| Sweet cherry(Prunus avium)fruit is very popular with consumers for its delicious organoleptic quality and rich in nutrients.However,sweet cherry fruit,with a concentrated harvest and ripening period,has a short storage period,and is prone to postharvest softening and rotting.This becomes the bottleneck of the fresh cherry industry.Postharvest precooling treatments are believed to be efficient in reducing the physiological activity of cherry,while suppressing corruption by microbial contamination,thus prolonging the storage life of cherry.The present research used “Red agate” sweet cherry fruit,which is widely cultivated and marketed in Shanxi province,to evaluate the efficiency and mechanism of air cooling,ice water cooling,and ozonated ice water cooling for storage life extension of sweet cherry.After precooling treatment,a forty days storage under modified atmosphere packaging,in 0±0.5 o C refrigeration house was carried out.The physiological characteristics,the activity of antioxidant enzymes,changes of sensory quality and development of superficial fungi were analyzed during the storage.Besides,significant changes in metabolites patterns responded to different precooling modes were detected.Thus giving a comprehensive understanding of the efficiency and mechanism of ice water and ozonated ice water cooling on postharvest physiology and quality retain,the main results are as follows:(1)Ozonated ice water treatment effectively reduced the stem browning index and rotting rate of“Red agate” cherry during cold storage.Compared with the other two treatments,the ice water treatment reduced the weightlessness rate,but increased the stem browning index and rotting rate.The air-cooling treatment was the most beneficial to delay the decrease of titratable acid(TA)content and soluble sugar content(SC)in the later period of cold storage.From the perspective of basic physical and chemical indexes,ozonated ice water cooling and air-cooling treatment were more conducive to inhibiting the senescence of sweet cherry fruit.Concerning organoleptic properties,ice water cooling had a detrimental effect compared with air cooling and ozonated ice water cooling.The cherry fruit treated by ice water had more serious fruit stalk browning,exhibited skin color darkening,after 30 days of storage,the sensory score was reduced to under acceptable score(6 points),suggesting a lost of the merchandise value.The sensory quality of cherry treated by air cooling and ozonated ice water was better.After 40 days of cold storage,the sensory scores of ozonated ice water and air cooling treated samples were between 6-7 scores,which still had commercial value.However,after 25 days of cold storage,the peel color of ozonated ice water treated fruit had an abrupt darkening.Within 20 days of storage,the cohesion,chewiness and elasticity of cherry fruit showed a similar trend as a first increase and a subsequent decrease.Ice water and ozonated ice water treatment increased cohesion and chewiness after 10 days of storage.Red agate cherry had a short shelf-life at room temperature,performed as a rapid decline in sensory quality.According to the sensory score,the shelf-life for the ice water cooling sample was 3-4 days,and prone to mildew,cherry fruit treated by air cooling and ozonated ice water cooling had a longer shelf-life of about 5 days.(2)The red agate cherry treated by air cooling,ice water and ozonated ice water cooling had remarkable different levels of antioxidant enzyme,oxidase.Compared with the air cooling treatment,ozonated ice water cooling improved the SOD activity during the whole storage,inhibited CAT and POD activity within the early 15 days,but increased their peak values.Ozonated ice water cooling may promote the accumulation of H2O2,thus improving the resistance of cherry fruit,by enhancing SOD levels and reducing the activity of CAT and POD.Compared with air cooling,ice water treatment reduced SOD activity in the early cold storage period,delayed the emergence of CAT peak,and significantly increased POD activity,which was not conducive to antioxidant ability and browning control of cherry fruits.In addition,ice water and ozonated ice water cooling notably reduced PPO level in the early 15 days,but increased PPO activity later,revealing a phrasal dependent manner of browning control.Ice water and ozonated ice water inhibited the activity of LOX,and may block fruit senescence mediated by LOX.Ozonated ice water treatment delayed the degradation of ascorbic acid(As A)and anthocyanin in sweet cherry pulp,compared with air cooling and ice water treatment.There was no significant difference in the total phenol content between cherries treated with different precooling modes.(3)Compared with the air-cooling treatment group,the ice water and ozonated ice water treatment lowered the abundance of fungal during the whole storage,and reduced the diversity of the initial fungal community,which lately increased at the late stage of cold storage.The initial dominant bacteria of “Red agate” cherry were the same regardless of the cooling mode,including Aureobasidium pullulans,Pleosporales(OTU 1),Alternaria(OTU 4)and Davidiellaceae(OTU 3).During the cold storage,the dominant fungi pattern varied in different samples.The Davidiellaceae(OTU 3)was the suspect for the decay of air-cooling samples.Ice water treatment enhanced the domination of biocontrol agent Aureobasidium fungus,however,the proportion of spoilage fungi as Pleosporales(OTU 1),Alternaria(OTU 4)and Davidiellaceae(OTU 3)reached a level of 8.95-15.28% after 30 days storage,and may contribute to the corruption.In the ozonated ice water treated samples,Pleosporales(OTU 1)occupied an absolute advantage and was believed to be the most possible spoilage fungus.Correlation analysis of fungal composition and physical-chemical indicators of cherries showed that the contents of titratable acid,anthocyanin and ascorbic acid had a high positive correlation with the abundance of Alternaria.Antioxidant enzyme activity(PPO,CAT and SOD)has a high positive correlation with the abundance of Dothioraceae and Cryptococcus.(4)The results of metabolome analysis showed that 148 compounds were identified from cherry samples of each treatment group,mainly including sugars and their phosphates,acids,alcohols and amino acids.There were significant differences(P<0.05)between ice water and air cooling,ozonated ice water and air cooling,and ozone ice water and ice water treatment groups with 5,9 and 2 metabolites,respectively.There was a strong correlation between the various metabolites in the ozone ice water treatment group and the air cooling group(P<0.05).The analysis of the metabolism pathway showed that the differentially expressed metabolites of the ice water and air cooling groups were mainly enriched in the TCA cycle,while the other comparison groups were not significantly enriched.Analyzing the specific differences and different metabolites shows that: ice water treatment and ozonated ice water treatment greatly reduced the glucose content in cherry fruits,and increased fructose(5586 times)and mannitol(355times),and maltose(444 times)and mannitol(278 times)respectively.Compared with the samples of the ozonated ice water treatment and the ice water treatment group,the increased ratio of salicylic acid is 878.Combining the trend of rising SOD and falling CAT,it is speculated that salicylic acid may be the response of red agate cherry to ozone treatment and improve the fruit cold storage period.The most important response factor for resistance.In conclusion,the results showed that the ozonated ice water is beneficial for the postharvest physiological activity of “Red agate” cherry and could postpone the decrease of storage quality.The results provide a strong basis for the application of ozonated ice water to prolong the postharvest storage period of sweet cherry,and provoked a new idea for postharvest freshness preservation of similar drupe. |
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