Litchi Postharvest Physiology And Enzymatic Degradation Of Phenolics Mediated By Epicatechin In Pericarp

Mature fruits of two cultivars of litchi(Litchi chinensis Sonn.),‘Hushanwanli'(storable)and 'Wuye'(unstorable)were utilized as material.The differences of storability between the two litchi cultivars and the effects of different storage environments on the fruit quality were compared;the changes of morphological pericarp structure,the fluctuations of peroxidase(POD)and polyphenol oxidase(PPO)were investigated in the process of storage;the main flavanols and anthocyanins in litchi pericarp,(-)-epicatechin(EC),(+)-catechin(C),type B procyanidins,type A procyanidins and cyanidin-3-O-rutinoside(Cya)were identified and determined by HPLC-MS/MS;POD and PPO were purified from litchi pericarp and the coupled oxidation mechanism of flavanols and cyanidin-3-O-rutinoside mediated by(-)-epicatechin was studied.The main results were as follows:The microstructure showed that,comparing with‘Wuye'litchi,'Hushanwanli' litchi had more obvious fissure slices of its exocarp,thicker palisade tissue of its mesocarp,more intimately arranged stone cells(group)with a higher content,better developed and ordered vascular bundles and more layers of papyroid cells with more intimately arranged in its endocarp.With the damage of fissure slices,cell gap reduction caused by water loss and cell boundaries becomes not obvious was found by observing its microstructure,the damage of pericarp tissue structure tend to increase during postharvest senescence and browning.Storage in package at 25?do less damage to pericarp tissue structure than storage without package at room temperature.Storage in package at 3±1? could keep the pericarp tissue structure intact for a long time.Besides,‘Hushanwanli'litchi pericarp showed a relatively slowly browning than `Wuye'.Browning index,cell membrane permeability,relative water loss rate of litchi pericarp,respiration intensity and weight loss rate of litchi fruit increased with the storage time prolonged,which was significantly higher in‘Wuye'than‘Hushanwanli'.Storage in package at low temperature could significantly delay the dehydration and browning of litchi pericarp.With storage time prolonged,the content of fruit soluble solids(TSS),titratable acid(TA)and vitamin C decreased,which was significantly higher in 'Hushanwanli' than 'Wuye'.Consistently,‘Hushanwanli'had a longer storage period than 'Wuye'.At the earlier stage of storage,the activity of soluble POD and bound POD in pericarp were gradually increased with browning of pericarp.At the later stage of storage,the activity of POD was declined caused by the damage of pericarp structure and mixture of POD and phenolic substance,which was significantly higher and changed greater in‘Wuye'than'Hushanwanli'.With process of the postharvest fruit aging and pericarp browning,the activity of PPO in the pericarp of 'Hushanwanli' was increased first and then decreased,the activity of PPO in the pericarp of‘Wuye'pericarp was constantly decreased.There were no significant differences of PPO activity in the pericarp at unpackaging,25? and 3±1?between‘Hushanwanli'and‘Wuye'.During the storaged at 10±1?environment,the activity of PPO in 'Hushanwanli' fas significantly higher than‘Wuye '.EC,C,procyanidins B1(B1),procyanidins B2(B2),procyanidins A2(A2),cyanidin-3-O-rutinoside(Cya),epiafzelechin(Afz)and proanthocyanidin trimers of main phenolic compounds in litchi pericarp were identified and quantified by HPLC-MS/MS.The main phenolic were arranged according to the content in litchi pericarp:EC>A2>B2>Cya>C>B1>Afz.This study found that the total content of different components of phenolic compounds in pericarp tend to decrease with the postharvest pericarp browning,which could be effectively inhibited by storage in package at 3±1? environment.The contents of EC,C,B1,B2,Cya and Afz in the pericarp of‘Hushanwanli' pre-rised and then decreased during storage,the contents of EC,C,B2 and A2 in‘Wuye' were decreased during the storage period.PPO showed high specificity for selection of substrate,PPO showed good catalytic activity for EC but could not recognized B1,B2 and A2.In the presence of H2O2,compared to the PPO,POD has a difference selection for substrates.In addition to EC,it also had good catalytic activity for A2.The metabolic transformation of POD and PPO to flavanols may follow the same reaction mechanism.EC catalyzed by PPO and POD produced semiquinone free radicals to promoted the oxidation of flavanols in nonenzymatic manner and brown compound was formed which led to the formation of litchi pericarp browning,but the metabolic transformation efficiency of POD to flavanols is far more than PPO.PPO could not directly catalyze the degradation of Cya,the degradation of Cya catalyzed by PPO depended on the PPO-EC-Cya reaction system,and need high concentration of PPO enzyme to drove the reaction of the system.In the presence of H2O2,POD could directly catalyzed the degradation of Cya,at the same time POD catalytic oxidation EC produced semiquinone free radicals to attacked Cya which led to increased in the rate of degradation,the catalytic reaction efficiency was greater than PPO.PPO could not be directly involved in the catalytic conversion of lignans during the conversion process,must mediated by EC,catalyzing the metabolic transformation of sinapinic alcohol.POD can directly catalyzed the oxidation of coniferyl alcohol and sinapinic alcohol.In vitro oxidation of the total crude extract of phenols in pericarp catalyzed by POD and PPO were analyzed.In different pH environments,the enzymatic degradation efficiency of flavanols and Cya in litchi pericarp catalyzed by POD was higher significantly than that by PPO.POD may play a leading role in the enzymatic degradation of phenolic compounds and the degradation of Cya was the leading reaction in the whole browning process,which driven by POD.POD plays a pivotal role in the process of browning in pericarp,which is more important than PPO.