experimental conditions and methods for adsorption of flavor substancesVegetable oil,3%acetic acid solution,10%ethanol solution and distilled water were used as food simulants.9 volatile flavor compounds（D-limonene,myrcene,ethyl butyrate,ethyl lactate,ethyl caproate,phenylethanol,linalool,2-heptanone and 2-nonanone）were used as solutes.By comparing the experiments,the adsorption unit model of liquid-solid experiment was determined,and the two-sided and one-sided membrane immersion adsorption model were constructed.The extraction of packaged film adsorbents was carried out by ultrasonic extraction with n-hexane solution.The ultrasonic water bath time was 1 h and the ultrasonic frequency was 60 Hz.According to the results of several pre-experiments and the boiling point of 9 flavor substances,the chromatographic conditions of flavor tests were determined,and the peak time and standard curve’s equations of nine flavor substances were obtained by external standard method.（3）Study on the influencing factors of the adsorption amounts of flavor substances in plastic packaging materialsThe types of flavor substances（terpenes,ketones,esters,alcohols）,initial concentration(100,200,500,1000 mg·L^-1),film thickness（40μm,50μm,80μm LDPE film）,storage temperature（4,23,40,64℃）,packaging materials（low density polyethylene（LDPE）,biaxially oriented polypropylene（BOPP）,cast polypropylene（CPP）and black-and-white PE）were studied and analyzed by two-sided immersion adsorption experiments.The effects of some factors on the adsorption amounts of flavor substances in plastic packaging materials were studied.The changes of packaging materials before and after adsorption were analyzed by single-sided film adsorption experiment.（4）Molecular dynamics simulation of adsorption and diffusion in plastic packaging filmThe adsorption and diffusion characteristics of D-limonene,myrcene,ethyl hexanoate,2-nonanone and linalool in LDPE and other plastic film materials were studied by MD simulation.The degree of polymerization（40,80,160,320）,single diffusion,co-diffusion,simulation temperature（277 K,296 K,313 K and 337 K）,types of polymers（PE,polypropylene（PP）,polyethylene teraphthalate（PET））,sub-mass fraction（2%,3%,4%,5%）were analyzed about the effects of the diffusion behavior of flavor molecules in amorphous polymer cells.The diffusion behavior of O₂ and H₂O before and after PE’s adsorption were further analyzed by combining the binding energy between molecules and the free volume of polymers.The adsorption solubility coefficients of 5 flavor molecules,O₂ and H₂O in PE at different temperatures were analyzed by MD and Grand canonical Monte Carlo（GCMC）simulation methods.（5）Comparison and analysis of important parameters in the process of adsorption and diffusionBased on Fick’s second law of adsorption and diffusion model,the diffusion coefficient and partition coefficient were studied and compared,which are important parameters for the adsorption and diffusion of flavor substances in polymer materials,obtained from prediction model（Limm-Hoffifield model and Brandsch model）,MD simulation and experimental data.In the diffusion coefficient analysis,the diffusion coefficient obtained by Brandsch model was smaller and more stringent than that obtained by Limm-Hoffifield model.In Brandsch model,the diffusion coefficient of the same polymer was mainly affected by temperature and molecular weight of flavor molecules.The diffusion coefficients of model prediction and MD simulation were higher than those of experiment fitting;the predicted diffusion coefficients were not affected by initial concentration;the MD simulation diffusion coefficients were not significantly affected by initial mass fraction,while the experimental diffusion coefficients were not affected by initial concentration in the range of 100-500 mg·L^-1;and the diffusion coefficients of flavor substances in LDPE were not affected by initial concentration.With the increase of temperature,the relationship between diffusion coefficient and temperature basically accorded with Arrhennius relationship,and the ratio of MD simulated diffusion coefficient to experimental value of flavor substances in BOPP was smaller than that of LDPE,CPP and black-and-white PE materials.In the analysis of partition coefficients,the partition coefficients obtained by experiment and normal solution method were mainly compared.The experimental values of partition coefficients were less than those obtained by normal solution method.Normal solution method mainly calculated the ratio of solubility parameter difference,and its predicted value need a lot of empirical data,which had certain limitations.