Study On The Control Techniques Of Peel Browning Of Postharvest Huangguan Pear Fruit

Huangguan pear is popular with consumers because of its delicate flesh,juicy and small stone cells.The Huangguan pear is usually stored at low temperature.However,in low temperature storage,the fruit is prone to chilling injury,resulting in browning of the peel.It is mainly manifested as the irregular brown curved texture on the surface of the peel.With the extension of storage time,the color of the lesion will gradually deepen and appear depression,commonly known as‘Chicken foot disease’,which seriously affects the appearance quality and consumer acceptability of the fruit.The disease greatly reduced the commodity value of Huangguan pear and limited the healthy development of Huangguan pear storage and transportation industry.In order to inhibit the browning of Huangguan pear peel,ethylene fumigation and chitosan oligosaccharide（COS）coating were selected in this study to investigate the inhibiting effect of ethylene and COS on the browning of Huangguan pear peel during storage and shelf life.The mechanism of its action was analyzed from the perspectives of energy metabolism,active oxygen metabolism and phenol metabolism,so as to provide technical guidance and theoretical support for inhibiting browning of Huangguan pear peel.（1）Exogenous ethylene fumigation treatment（0,5,20,100μL/L）combined with 0℃refrigeration and low temperature conditioning（reduced by 1℃every 24 h for postharvest30℃Huangguan pear,reduced to 0℃after 30 d）were used to treat Huangguan pears.After300 days of storage,the incidence and incidence index of the control group（0μL/L exogenous ethylene）reached 34.5%and 10.7%,respectively.The incidence and incidence index in the low temperature conditioning group were only 5.1%and 2.1%.At the end of the shelf life,the incidence and incidence index of the control group（0μL/L exogenous ethylene）were as high as 93.0%and 37.6%,while those of the low temperature conditioning group were only 13%and 10%.The ethylene treatment could completely suppress the peel browning symptoms caused by chilling injury during the shelf life of Huangguan pear,and the incidence was 0%.Meanwhile,the water content,firmness,soluble solid contents,titratable acid contents,soluble sugar contents and ascorbic acid contents of Huangguan pear fruit during shelf life could be maintained by ethylene treatment.Exogenous ethylene treatment could increase the contents of immobile water and bound water in Huangguan pear fruit by changing water distribution,inhibit the migration of the two kinds of water to free water,reduce the activity of water in tissue cells,strengthen the binding force with starch,protein and other non-water components,reduce the metabolic rate,slow down the degradation of polymer substance,and reduce the free water content.It could avoid forming ice crystals and puncturing cell membrane due to low temperature,thus inhibiting the browning of Huangguan pear peel during shelf life and improving the storage quality of Huangguan pear.（2）The 20μL/L ethylene fumigation treatment combined with 0℃cold was use storage to treat Huangguan pear fruits.Through the determination of the incidence of browning,physiological indexes,sensory evaluation,energy level,enzyme activity related to energy metabolism,total phenol content,and enzyme activity related to phenolic metabolism of fruits,we were found that 20μL/L ethylene treatment completely inhibited the occurrence of peel browning.Although there was no significant difference in sweetness,acidity and juiciness between the ethylene treatment group and the control group,the fruit of the ethylene treatment group had better appearance quality,so the consumer acceptability was much higher than that of the control group（P<0.05）,ensuring the commodity value of the fruit.Further,we analyzed the mechanism of ethylene inhibiting peel browning from the perspectives of energy metabolism and phenol metabolism,and found that,（1）Exogenous ethylene treatment significantly increased H⁺-adenosine triphosphatase（H⁺-ATPase）,Ca²⁺-adenosine triphosphatase(Ca²⁺-ATPase),succinate dehydrogenase（SDH）and cytochrome oxidase（CCO）activity,maintaining high concentrations of adenosine triphosphate（ATP）and adenosine diphosphate（ADP）in the peel.It inhibited the increase of adenosine monophosphate（AMP）concentration,significantly improved the energy state of peel,and played an important role in ensuring the integrity of cell membrane,thus avoiding the occurrence of cold injury of Huangguan pear fruit.This indicated that exogenous ethylene could regulate the activities of enzymes related to energy metabolism.It can improve cold tolerance and storage quality of Huangguan pear.（2）Exogenous ethylene treatment enhanced glucose 6-phosphate dehydrogenase（G6PDH）,shikimate dehydrogenase（SKDH）and phenylalaninammo-nialyase（PAL）are active in phenolic anabolic enzymes.Also,it could inhibit the activity of polyphenol oxidase（PPO）and peroxidase（POD）,which have antagonistic effect on phenolic anabolism.Therefore,the content of phenolic substances in pear peel after exogenous ethylene treatment is always maintained at a high level,so that it could improve the membrane antioxidant capacity and induce cold tolerance of Huangguan pear peel by removing reactive oxygen species.（3）Huangguan pear was coated with COS solution with different mass fractions of0.25%,0.5%and 1%,and the surface moisture was air-dried.After sealing and packaging with plastic wrap,Huangguan pear was stored in cold storage at 0℃.In the control group,the incidence and incidence index were 93%and 40%,respectively.It was found that the incidence and incidence index of browning of Huangguan pear peel with COS coating treatment of different mass fractions were significantly inhibited.Among them,the incidence and incidence index of 0.5%COS treatment group were 12%and 9%,respectively.COS treatment significantly inhibited the decrease of firmness,soluble solids content and titratable acid content,and maintained the unique fine taste and delicious flavor of Huangguan pear.The activities of PPO and lipoxygenase（LOX）in peel were significantly inhibited by 0.5%and 1%COS treatments.The enzyme activity of catalase（CAT）,ascorbate peroxidase（APX）,superoxide dismutase（SOD）,PAL and other active oxygen metabolism enzymes were maintained at a high level.The contents of phenols and ascorbic acid in peel could be maintained at a high level by COS treatment with mass fraction of 0.5%and 1%,and the excessive accumulation of hydrogen peroxide（H₂O₂）and malondialdehyde（MDA）could be reduced.The results showed that 0.5%COS treatment could improve the activity of reactive oxygen metabolizing enzymes and reduce the conversion rate of phenolic substances.By affecting the accumulation of reactive oxygen metabolism and antioxidant substances,it could inhibit the browning of Huangguan pear peel during real low temperature storage.