| According to the National Bureau of Statistics,China’s fruit production reached nearly 300 million tons in 2021,ranking first in the world.However,fruit still maintains vigorous life activities after picking.Under the combined action of its own respiratory metabolism,microorganisms,ethylene and external mechanical losses,fruit is easy to rot and deteriorate,leading to the shortening of its shelf life.Fruit preservation materials have the functions of water blocking,air regulating preservation and bacteriostasis.At present,the commonly used food preservation material on the market is a kind of polymer plastic packaging film,which takes petroleum-based polymers as raw materials（such as polyvinylidene chloride and polyethylene）.Although it has the advantages of low price and convenience,it cannot be recycled and difficult to degrade,causing serious environmental pollution.Therefore,it is of great significance to develop green and efficient biological fruit cling film.zein has attracted extensive attention in the field of food packaging because of its excellent film forming and hydrophobicity.Our previous study found that zein can adsorb ethylene through its active functional groups（such as sulfhydryl groups,etc.）and prolonged the shelf life of fruit.However,the dense structure and poor processing performance of natural zein restrict its ethylene adsorption properties and its application in food processing.It is urgent to stretch the structure of zein in a green and safe way to expose more active functional groups and improve its ethylene adsorption and processing properties.In addition,microbes can also accelerate fruit decay.Natamycin,as a broad-spectrum antibiotic and food preservative,can effectively inhibit the growth of microorganisms.Based on the above background,in this paper,80%acetic acid and 80%ethanol were used as solvents to systematically study the synergic modification of zein with different ultrasonic intensity and two solvents,screen out the best process parameters to stretch the structure of zein,and investigate the vinyl adsorption properties and fruit preservation properties of modified zein film.In addition,the modified zein-natamycin composite film was prepared by combining with natamycin,which had wide bacteriostatic and high safety,so that it had good ethylene adsorption and bacteriostatic properties,and extended the shelf life of fruit.The research results have a general theoretical guidance and reference value for extending the storage life of fruit and improving the quality of fruit preservation.The research contents are as follows:（1）The change rule of zein molecular aggregation state under different ultrasonic time（0 min,5 min,15 min and 30 min）and ultrasonic power（0 W,200 W,400 W and 600 W）in 80%acetic acid solvent was systematically studied,and the preparation method of zein based vinyl adsorption film assisted by ultrasound was defined.The internal relationship between protein structure changes and ethylene adsorption properties,mechanical properties,gas permeability and fruit preservation properties（banana skin browning rate,banana hardness and banana weight loss rate）of zein film was explored,providing new ideas and theoretical basis for effectively extending the shelf life of bananas.The results showed that when the ultrasonic power was 200 W,with the extension of ultrasonic time（0 min,5 min,15 min and 30 min）,the structure of zein gradually stretched,the active groups in the protein were exposed,and the content of sulfhydryl group increased.When the ultrasonic power was 400 W and 600 W,with the extension of ultrasonic time（0 min,5 min,15 min and 30 min）,the zein structure showed a change of first stretching and then aggregation.These results indicated that moderate ultrasound could stretch the zein structure,while excessive ultrasound could lead to reaggregation of zein.When the ultrasonic intensity was 400 W and 15 min,the average particle size of zein was the lowest（1013.3±6.9 nm）.The proportion ofα-helix structure was the lowest（45.86%）.The proportion of β-sheet structure was the highest at 12.20%.At the same time,compared with zein-0 film（without ultrasonic treatment）,the ethylene adsorption performance of the optimized zein15 film increased by 9.486 mg·m-3·h-1,the oxygen resistance performance increased by 0.75×10-16 kg·m·m-2·s-1·Pa-1,which effectively reduced the surface browning degree of bananas（44.72%lower than the control group）,the weight loss rate（73.03%lower than the control group）,and improved the banana flesh hardness（28.65%higher than the control group）,and prolonged the shelf life of banana.（2）The influence of ultrasonic time（0 min,5 min,15 min and 30 min）on zein structure was systematically studied in 80%ethanol solvent,ultrasonic power was 400 W,and the preparation method of D-zein film assisted by ultrasound was defined.The relationship between the protein structure changes and the ethylene adsorption properties,mechanical properties and gas permeability of zein film was explored.The internal relationship between D-zein film and the shelf life of banana（banana skin browning rate,banana hardness and banana weight loss rate）was studied by coating the prepared zein solution on the surface of banana.It provides a new idea and theoretical basis for effectively extending the shelf life of banana.The results showed that when the ultrasonic time was 15min and the ultrasonic power was 400 W,the α-helix content of zein was the lowest（40.20%）,and the β-sheet structure content was the highest（19.40%）.The protein structure was stretched and more active groups（the content of sulfhydryl group）were exposed.The prepared zein film has good ethylene adsorption properties,mechanical properties（tensile strength and elongation at break）and oxygen isolation properties.They were 28.23±0.43 mg·m-3·h-1,3.89±0.22%,25.25±2.10 MPa and 1.72±0.13×10-16kg·m·m-2·s-1·Pa-1,respectively,which could effectively delay the changes of skin browning rate,the bananas pulp hardness was the highest,67.04±2.3g;the lowest weight loss rate was 13.50%,and prolonged its shelf life.（3）The depolymerization-zein-natamycin（D-Z-Nt）film with both ethylene adsorption and antibacterial properties was prepared by the synergistic action of the depolymerization zein and natamycin.The effects of natamycin on mechanical properties,contact Angle,water vapor transmittance,oxygen transmittance,ethylene adsorption efficiency and antibacterial properties of zein film were investigated.The results showed that with the increase of natamycin content（0,0.2,0.4,0.6,0.8 and 1.0 mg/mL）,the water contact angle increased,the water vapor permeability decreased,the oxygen permeability decreased and the ethylene adsorption efficiency increased.When the content of natamycin was 1.0 mg/mL,the lowest water vapor permeability was 0.89±0.09×10-16 kg·m·m-2·s-1·Pa-1.The lowest oxygen transmittivity was 0.70±0.05×10-16 kg·m·m-2·s-1·Pa-1.The highest ethylene adsorption efficiency was 18.86±0.28 mg·m-3·h-1.In addition,D-Z-Nt film could effectively inhibit Geotrichum candidum and Wickerhamonyces anomalous from banana.When natamycin content was 1.0 mg/mL,the inhibition zone diameter of D-Z-1.0Nt film against Geotrichum candidum was（2.2±0.14）cm.The diameter of inhibition zone against Wickerhamonyces anomalous was（2.45±0.05）cm.At the same time,the shelf life of bananas coated with D-Z-Nt films was longer than that of bananas in the control group.After 7 days under the same conditions,the skin browning rate of bananas coated with D-Z-1.ONt was the lowest,the hardness of banana peel reached the maximum（92.94±2.59）g,and the hardness of banana pulp reached the maximum（90.32±3.35 g）.In conclusion,D-Z-Nt films can cooperate with natamycin for ethylene adsorption and bacteriostatic properties,limited to extend the shelf life of fruit. |
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