Study On The Mechanism Of Browning Of Pear

Pear is one of the major fruit tree species in China,pear fruit has high nutritive values and is beneficial to human health.In recent years,fresh-cut pear consumption becomes popular in Europe,Japan and other countries for its nutrition,convenience,edible rate of 100%.It is also widely provided in senior hotels,restaurants,airplanes and other places in China.However,it often occur such problems as browning,loss of fresh,quality deterioration,short shelf life in the processing of fresh-cutting,which affecting the industrial production of fresh-cut pears.In this paper,pear varieties with different natural browning level were selected to analyze the relationship between the polyphenol content,enzymatic browning enzyme,browning inhibitors and cell membrane fatty acid component,aiming to reveal browning mechanisms of fresh-cut pear.Based on the fact that it has been a focus to exploit and utilize the antioxidant bioactive compounds of fruits,this paper also investigate the antioxidant potentials of 37 pear varieties,then,we use two methods to reveal the antioxidant bioactive compounds in the pears.One method is to study antioxidant ability of the monomer phenol,the other is to analyze the correlation coefficients of antioxidants with respect to phenols of pears.The main results were as follows:1.The degree of browning were different among pear varieties,in which,‘Mantianhong' browning the most,followed by ‘Yali',‘Xinli 7' and ‘Xueqing' showed slowest.Under cold storage conditions,the degree of browning reduced in pear flesh,however,core browning became heavier.2.The activity of PPO or POD was not related to the degree of browning in different varieties,while the activity of LOX was consistent with the degree of browning.The LOX activity of ‘Mantianhong' pear was highest accompanied the heaviest degree of browning,while LOX in ‘Xueqing' pear lowest and browing the slightest.The LOX activity of pear flesh decreased after cold storage for a short time,while the LOX activity of pear core increased,the same trend as the degree of browning.The results showed that the highter of LOX activity,the severer of browning.As LOX was correlated with the peroxidation of membrane,so it can be assumed that the stability of the cell membrane plays an important role in browning of pears.3.The main components of fatty acids in pear were palmitic acid?16:0?,stearic acid?18:0?and linoleic acid?18:2?.And the content of palmitic acid and stearic acid was higher,while the content of linoleic acid was lower.The higher the unsaturated fatty acid content in the cell membrane,the better stability of cell membrane,and the pear was not easy browning.Cold storage and calcium treatment could increase the unsaturated fatty acid content in different pear varieties,and enhance the stability of cell membrane,reduce the degree of browning of pear.4.Ca²⁺-ATP enzyme of pear was mainly located in mitochondria,while the activity of that enzyme in chloroplasts and cytosol was less.The Ca²⁺-ATP activity among different varieties ranged from high to low was as follows: ‘Xueqing',‘Xinli 7',‘Yali',‘Mantianhong'.The activity of Ca²⁺-ATP was higher the stability of the cell membrane was better,thus the browning was slighter.5.Pear fruit cell structure change significantly after fresh-cutting.Wherein ‘Mantianhong' was easy browning,its cell wall dissolved after slicing 5min;its cell wall dissolved further after being cutted 10min;30min later,its cell wall showed white universal with cell membrane disappears.6.The effective browning inhibitor were calcium salts / citric / ascorbic acid / oxalic acid/ cysteine.And 1% calcium chloride,1% oxalic acid,5% citric acid,5% ascorbic acid,5% sodium ascorbate,1% cysteine can effectively reduce the browning of pear.7.The antioxidant properties of the fruits extracts of ten varieties were evaluated for their potential antioxidant capacity by several assays?ABTS,DPPH,?-carotene-linoleic acid emulsion method,FRAP?,the results showed that the oxidant potential of ‘Anli',‘Nanguoli',‘Bartlett' was higher,while the anti-oxidant properties of ‘Kuerle' and ‘Whangkeumbae' was lower.Among the methods used for quantifying antioxidant capacity,there were significant correlations among the four assays.For FRAP was cheaper and simple,which is a suitable method to evaluate antioxidant properties of pears.8.We evaluated the anti-oxidant properties of 37 pear varieties,and the results showed that,the antioxidant of pear peel was higher than that of the pear pulp.Antioxidant capacity of ‘Longxiangli' and ‘Yangli3' was higher,while the antioxidant capacity of ‘Xinxing',‘Kousui',‘Ganquan',‘Josengwhangkeum Pear',‘Manfeng',‘Xuehua',‘Chili',‘korla fragrant pear',‘Dangshanli',‘Bartlett',‘Bosc' was lower.9.The contents of polyphenol showed highly correlation with the total antioxidant capacity of pear fruit.Correlation analysis was used to explore the relationship between phenolic compounds and antioxidant activity of pear fruit.The results showed that the antioxidant activity was significantly correlated with chlorogenic acid,vanilla acid,arbutin,caffeic acid,catechin,epicatechin,erucic acid,while there were no correlation between antioxidant activity and contents of rutin,coumaric acid,quercetin,gallic acid and ferulic acid.10.The anti-oxidation performance test of phenolic compounds showed that the antioxidant capacity of p-coumaric acid,vanillin,arbutin,cinnamic acid,vanillic acid was very weak,while the antioxidant capacity of quercetin,proanthocyanidins,gallic acid,caffeic acid,erucic acid,chlorogenic acid,rutin was strong.