Effect of different O2/CO2 proportions on the physiological characteristics of ‘Fuji’ apple fruit during modified atmosphere storage

Abstract:【Objective】In this test,‘Fuji’apple fruit was used as the material and stored at (1±0.5) °C to study the effect of different ratios of O2/CO2 treatment on fruit quality, including firmness, titratable ac- id and soluble solids content, relative conductivity, MDA, ATP and H2O2 contents, O2- production rate, PLD, SOD, mitochondrial H+-ATPase, mitochondrial Ca2+-ATPase, SDH, and CCO activity during modified atmosphere storage. In order to explore the causes of CO2 damage to‘Fuji’fruit with different ratios of O2/CO2 treatment from the perspectives of lipid and energy metabolisms, the optimum O2/CO2 ra- tio of the‘Fuji’apple during modified atmosphere storage was determined, which provided a theoreti- cal basis for‘Fuji’apple storage after harvest.【Methods】‘Fuji’apples were picked from the Baoji Qianyang Apple Experimental Farm of Northwest A & F University, China, on 23 October 2018. After standing at room temperature for 24 hours to dissipate the field heat, 600 healthy fruits with the same maturity and sizewithout diseases, pests and mechanical damage were selected for subsequent experi- ments. The fruits were then divided into four groups and 150 fruits were contained per group. The fruits of four groups were put into sealed plastic buckets with a volume of 120 L, and stored in a refrigerator at (1±0.5) °C and with 85%-90% relative humidity. The gas ratios of four groups included treatment I 3%CO2+12%O2+85%N2; treatment II 5%CO2+10%O2+85%N2; treatment III 10%CO2+5%O2+85%N2; and CK was just common air.‘Fuji’apple samples were taken out every 30 days, and 10 fruits were taken for each treatment. Relevant indexes were determined and some samples were saved for subse- quent tests. The O2/CO2 ratio was readjusted after sampling.‘Fuji’apples were stored for 150 days.【Results】During the long-term storage of‘Fuji’apples, compared with ordinary refrigeration, simple air-conditioning with different ratios of O2/CO2 treatment can effectively maintain fruit quality, includ- ing hardness, titratable acid content, and soluble solids content. Among them, Treatment I had the best effect on delaying fruit weight loss. At the same time, the effects of the three treatments on the soluble solids and titratable acid contents were not significantly different. The CO2 injury of‘Fuji’apple was visualized as flesh browning. CK and Treatment I fruits did not have CO2 damage during the long- term storage. Treatment I had no significant effect on the membrane integrity. When the O2 concentra- tion was less than 10% and the CO2 concentration was more than 5% during the long-term storage, the flesh of‘Fuji’apple browned worse with the decrease of the O2/CO2 ratio. Cell membrane lipid metabo- lism and energy level were the main factors that affected browning. The energy level of cells played an important role in maintaining cell membrane integrity. In this study, during the long-term storage of‘Fuji’apples, Treatment I had no significant effect on the mitochondrial H+-ATPase and CCO activi- ty. When the O2 concentration was less than 10% and the CO2 concentration was more than 5%, mito- chondrial H+-ATPase and CCO activity decreased. At the same time, with the decrease of the O2/CO2 ra- tio, mitochondrial Ca2 +- ATPase and SDH activity as well as the ATP content of‘Fuji’apple fruit de- creased. Damage to membrane integrity was considered to be one of the main causes of browning. The lipid metabolism of cells was related to PLD and SOD. In this study, during the long-term storage of‘Fuji’apples, Treatment I can maintain lower O2- production rate, as well as H2O2 and MDA contents. When the O2 concentration was less than 10% and the CO2 concentration was more than 5%, O2- produc- tion rate and H2O2 content increased. With the decrease of the O2/CO2 ratio, SOD activity decreased, PLD activity, relative conductivity and MDA content increased, and the flesh of‘Fuji’apple browned worse【. Conclusion】In this study, the best preservation effect on ‘Fuji’ apple fruit during modified at- mosphere storage was Treatment I(3%CO2 +12%O₂ +85%N2). During the long-term storage of‘Fuji’apples, compared with ordinary refrigeration, simple air-conditioning with different ratios of O2/CO2treatment can effectively maintain fruit quality, including hardness, as well as titratable acid and soluble solids contents. However, when the O2 concentration was less than 10% and the CO2 concentration was more than 5% during the long-term storage, the flesh of‘Fuji’apple browned worse with the decrease of the O2/CO2 ratio. The mitochondrial H +- ATPase, mitochondrial Ca2 +- ATPase, SDH and CCO activi- ties related to energy production in the fruit decreased, resulting in decreased ATP content. Cells main- tained lower energy levels. This led to a decreased SOD activity and increased production of O2- and H2O2, and PLD activity was activated. This caused the membrane integrity to be destroyed and CO2 inju- ry to occur.