| Fresh fruits and vegetables have good taste and flavor,and are rich in vitamins,polyphenols,minerals,and dietary fiber,which are important components of the daily human diet.Due to mechanical damage,microbial infection and post-harvest physiological reactions,fruits and vegetables can cause a lot of waste during storage and transportation.Edible films/coatings are safe,non-toxic and easily degradable,as well as an eco-friendly method of preservation by reducing water loss,respiration rate,preventing mechanical damage and improving the appearance of fruits and vegetables to extend shelf life.However,the low mechanical and barrier properties of edible films/coatings prepared from natural polymers and the volatility of active substances have been limited their application and development.In this study,corn starch and konjac gum were used as film-forming substrates,and citric acid was added to cross-link with corn starch and konjac gum in order to improve the physical properties of corn starch/konjac gum composite films.Meanwhile,β-cyclodextrin/thymol microcapsules（TCD）were prepared and incorporated into the composite film withβ-cyclodextrin as the wall material and thymol as the core material to achieve the continuous release of thymol in the composite film.Finally,the composite films were applied to the preservation of cherry tomato fruits to explore the effects of composite films on the physicochemical properties and nutritional values of tomato fruits.The main studies were as follows:（1）The citric acid cross-linked composite films were prepared by the solution casting method using corn starch and konjac gum as raw materials and adding citric acid for the cross-linking reaction.Firstly,the film formation mechanism of the cross-linked composite film was characterized and analyzed by Fourier infrared spectroscopy（FTIR）,X-ray diffraction（XRD）,thermal stability analysis（TGA）and scanning electron microscopy（SEM）techniques.Secondly,the effects of citric acid cross-linking on the water vapor transmission rate,moisture content,swelling,solubility,mechanical properties,opacity,and the effects of citric acid cross-linking on the performance of corn starch/konjac gum composite films were investigated.The results showed that the esterification reaction between citric acid and corn starch and konjac gum formed a highly entangled network structure,and the surface pattern of the composite film was more complex,which improved the thermal stability of the composite film,but the addition of citric acid did not change the crystallinity of the composite film.In addition,the addition of citric acid improved the water vapor transmission rate,opacity,mechanical properties,moisture content and swelling of the composite film,but improved the solubility of the composite film.（2）TCD was prepared by co-precipitation method and added to the film-forming solution to construct the active composite film system.Using the embedding rate as the evaluation index,the optimal embedding conditions were determined by orthogonal experiments as follows:thymol andβ-cyclodextrin molar ratio of 1:1,embedding temperature of 35℃,and embedding time of 4 h.At this time,the maximum embedding rate of thymol was 69.75±0.39%.（3）TCD was added to the citric acid cross-linked composite film to prepare SK-AC composite film,and the interaction between TCD and composite film molecules was found to be hydrogen bonding interaction by FTIR,XRD,TGA and SEM,and the addition of TCD improved the crystallinity of the composite film and overall improved the thermal stability of the composite film.And the appropriate amount of TCD had good compatibility with the composite film matrix,but the excessive amount of TCD showed microcapsule coalescence phenomenon,which had a negative impact on the composite film.The results of the analysis of the performance of the active composite film showed that in the concentration range of5%-15%,the addition of TCD reduced the water vapor transmission rate of the composite film,and when the TCD content was 10%,the composite film had the lowest water vapor transmission rate of 0.69±0.1·10-6·g·m-1·h-1·pa-1;However,when the concentration was 40%,the water vapor barrier performance of the active composite film was lower than that of the composite film without TCD addition;with the increase of TCD concentration,the water content of the active composite film decreased to 78.67%of the original,while the swelling and solubility increased to143.16%and 140.97%.The TCD-added composite films showed good antibacterial properties against E.coli and S.aureus,but the antibacterial properties decreased with increasing time.Overall,the addition of TCD improved the mechanical properties,water vapor barrier properties,light barrier properties,thermal stability,moisture content,swelling and bacterial inhibitory activity of the composite films,and achieved the sustained release of thymol from the composite films.（4）Citric acid cross-linked corn starch/konjac gum composite films loaded with TCD were applied to the preservation of cherry tomato fruits.The results showed that the composite film of SK-AC-TCD treated tomato fruits by dipping and coating effectively alleviated the tissue softening and water evaporation during post-harvest ripening,while promoting the accumulation of soluble solids,phenolics and total flavonoid compounds in the fruits and improving the antioxidant properties of the fruits during storage at room temperature for 8 days.Thus,SK-AC-TCD coating treatment of cherry tomatoes maintained the nutritional value of the fruits and extended the shelf life of the fruits.The SK-AC-TCD significantly improved the physical properties of the composite film,while solving the problems of hydrophobicity and volatility of thymol,achieving the continuous release of thymol in the composite film.The composite film can effectively extend the shelf life of tomato fruits and maintain the nutritional value of the fruit.This study provides theoretical support and reference for the research of edible film/coating in the preservation of fruits and vegetables. |
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