Quality Detection And Fresh-keeping Effect Of Water Used In Postharvest Handling Of Main Citrus Production Areas

China is the largest producer and consumer of fresh citrus fruit in the world.Washing,decay protection and fresh-keeping of fruit,which involve the use of large amounts of water,play crucial roles in the postharvest handling of fresh citrus,while water quality directly affects the production cost,decay protection and fresh-keeping effect.Practical investigation of main citrus producting regions such as Hubei Province,Jiangxi Province and Yunnan Province has demonstrated that large quantities of chemical antiseptic agents are used with little effect in citrus postharvest fresh-keeping treatment.The scale formation in cleaning brush of sorting line frequently occurs,and production wastewater was directly released without harmless treatment.The above problems are all related to the quality of production water.To date,the basic data of water quality of postharvest production water in main citrus production areas in China are still not available,and there has been little systematic research on the effects of water quality on decay protection and fresh-keeping.Besides,decay protection and fresh-keeping treatment of fruits mainly rely on practical experience.These practical problems directly limit precise and targeted postharvest decay protection and fresh-keeping production of Chinese citrus industry.To meet the realistic demands of postharvest handling of citrus industry,production water in the main citrus production areas in China was studied in this research.PH,water hardness and fungal colony count were detected and analyzed as the main evaluation indicators,aiming to provide optimal guideline for postharvest handling lines based on different water quality or different areas.‘Newhall’ navel orange(Citrus sinensis Osbeck),Satsuma mandarin(C.unshiu Marc)and Ponkan(C.reticulata Blanco)were used as experimental materials.The samples were dipped in mixed chemical antiseptic agents aqueous solutions prepared with different pH and hardness water,so as to explore the effect of water used in production on citrus decay and fresh-keeping as well as the optimal water quality conditions of postharvest commercial treatment of fresh citrus fruit.In additions,to investigate how improved water quality affects the current decay rate and fresh-keeping,the effect of food additive EDTA·2Na to improve water quality on citrus decay and freshkeeping was investigated.The main findings are shown below.1.The pH of water used in postharvest handling in China’s main citrus production areas showed a normal distribution between weak alkaline and alkaline.The water hardness presented a right skewness distribution,which was generally below medium-hard.In terms of the main production provinces,pH showed no obvious geographical distribution characteristics,but water hardness was higher in the north-western territory of the main production areas.PH showed significant weak correlations with water hardness and fungal colony count,with the correlation coefficient of 0.313 and 0.202,respectively.Water hardness had no significant correlation with fungal colony count.2.Neutral pH =7.00 treatment could effectively reduce fruit decay rate and maintain postharvest quality of citrus fruit during storage.Experiments of decay protection and fresh-keeping treatments with production water with pH of 6.00,7.00,8.00 and 9.00 were carried out,followed by uni-package and room temperature storage.Among which pH=7.00 treatment showed the best performance,which significantly reduced fruit decay rate.The fruit decay rate under pH =7.00 treatment was 2.00%,which was far lower than the9.00%,9.50% and 6.00% respectively in other three treatments after storage for 150 days for ‘Newhall’ navel orange in 2020.The decay rate under pH =7.00 treatment was 6.11%,while that under other treatments was 7.78%,8.33% and 10.00%,respectively,after storage of 60 days for Satsuma mandarin.Similarly,pH =7.00 treatment showed a lower decay rate of 1.11% as compared with other treatments,which were 2.22%,2.22% and 1.67%,respectively,after 90 days of storage for ‘Newhall’ navel orange in 2021.PH =7.00 treatment had a decay rate of 2.78%,which was much lower than the 7.22%,3.33% and5.00% of other three treatment after 90 days of storage for Ponkan.In addition,pH =7.00 treatment reduced the postharvest decay rate of citrus fruit by 0.55-7.50 percent point.Pure water treatment could improve peel firmness of ‘Newhall’ navel orange and Ponkan,slow down the decrease in total soluble solid(TSS)content of Ponkan and titratable acidity(TA)of ‘Newhall’ navel orange,and reduce the accumulation of off-flavor substances in Ponkan fruit.In brief,pH =7.00 treatment exhibited a priming effect to improve postharvest storage quality of citrus fruit.Experiments of decay protection and fresh-keeping treatments of citrus fruit with different water hardness were carried out.The results indicated that harder water used in postharvest handling had a negative effect on postharvest decay protection and fresh-keeping of citrus fruit.Besides,harder water tended to result in the loss of TSS of ‘Newhall’ navel orange in the early stage of storage and Ponkan throughout the whole storage,as well as the accumulation of off-flavor substances in fruit.3.EDTA·2Na could effectively improve water quality of hard water,significantly reduce the postharvest decay rate and improve the postharvest quality of citrus fruit.Experiments of 4 mmol/L hard water with equal concentration of EDTA·2Na treatment followed by decay protection and fresh-keeping treatment were carried out to solve the problem that hard water accelerated fruit decay during postharvest storage,which could decrease the decay rate from 7.78%,2.22% and 7.22% to 2.78%,0.56% and 0.56%,respectively,in Satsuma mandarin,‘Newhall’ navel orange and Ponkan.Although the use of EDTA·2Na in hard water increased the water loss of fruit and the content of off-flavor substsances in fruit at the end stage of storage for 90 days,it could delay peel softening,reduce respiratory rate of fruit,and slow down the decrease in TSS content in the middle of storage.Interestingly,application of EDTA·2Na in pure water had no significant effect on the decay rate of ‘Newhall’ navel orange and Ponkan,and accelerated water loss and peel softening of fruit.In conclusion,the pH of water used in postharvest handling of China’s main citrus production areas showed a normal distribution between weak alkaline and alkaline.Water hardness was generally below medium-hard.The optimal parameters of water used in citrus postharvest decay protection and fresh-keeping handling are a neutral pH of 7.00 and a water hardness of c=0 mmol/L.Increase of water hardness is not beneficial to the postharvest decay protection and fresh-keeping of citrus fruit.Furthermore,EDTA·2Na could effectively improve the quality of hard water,reduce postharvest decay rate and improve the fruit quality.This research provides a theoretical basis and technical guidance for the green,targeted and scientific development of citrus postharvest handling.