Analysis On Water Retention Capability Of Citrus Fruit Epicuticular Wax And Development Of Novel Fruit Wax Coating

Citrus is the most important fresh-eating fruit in the world.Postharvest water loss is one of key factors affecting its fruit freshness.It is extremely important to clarify relevant factors affecting postharvest water loss for fruit freshness maintenance,storage and transportation.Epicuticular wax is a continuous hydrophobic membrane covering plant surface,and has considerable influence on water loss.However,there has been a lack of in-depth studies of the correlation between postharvest water loss and complex epicuticular wax components.Besides,structure and components of natural wax layer on fruit surface can be damaged during cleaning and are difficult to recover in a short time after continuous washing in the postharvest processing line.Therefore,it is necessary to treat fruit with synthetic coatings in postharvest processing line to prolong its fresh-keeping life.Currently,commercial coatings are usually used to increase the glossiness and shelf life.Wax coatings widely used in main citrus producing areas of China are mainly imported from foreign countries such as United States or Spain.However,practices have shown that these coatings are not suitable for citrus production in China,with some prominent problems such as short shelf life,accumulation of off-flavor substances,poor flavor and rapid loss of market competitiveness.In order to satisfy the long-term storage demand of citrus fruit,polyethylene single-film packaging is used as the main preservation method in main citrus producing area of China for nearly 40 years.Single fruit bagging has obvious freshness-keeping effect,but is labor-intensive and time-consuming due to low degrees of mechanization in production.In addition,the film is not recyclable,resulting in serious environmental pollution.Thus,the main research contents and results are as follows:1.Composition and morphology of epicuticular wax in the natural population of citrus were different among varieties.Aldehydes in epicuticular wax were significantly negatively correlated with fruit postharvest weight loss rate,and synthetic C24/C26 aldehydes have good water retention and fresh-keeping effects after treatment of satsuma mandarins.A total of 75 species of mature citrus fruit,including mandarins,tangerines,oranges and pomelos,were selected from the National Citrus Breeding Center as experimental materials.The water retention capacity of epicuticular wax in different citrus varieties was measured based on the difference in weight loss during storage.Crystal morphology of epicuticular wax was observed;components of epicuticular wax were detected and their correlation with fruit weight loss was analyzed.Scanning electron microscopy（SEM）observation revealed that the weight loss rate of fruit variety with flaky waxy crystal structure was significantly lower than that of other crystal structures,while stomata number exhibited no relevance to fruit weight loss rate.Epicuticular wax content in citrus fruit was 0.50–11.45μg·cm^-2,in which aldehydes accounted for the largest proportion（>40%）,followed by alkanes and alcohols,and the proportion of fatty acids was the lowest.These results demonstrated that aldehydes had an important influence on postharvest water loss of citrus fruit.Satsuma mandarins（Citrus unshiu Marc）treated with coatings of 1%C24aldehydes and 1%C26 aldehydes showed a significantly lower fruit postharvest weight loss rate,which could also effectively maintain peel firmness and color index as well as reduce respiration rate.These results indicated that both C24-aldehyde and C26-aldehyde have good water retention and fresh-keeping effect on satsuma mandarins.2.Coating prepared based on candelilla wax-beeswax had good fresh-keeping effect for long-term preservation of citrus fruit and exhibited potential to replace polyethylene single film packaging.Candelilla wax and beeswax were found to be the most appropriate components for coating as indicated by measurement of water retention capacity and detection of off-flavor compounds of citrus fruit under treatment with different materials.Then,a series of coatings with different ratios of candelilla wax and beeswax were prepared to treat citrus fruit,and the emulsion state and fresh-keeping capacity were evaluated.No.15“BC”coating（Beeswax-candelilla wax based coating）was selected to treat satsuma mandarins and Newhall navel oranges（C.sinensis Osbeck）.Long-term storage preservation performance after coating was evaluated and feasibility of substitution for polyethylene in single film packaging was analyzed.It was demonstrated that“BC”coating had a strong fresh-keeping effect.“BC”coating could greatly decrease the weight loss rate of satsuma mandarins（28 d）and Newhall navel oranges（56 d）and effectively maintain peel firmness and color index.Concentration of CO₂ in satsuma mandarins showed a moderate increase after“BC”coating treatment,and respiratory rate dramatically decreased from 43.36 m L·kg^-1·h^-1to 16.75 m L·kg^-1·h^-1.In addition,“BC”coating could significantly delay changes in the TSS（Total Soluble Solid）and TA（Titratable Acid）in treated fruits,as well as the increase in contents of TPC,TFC and primary metabolites in peel,which promoted the sensory quality of satsuma mandarins compared with the control,with general satisfaction score increasing from 3.09 to 3.69.Detection of off-flavor compounds showed that the ethanol concentration of“BC”coating-treated satsuma mandarins(777.90 mg·L^-1)was apparently lower than that of the fruit treated with imported commercial coating(2021.20 mg L^-1),and the ethanol concentration of“BC”coating-treated Newhall navel orange(106.66 mg·L^-1)was also significantly lower than human perception threshold(1500.00 mg·L^-1).No significant difference was found in internal quality,appearance quality and multiple biochemical indicators between the polyethylene film-packaged and“BC”coating-treated satsuma mandarins.Test results on the postharvest processing line demonstrated that“BC”coating has a good compatibility with existing fresh-keeping equipment.Above results suggest that“BC”coating can be a promising alternative to polyethylene film artificial single fruit packaging.3.Shellac-resin based coating could increase fruit glossiness and shelf life when used for fresh sale fruit,and its combination with“BC”coating may be an effective way to improve the commodity value of citrus fruit.Considering the fresh-keeping needs of citrus fresh fruit on sale,coating“SC”（Shellac-resin based coating）produced with an optimal ratio of shellac,resin and Hydroxypropyl methyl cellulose was used to treat satsuma mandarin and Lane Late navel orange（C.sinensis Osbeck）and then evaluate the improving effect on the commercial value of citrus fruit.It was found that“SC”treatment could markedly enhance surface glossiness and reduce weight loss rate and peel shrinkage during shelf life.Moreover,“SC”was found to be the most effective treatment to maintain the glossiness of Lane Late navel orange among all treatments.In addition,“SC”treatment could delay color change of the peel,reduce respiration rate,increase TSS and TA contents and control off-flavor under human perception threshold during helf life.These results indicate that“SC”coating can significantly improve citrus fruit appearance quality and maintain their internal quality during shelf life.Combined treatment of“BC”coating and“SC”coating on satsuma mandarins could notably delay water loss and firmness decrease,reduce fruit respiration rate,maintain the concentration of TSS/TA,control the content of off-flavor compounds and effectively inhibit the color change.Treatment of fruit with SC coating for a second time at 21 d of storage after BC coating treatment could confer better glossiness to the peel.Combined treatment with two types coatings can effectively prolong the storage life of satsuma mandarins from 11 d to 38 d and promote the commodity value of fruit during shelf life.In conclusion,this study revealed key epicuticular wax components affecting the postharvest water loss of citrus fruit,and preliminarily verified their fresh-keeping potential,providing a theoretical basis for development of specific citrus fruit coating for different citrus varieties.In addition,development of two different types coating for citrus fruit long-term storage or fresh fruit sales,and test results of the secondary coating treatment provide important theoretical support for the industrial application of new coatings and innovation of coating methods.