Study On Influence Factors And Mechanism Of Browning Of Yali Pear Fruit After Harvest

Yali pear (Pyrus bretschneideri Rehd) originated in our country is one of the major varieties of white pear lines. Browning heart has been one of the most severe postharvest physiological injury in storage. Although there are many reports about the reason why this kind of physiological injury happened, and yet most opinions only stressed the biochemical reactions of phenol catalyzed by PPO, the lipid peroxidation of cell membrane and the physiological affecting factors such as maturity, temperature, high CO2, ethylene, low Ca, and etc. Presently, the relationship between these indexes and their interactions during browning of Yali pear have not been comprehensively elucidated yet.The harvest maturity and cooling methods were the main influence factors of browning of Yali pear fruit. In this paper, both early harvested Yali pear (picked in the first ten days of September) and the late harvested Yali pear ( picked in the last ten days of September) were treated with slow cooling treatment (cooling from room temperature directly to 0 ℃) or rapid cooling treatment ( storage at 12 ℃ and cooling them to 0 ℃ over 30 d by reducing temperature by 2 ℃ every 5 d). The effects of the two cooling treatment on the pear's ability against stress, polyphenol content, membrane lipid's peroxidation, membrane fatty acids' composition, dissolved CO2 amount and pear seed physiology were systematic--ally studied. So did the relationship between ripening and browning. The obtained main results are as follows.1. Late harvest increased the content of soluble solid, reduced sugar, vitamin C, GSH, and soluble protein, linoleic acid, linolenic acid content and U/S value of flesh and core of Yali pear. The phase transition temperature was reduced. The pear's ability against stress was improved. However, the late harvested fruit had vigorous respiration and ethylene metabolism so that the fruit hardness, titrated acid content, soluble solid content, reduced sugar content, soluble protein content, vitamin C content and GSH content decreased very quickly. Moreover, the decrease of SOD activity and increase of LOX activity caused the accumulation of hydrogen peroxide and superoxide radical, increase of MDA content and severe oxidation of fruit membrane lipids. Resultantly, the permeability of cell membrane was enhanced, which lead to the enzymatic activity increase of PPO and PPO isoenzyme and content decrease of phenols. All these changes mentioned above finally made the late harvested fruits much easier to get browning. Moreover, the more severe browning was found in pear's core than in its flesh because of the more acute changes in its core.2. Rapid cooling induced cold injury to early harvested pears and especially to their cores. Compared with rapid cooling treatment, slow cooling treatment obviously increased the pear flesh and core's soluble protein content, linoleic acid content, linolenic acid content, and U/S value, reduced its phase transition temperature and resultantly enhanced the Yali pear's ability against stress. Additionally, slow cooling slowered the metabolism of respiration and ethylene, enhanced the SOD activity, delayed the decrease of fruit hardness, titrated acid content, soluble solid content, reduced sugarcontent, soluble protein content, vitamin C content and GSH content in late stage of storage. The increase of enzymatic activity of LOX, PPO and PPO isoenzyme, the accumulation of proline, hydrogen peroxide and superoxide radical and cell membrane lipid peroxidation was reduced or attenuated. Resultantly, slow cooling well restrained the browning of early harvested pear and especially exerted better protective effect on pear core browning.3. Fruit peel contributed to more than 90% of the total respiration of the whole fruit. Dissolve of CO2 decreased from inner to outer tissue of the fruit. Results showed that dissolve of CO2 in tissue was correlated with temperature, CO2 concentration in environment and physiological status of the tissue. As temperature decreased, CO2 concentration increased, and se...