| Citron(Citrus medica L.)is a characteristic local citrus fruit in Yunnan Province.Citron industry is a characteristic and advantageous industry in western Yunnan border region to consolidate the achievements of poverty alleviation and rural revitalization.In recent years,the planting area of Yunnan citron has been increasing continuously,but due to the single variety,highly concentrated maturity stage,and the cultivation area mostly belongs to poor mountainous areas,the postharvest preservation and commercialization treatment technologies are seriously lagging behind.The pulp of citron has high moisture content and is extremely perishable in the process of storage and transportation,decay loss has become a prominent problem restricting the development of citron industry,and it is urgent to adopt appropriate postharvest preservation treatment to reduce decay loss.The biggest difference between citron and other citrus varieties is that the peel and pulp are eaten together,considering the health of consumers,conventional chemical preservation methods have been unable to meet the actual production demand,so it is urgent to develop green preservation technology to meet the high-quality development needs of citron industry.Heat treatment is widely used in postharvest treatment of various fruits and vegetables due to its environment-friendly,no residue of chemicals and easy operation,and its effectiveness and safety have been widely recognized.However,there has been no report on the application of heat treatment in postharvest treatment of citron fruits.Based on the above problems,this study optimized the technical scheme of postharvest hot water treatment for Yunnan citron,aiming to screen out the optimal heat treatment conditions and clarify its influence on physiological metabolism.The main findings are as follows.1.Hot water treatment at 55℃ for 1 min had good preservation effect for Yunnan citron.Yunnan citron fruits were treated at 50℃ for 3 min,55℃ for 1 min and 60℃ for 20 s,then stored at 5-7℃ cold storage.The effects of different heat treatment conditions on storage performance were compared.It was found that the fruits in 50℃ treatment group could maintain good color,brightness and peel hardness,and slightly increase the fruit water loss,effectively reduce fruit rot.Treatment at 55℃ can maintain better color,brightness and peel hardness,reduce fruit weight loss,and reduce fruit rot effectively.Treatment at 60℃ accelerated the fruit turning red,reduced the fruit brightness and hardness,increased the fruit water loss,the decay rate was not different from the control group.Based on the above indexes and considering that 55℃ treatment can reduce time cost and improve postharvest production efficiency,it is determined as the best treatment condition.2.Hot water treatment at 55℃ for 1 min promoted the accumulation of flavor and aroma substances,and had a positive effect on sensory quality of Yunnan citron fruits.Analysis of primary metabolites of fruits showed that 55℃ treatment significantly increased the accumulation of soluble sugar content in pulp during storage,delayed the decrease of L-alanine,L-proline and serine contents in peel,and increased the contents of scyllo-inositol and myo-inositol in peel.The contents of moniterpene aroma substances accounted for more than 89.40% of the total aroma substances,among which the contents of d-limonene accounted for more than 64.50% of the total aroma,and the contents of γ-terpinene accounted for more than 17.99% of the total aroma.The contents of d-limonene and γ-terpinene in fruits of 55℃ treatment group were significantly higher than those of control group during storage.The contents of monoterpene aldehydes and monoterpene esters were also significantly increased under the influence of 55℃ treatment.In conclusion,55℃ treatment is conducive to the accumulation of sugars during storage,which makes the fruit flavor more intense,delays the reduction of aroma substance content,keeps the fruit fragrance better,and enhances its sensory quality.3.Hot water treatment at 55℃ for 1 min can directly inhibit the growth of pathogenic bacteria,make the waxy structure uniform and smooth,induce the accumulation of resistant substances,and endue the fruit long-term resistance.The number of microorganisms on the fruit surface showed a significant difference after heat treatment,which proved that 55℃ treatment had a direct inhibitory effect on pathogenic bacteria.The microorganisms on the fruit surface in the 55℃ treatment group were still significantly lower than those in the control group at the later stage of storage,indicating that heat treatment induced long-term resistance in the fruit.The waxy structure of fruit surface was observed,and it was found that the waxy structure of fruit in 55℃ treatment group was uniform and smooth until the 52 nd day of storage,which was beneficial to resist the invasion of microorganisms.The contents of secondary metabolites in pericarp were determined.Some substances reported to have antibacterial and antioxidant effects were significantly up-regulated accumulation under the influence of 55℃ treatment,such as fraxin,salicylic acid,sakuranetin,coumarin,scopoletin,nobiletin,shikonin,hesperidin,ferulic acid and phloretin,which endored fruits with stronger resistance.In summary,this study compared the effects of different heat treatment conditions on the storage performance of Yunnan citron fruits,and selected 55℃ for 1 min as the best heat treatment condition,and then identified the favorable effects on the flavor and fragrance of citron fruits.In addition,the reasons for the decrease of fruit decay rate were analyzed from the aspects of direct inhibition of pathogenic bacteria,effect of heat treatment on wax structure and induction of resistant substance accumulation.The results of this study could help improve the postharvest production capacity of Yunnan Citron. |
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