Changes And Regulation Of Main Flavor Quality In The Fruit Of Citrus Sinensis Osbeck Cv.Jincheng During Postharvest Storage Time

The fruit of Jincheng(Citrus sinensis L.Osbeck cv.‘Jincheng 447')is rich in antioxidants,which can prevent the occurrence of some human diseases,such as diabetes,cardiovascular disease and cancer.During storage time,in addition to be susceptible to pathogenic bacteria,there will be many changes in citrus fruits.For example,the fruits'skin become softer and drier,the odor gets weaker,and the content of sugar and acid decrease obviously,which deeply affect the commercial value of citrus fruits.Hence it is the superior storage fruit quality that plays an important role in preventing the unmarketable of citrus fruits.Using‘Jincheng 447'citrus as material,the experiment compared the citrus'sensory qualities,ethanol and acetaldehyde contents,and volatile flavor components before and after storage,and then analyzed the dynamic changes of soluble solids,organic acids,ascorbic acid,soluble sugars,and respiratory intensity of the citrus fruits during postharvest storage time.The change in expression levels of volatile flavor components-related genes were determined during storage time.Furthermore,the effect of postharvest treatments by melatonin and oxalic acid on the flavor quality of Jincheng fruit were researched,the regulating effect and mechanism were explored as well.Research content also included the optimization of sodium alginate coatings.The sodium alginate and Ca Cl₂compositing coat treatment on the citrus flavor quality were discussed.The main results are as follows:(1)The content of ethanol in the Jincheng fruits was 37.99?g·g^-1,and the content of acetaldehyde was 0.02?g·g^-1,and there are 48 substances could be detected by GC-MS before storage,and the content of terpenes accounted for 90.36%of the total volatile components.The main volatile aroma components in the fruit contained linalool,octanol,4-terpineol,citronellol,decaldehyde,2-octenal,octanal,(Z)-2-undecenal,neral,isobutyl acetate,ethyl octoate,ethyl hexanoate,geranylacetone,2-octanone,DL-carvone,hexanone,D-limonene,myrcene,?-zingerene,alloaromadendrene and?-terpinene,et al.These substances'aroma characteristics are mainly grassy,fatty,floral and fruity.(2)The flavor of the fruit deteriorated significantly compared with that before storage,but the TSS content,ASA content and respiratory intensity of fruit had no significant differences,while the TA and soluble sugar contents declined obviously.At the end of storage,the ethanol content in the pulp was 884.63?g·g^-1,and the acetaldehyde content was 14.45?g·g^-1.The total content of ketones,alcohols,aldehydes,terpenes in the fruit decreased sharply,and a large amount of esters accumulated in the pulp,which eventually led to a major flavor of fruity(caused by esters),woody,herbal,spice,and turpentine in citrus fruits.(3)Compared with storage temperature at 13?,storage condition at 7?could significantly improve fruits'sensory quality,reduce the accumulation of ethanol,acetaldehyde,and the content of volatile aroma substances,decrease the aroma of fruity(caused by esters),woody,herbal and fatty.The expression level of LOX,HPL,ADH,AAT and PDC genes were significantly down-regulated,which might be the reason for the visible reduces in the content of alcohols,aldehydes,esters,and ethanol and acetaldehyde in fruits.Although the expression of terpene synthesis-related genes,DXS,DXR,GGPS,FPPS and CCD₄,were promoted,the total content of terpene was still low,indicating that terpenes'content is not completely determined by the expression level of these 5 genes.(4)After treatment with melatonin at the concentration of 3.0 g·L^-1,the Jincheng fruits'sensory quality was improved after storage,with increase in the total amount of volatile,TSS and ASA contents in the pulp,the fruits'respiration intensity was also inhibited.It was found that melatonin treatment could improve the expression of HPL,LOX,ADH,AAT,PDC,CCD₄,DXR,DXS,FPPS and GGPS genes in fruits,which may cause the promotion of the synthesis of volatile.(5)The post-harvest citrus fruit treated with oxalic acid at a concentration of 1.0g·L^-1 had obviously improved their flavor and taste after storage time,and the ethanol content in the fruit reduced to 7.14%of the control group.The content of volatile components increased significantly,among which 44 kinds of volatile aroma components'contents were enhanced,with the smell of herbal,fruit,woody and floral.At the same time,the contents of DL-carvone,4-terpineol,methyldihydrofuranthiol,?-phellandrene and?-copaene reduced remarkablely.During storage time,oxalic acid treatment could significantly inhibit the respiration intensity of Jincheng orange,and increase the fruits'TSS and ASA content,the expression of LOX,GGPS and FPPS genes were improved as well,while the expression level of HPL,ADH,PDC,AAT,DXS,DXR and CCD₄gene were suppressed.(6)Before coated with 5.0 g·L^-1 sodium alginate,the citrus fruits were soaked in2.0 g·L^-1 Ca Cl₂ solution and dried in atmosphere.After a period of 120 days'storage time,not only the fruits'sensory quality was improved,but also the contents of ethanol and esters in the body were respectively reduced by 23.63%and 42.58%of the fruit which coated by 5.0 g·L^-1 sodium alginate alone,and the total contents of aldehydes,alcohols,and terpenes were added additionally.The substances whose contents decrease significantly in pulp were mainly owned by esters and ketones,exhibiting a flavor of fruity,fatty,spice,and wood.After Ca Cl₂ composite coating,the fruit could further increase the ASA content in the fruit,reduce the fruit's respiratory intensity during storage time.The accumulation of ethanol and esters could be cut by inhibiting the expression of LOX,HPL,ADH,AAT,and PDC genes,while the expression levels of DXS,DXR,GGPS,FPPS and CCD₄ were boosted,which could promote the synthesis of terpenes and then improve the overall fruit quality.