| Edible coating is a green technology of keeping fruits fresh by coating edible macromolecule on the surface of fruits to form a semi-permeable coating to regulate the postharvest respiration and prolong the storage period of fruits.At present,researchers mainly improve the effect of coating by adding active substances such as anti-bacterial and anti-oxidation,but the surface of fruit is usually covered by non-polar waxy layer,it is difficult to form good wetting and spreading on the surface of fruit because of the strong polarity of polysaccharide and protein coating,which affects the uniformity and stability of the film-forming structure.Plant essential oil is a kind of natural and high effective bacteriostatic agent and antioxidant.It can also reduce the surface tension of hydrophilic natural polymer solution as a lipid mixture containing many kinds of non-polar components,improves the interface between the fruit and the fruit.Therefore,in this study,citral,the main component of lemongrass essential oil,was prepared into nanoemulsion and then added to chitosan film,the effects of nanoemulsion content and fruit surface characteristics on the wetting and spreading behavior of the composite coating solution on the fruit surface were investigated.The main research findings and findings are as follows:1.Citral nanoemulsion was prepared by using Tween 80 as emulsifier,and the coating was prepared by adding citral nanoemulsion into chitosan with different concentration(0.1%,0.2% and 0.3% citral),the interaction between citral nanoemulsion and chitosan,and the effect of citral nanoemulsion on the microstructure,tensile properties and permeability of the films were investigated.The results showed that the particle size,polydispersity index and Zeta potential of the nanoemulsion were131.6 ± 1.52 nm,0.217 ± 0.007 and-22.9 ± 1.3 m V,respectively.Citral nanoemulsion was physically embedded in the chitosan molecular chain,which weakened the interaction between the chitosan molecules,promoted the flow of chitosan coating solution and was beneficial to the spreading of chitosan coating solution.Besides,the citral nanoemulsion played a “Plasticizing” role,enhanced the film’s plasticity,and it can also enhance the film on water and oxygen permeability.2.The effects of different concentrations of Tween 80(The content of Tween 80 was 0.015%,0.075%,0.15%,0.225%,0.375%),citral/Tween 80(The content of citral was 0.02%,0.1%,0.2%,0.3%,0.5%,citral : Tween 80 =5:4)and citral nanoemulsion(The content of citral was 0.02%,0.1%,0.2%,0.3%,0.5%)on the wettability and spreading properties of chitosan coating solution were investigated.The results showed that the surface tension of chitosan coating solution decreased from 58.45 m N/m to 43～ 34 m N/m,and the contact angles of coating solution on the surface of tangerine fruit decreased.Citral nanoemulsion can improve the spreading ability of chitosan coating solution by reducing the work of cohesion of chitosan coating solution.Compared with chitosan coating solution containing Tween 80,the composite chitosan coating solution containing citral nanoemulsion had a better spreading effect on the peel of tangerine.3.The surface properties of blueberry,cherry tomato,loquat and shatang mandarin were characterized,and the wettability of citral nanoemulsion/chitosan composite coating solution on four kinds of fruits were investigated.According to the surface free energy,the four kinds of fruits were as follows: shatang mandarin(32.36 m N/m),loquat(28.28 m N/m),cherry tomato(27.48 m N/m),blueberry(21.13 m N/m);According to the order of Surface roughness from large to small,they are shatang mandarin(10.76μm),loquat(8.9 μm),blueberry(1.9 μm)and cherry tomato(1.45 μm).Among them,shatang mandarin which has the largest surface free energy and the roughest surface was the most easily wetted by the composite coating solution,while the surface free energy is the lowest,the relatively smooth blueberry was the least easily wetted.The wettability depends not only on the surface free energy of the fruit,i.e.the hydrophobic interaction between the fruit surface and the coating solution,but also on the surface microstructure.4.Citral nanoemulsion/chitosan composite coatings with citral content of 0.1%,0.2% and 0.3% were applied to shatang mandarin and cherry tomato,respectively.The results showed that micropores appeared on the surface of stomata of shatang mandarin peel,but the size of micropores decreased with the addition of citral nanoemulsion.Shatang mandarin had higher respiratory rate after harvest,which could be significantly reduced by composite coating(p<0.05),so the decay rate was effectively reduced,especially when the content of citral was 0.2%.The respiratory rate of cherry tomato decreased sharply with the prolongation of storage time,and the coating had no significant effect on it(p> 0.05).Although the composite coating could reduce the mildew of the fruit,it could make the fruit soften and deteriorate.It may be that the skin of cherry tomato was thinner,it was difficult to tolerate citral stimulation on pericarp cells,but 0.1% citral nanoemulsion/chitosan composite coating could maintain the nutrient content of cherry tomato.To sum up,the citral nanoemulsion reduces the cohesion and surface tension of the chitosan coating solution by reducing the intermolecular force of chitosan and enhancing its non-polarity,thus enabling it to spread better on the fruit surface,but the improvement effect is also related to the surface free energy and micro-morphology of the fruit.Citral nanoemulsion can enhance the effect of chitosan coating on granulated tangerine,but its effect on cherry tomato is limited because of citral’s irritability. |
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