| Environmental pollution and shortage of petroleum resources are two major challenges facing human beings in today's society.Developing degradable packaging materials to replace traditional petroleum-based packaging materials will become a trend in food packaging industry.So more researchers begin to pay attention to biodegradable packaging materials.Based on previous research on preparation conditions of zein films,glycerol and citric acid were selected as plasticizers to prepare high barrier zein films,and the mechanical properties,hygroscopicity,water resistance,oxygen resistance,oil resistance and water solubility of films under different temperatures(5?,20?,35?)and different relative humidity(RH34%,RH54%,RH80%)were studied.Besides,the mechanism of properties change of zein films was obtained.Finally,the mechanical properties and barrier properties of zein films in different conditions were verified by the application test.Firstly,properties of zein films with different ratios of glycerol and citric acid at different temperatures and relative humidity were studied.With the time increased,the tensile strength(TS)of the 2:3 film(glycerol:citric acid)and the 1:1 film increased while the elongation at break(EB)decreased gradually.The films stored at a high temperature and low relative humidity had poor stability of mechanical properties.The films stored at 5?RH80%had less change in mechanical properties than others,and the films stored at RH34%and 35?RH54%became brittle and films at 35?RH80%were sticky.All films stored at RH34%were brittle,and 35?was negative to properties change of zein films.The results of moisture absorption rates(Ra)showed that different films at RH54%were different from that at RH80%.At the same temperature,the Ra of films at RH80%was lower than that at RH54%;at the same relative humidity,the Ra of films at 20?was higher than that at 5?.The higher glycerol addition ratio caused the higher moisture absorption rate of films.The results of water vapor permeability(WVP)and oxygen permeability(OP)showed that the WVP and OP of the films decreased and increased gradually after 20 weeks.During storage,the films stored at 5?RH54%and 20?RH80%had less change in water vapor permeability and oxygen permeability than others,respectively.The 1:1 film had better storage stability than the 2:3 film in water and oxygen resistance.The results of oil permeability(PO)shows that the PO of films decreased gradually during storage.Films stored at 5?had better storage stability in oil resistance than others.Films stored at 20?RH54%had best oil resistance than others,and the PO of the 2:3 film and the 1:1 film were0.260g·mm·m-2·d-1and 0.210g·mm·m-2·d-1,respectively.Besides,the results showed that water solubility of all films decreased gradually with the time increased,and films stored at20?RH80%had the worst storage stability in water solubility.Secondly,in order to explore the mechanism of properties change of zein films under different storage conditions,the glycerol migration,moisture content and free sulfhydryl content of films were studied.Films stored at 5?had higher moisture content than others and the films with more glycerol addition had higher moisture content.With the time increased,the glycerol migration and free sulfhydryl content of films increased and decreased gradually,respectively.Films stored at 20?RH80%had the higher glycerol migration and lower free sulfhydryl content than others.Correlation analysis showed that the free sulfhydryl content and the glycerol migration of films were related to its mechanical properties at RH54%,and the moisture content and glycerol migration of films were related to its mechanical properties at RH80%.Analysis of barrier properties showed that glycerol migration and moisture content were related mainly to the WVP of films,and the free sulfhydryl content was related mainly to the OP and PO of films.SDS-PAGE showed that more large molecular weight protein polymers were formed in films at a higher storage temperature,and the main force was disulfide bond.The intermolecular interaction results showed that the hydrophobic interaction and hydrogen bonding of films decreased in all conditions.The effect of disulfide bond increased significantly during storage and more disulfide bonds were formed in films at a higher temperature.The FTIR results showed that the zein films storaged at a higher temperature had a weaker hydrogen bonding than others.Finally,2:3 films and 1:1 films were used for the packaging of sauce,and plastic films were used as the control group.The seasoning bags were stored in a sealed bag at different temperatures.The zein films stored at 35?were broken,but these bags at 5?had better flexibility and a higher moisture content of sauce than others.The acid value and peroxide value of sauce in zein films stored at 35?were higher than the the national standard at 24weeks.The results showed that there was no significant difference in oil resistance between the two zein films.The 2;3 film had the better water and oxygen resistance,and zein films were suitable for the packaging of sauce at a lower temperature. |
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