Regulatory Mechanism Of High Carbon Dioxide Treatment On Quality Of Fresh-cut Pears

Fresh-cut pears tend to brown during storage,which leads to reducing of fruit quality and shortening of shelf life.For fresh-cut fruits and vegetables,reducing browning is a top priority to maintain their quality and commercial attributes.CO₂ is generally used as a modified atmosphere treatment to extend the shelf life of fresh-cut fruits and vegetables.CO₂ is widely used in the preservation of fresh-cut fruits and vegetables due to its convenience and safety.In present study,fresh-cut pear fruits were used to study the effects of high-concentration CO₂ treatment on the quality,sugar metabolism,active oxygen metabolism,membrane lipid metabolism,proline,?-aminobutyric acid(GABA)metabolism,respiration and energy metabolism of fresh-cut pear fruits.Below are key research findings:1.The treatment results of three concentrations of CO₂(5%,10%,15%)showed that 10%CO₂ treatment had the most significant inhibitory effect on the browning index,malondialdehyde(MDA)content and electronic leakage of fresh-cut pears.The10%of CO₂ maintains the color of the fruit,delays the decrease of titratable acid(TA)and promotes the accumulation total soluble solids(TSS),and significantly inhibits the growth of total bacterial counts(TBC),mold,yeast and coliform bacteria.Overall,10%of CO₂ maintains the quality and inhibites browning of fresh-cut pear fruit.On the fifth day of storage,the browning index of the CO₂ treatment group was reduced by 46%compared with the control group.The respiration rate and ethylene production of the fruit were also significantly inhibited.The results of principal component analysis(PCA)analysis showed that respiratory rate,ethylene production,TA,color,MDA,electronic leakage,and browning index(BI)were the main quality indicators,while CO₂ treatment mainly affected BI,MDA,conductivity,ethylene,TSS,and microorganisms.2.CO₂ treatment induces increased Sucrose synthase-synthesis(SS-synthesis),sucrose phosphate synthase(SPS)enzyme activity and gene expression,whileamylase,acid invertase(AI),neutral invertase(NI),Sucrose synthase-cleavage(SS-cleavage),fructokinase(FK),hexokinase(HK),NI,Sorbitol oxidase(SOX),NAD Sorbitol Dehydrogenase(NAD-SDH),NADP Sorbitol Dehydrogenase(NADP-SDH)activity and corresponding gene expression were suppressed.On the 5th day after treatment,the activities of SS-synthesis and SPS were increased by 19%,14.8%in the CO₂-treated group,respectively.The activities of amylase,FK,HK,SOX,NAD-SDH,NADP-SDH,and enzyme activity at the end of storage decreased by 20.6%,9.2%,15%,6.4%,12%and 7.7%compared with the control group at the same period.It indicated that CO₂ could regulate the sugar metabolism of fresh-cut pear fruits by inducing the accumulation of glucose,fructose,sorbitol and sucrose,which had an effect on the quality of the fruits.At the end of storage,the contents of sucrose,glucose,fructose,and sorbitol in the CO₂ treatment group were increased by9.2%,43%,8.5%,and 36.7%in the CO₂-treated group,respectively.Correlation analysis results show that the role of CO₂ in sugar metabolism is to regulate the conversion of various sugar components in the fruit,in order to induce the synthesis of glucose,fructose,sucrose and sorbitol to affect the quality of the fruit.3.CO₂ treatment has a significant regulation effect on reactive oxygen species (ROS),mainly manifested in(1)it can significantly inhibit the fruit H₂O₂ content,O₂^-production rate and intracellular ROS level,in which H₂O₂ content and O₂^-production rate were significantly increased on the second day and the third,respectively,when the H₂O₂ content and O₂^-production rate were 1.3 times and 1.6 times of the control;(2)it can significantly inhibit the activity of NADPH oxidase(NOX),on the fifth day,NOX activity was 24%lower than that of the control group at the same time,and related gene expression was also suppressed;(3)CO₂ treatment induced the accumulation of total phenol and total flavonoids,and DPPH,ABTS and FRAP also increased significantly;(4)CO₂ treatment has a significant effect on the ROS removal system,in which the activity of superoxide dismutase(SOD),catalase(CAT),peroxidase(POD)increased significantly,and the activity was 1.4 times,2.4 times and 1.7 times that of the control group on third and fourth days,respectively.The activity of glutathione peroxidase(GPX),aseorbateper oxidase(APX),monodehydroascorbate reductase(MDHAR),glutathione reductase(GR)and the expression of corresponding genes were also induced.However,the corresponding gene expression levels of polyphenol oxidase(PPO)and dehydroascorbate reductase(DHAR)activities were significantly inhibited,the accumulation of ascorbic acid (As A)and glutathione(GSH)was induced while glutathiol(GSSG)was inhibited.Correlation analysis shows that CO₂ mainly regulates ROS metabolism by directly acting on the As A-GSH pathway and indirectly affecting other substances,enzyme activities and genes in the antioxidant system.It shows that CO₂ can reduce the oxidative damage caused by ROS by inducing the accumulation of antioxidant substances and the increase of antioxidant enzyme activity in the fruit,so as to maintain the quality of fresh-cut pears.4.For fatty acid metabolism,the regulation of CO₂ is mainly in reducing saturated fatty acids and increasing the proportion of unsaturated fatty acids.At the same time,CO₂ treatment induced an increase in orn-?-aminotransferase(OAT),?-pyrroline-5-carboxylate synthase(P5CS),glutamic acid decarboxylase(GAD),GABA-transaminase(GABA-T)activity and corresponding gene expression,while lipoxygenase(LOX),phospholipases D(PLD),PDH activity and corresponding gene expression were suppressed.Also,the accumulation of pecithin(PC), phosphatidylinositol(PI),proline and GABA in the fruit is also promoted.At the end of storage,the contents of PC,PI,proline,and GABA in the CO₂-treated group was31%,34%,36%,and 16%higher than the control group,respectively.Correlation analysis results show that fatty acid metabolism pathway,proline pathway substances and GABA pathway influence each other.It shows that CO₂ can reduce the oxidative damage of membrane lipids,maintain the function and integrity of membrane structure,and inhibit the browning of fruits by affecting the changes of related genes and enzymes,increasing the proportion of unsaturated fatty acids,and promoting the accumulation of proline and GABA.5.For respiratory metabolism,CO₂ treatment reduces the total respiration rate of fruits,inhibits cytochrome c oxidase pathway(CP)pathways and promotes tricarboxylic acid cycle(TCA),pentose phosphate pathway(PPP),alternative oxidase pathway(AP),and early embden meyerhof parnas(EMP)pathway.CO₂ treatment maintains mitochondrial membrane and structural integrity by inhibiting increased mitochondrial membrane permeability and keeping mitochondrial cytochrome c/a content.At the end of storage,membrane permeability of CO₂-treated fruits increased by 18.6%compared with the control group,and the cytochrome c/a content was 17.4% higher than the control group.In addition,CO₂ treatment induced NAD kinase(NADK),cytochrome c oxidase(CCO),succinate dehydrogenase(SDH),pyruvate kinase(PK),phosphofructokinase(PFK),H⁺-ATPase,Ca²⁺-ATPase,Glucose-6-phosphate dehydrogenase(G-6-PDH)and 6-phosphogluconic dehydrogenase(6-GPDH)activity and corresponding gene expression.Also,adenosine triphosphate disodium(ATP),adenosine diphosphate(ADP),nicotinamide adenine dinucleotide(NAD),nicotinamide adenine dinucleotide phosphate(NADP)and NADPH are induced,while the accumulation of adenosine monophosphate(AMP)and nicotinamide adenine dinucleotide(NADH)is inhibited.On the second day,the CCO and SDH activities of the CO₂-treated group were 1.6 and 1.3 times that of the control group,respectively,while the H⁺-ATPase and Ca²⁺-ATPase activities were 1.9 times and 1.2 times that of the control group,respectively.Correlation analysis shows that the effect of CO₂ is mainly to regulate the energy substance by regulating the proportion of various respiratory pathways in the fruit,thereby adjusting the energy state of the fruit to adapt to and reduce oxidative stress.Thus,slow down the membrane lipid peroxidation,reduce senescence and browning of fresh-cut pear fruits.