Research On Mechanisms Of NaCl Regulating Aroma Of Fresh-cut Apple

Smell is one of the key factors that affects consumers' willingness to buy the same product repeatedly.During the processing and preservation of fresh-cut fruits and vegetables,some handling methods,such as disinfection,peeling,cutting,dressing and packaging,could cause damage to pericarp and cells,and further accelerate a series of secondary metabolisms.Other factors,the synergistic effect of variety,optimal processing maturity,initial maturity,fruit storage period,product shelf life,temperature management,gas environment in packaging,chemical reagents applied and other odor contamination,play a synergistic role in aggravating the loss and deterioration of aroma of fresh-cut apples.But there is little in-depth discussion about how to maintain and improve the aroma of fresh-cut apples.In this study,fresh-cut ‘Fuji' apples were treated with 0.1 mol/L NaCl and then stored in low-density polyethylene(LDPE)bags at 5? for 12 d to explore the mechanisms of how NaCl affected the aroma of fresh cut apples at the transcriptional and protein levels.The results are as follows:1.After NaCl treatment,the aroma evaluation score was higher,e-nose results appeared obvious difference,and ethanol concentration was lower.It illustrated that the aroma of freshcut apple was improved after NaCl treatment.According to the harvest date(October 13,2020,October 3,2021),apples was defined as apple-2020 and apple-2021 respectively.NaCl treatment changed the proportion of esters in fresh-cut apples.As for apple-2020,the methyl esters(methyl butyrate,methyl 2-methylbutyrate and methyl hexanoate)content were increased after NaCl treatment,whereas propyl esters(propyl butyrate)and butyl esters(butyl butyrate)content were decreased.As for apple-2021,the differences of volatile gas after NaCl treatment were different from apple-2020,but the variation pattern was similar: after NaCl treatment,the relative contents of methyl esters(methyl butyrate,methyl caproate and methyl 2-methylbutyrate)were increased,however the relative contents of propyl ester(propyl butyrate),butyl ester(butyl butyrate),ethyl butyrate,and ethyl 2-methyl-butyrate were decreased.2.After NaCl treatment,the cell structure of fresh-cut apples maintained well,and the total number of bacteria was inhibited during the late storage period.However,no E.coli was detected in both the control and NaCl treatment group.There was no regular change in ethylene concentration in the headspace air body of the packaging bag after NaCl treatment.3.After NaCl treatment,fresh-cut apples could selectively synthesize volatile aroma substances by regulating the relative expression of MdAAT1 at the transcriptional level.Linoleic acid content increased with NaCl treatment.And Md LOX1 a and MdAAT1 were significantly upregulated on 1 d,MdADH was upregulated on 1 d,but downregulated on 4 d.Md PDC1,a key enzyme gene in ethanol synthesis pathway,was downregulated with NaCl treatment.After the transformation of TRV2-MdAAT1-S and PBI121-MdAAT1,the proportion of aroma substance in apple pulp changed significantly.With the transient overexpression of MdAAT1,the relative contents of methyl ester(methyl caproate)were increased,while the relative contents of propyl ester(propyl butyrate)and butyl ester(butyl acetate)was decreased.The relative content of ethyl butyrate was decreased and increased in MdAAT1 transient overexpressed and sliced apples.Compared with the changes of esters in fresh-cut apple after NaCl treatment,the above results suggested that NaCl treatment regulated the expression of MdAAT1 at the transcriptional level.After the transformation of exogenous MdAAT1,the relative content of esters in tobacco leaves was increased significantly.It was indicated that MdAAT1 had a wide range of substrate selectivity.And the high expression of MdAAT1 also affected the synthesis of other volatile substances in the fatty acid pathway of tobacco.4.NaCl treatment increased the aroma of fresh-cut apple by increasing the ?-glucosidase(BGL)activity.NaCl treatment increased the BGL activity during the earlier storage,which may lead to the volatilization of binding aroma.With the increase of NaCl concentration in the reaction system,the activity of BGL crude enzyme from apples increased.On the contrary,the alcohol-acyltransferase(AAT)activity was not changed because of the change of NaCl concentration,so the direct interaction between NaCl and AAT was excluded.After pET30 aMd BGL40 was constructed and transferred into E.coli,the activity of recombinant protein(MpBGL40)also was increased with the increase of NaCl concentration.These results indicated that NaCl could directly interact with MpBGL40 to improve its activity.