Study on the factors influencing postharvest browning of the pomegranate peel

Abstract:【Objective】Pomegranate (Punica granatum L.) is popular among consumers because of its delicious flavor, high nutritional value and medicinal benefits. However, the fruits senesce rapidly after harvest with rapid peel browning, resulting in the reduction of commercial value. The white peel pomegranate cultivar‘Baihuayushizi'was used as test material to understand the mechanism of peel browning during storage.【Methods】Pomegranate fruits were harvested from a commercial orchard in Huaiyuan county, Anhui province. The fruits were precooled immediately at 4 ℃ after harvest and then were transported in the foam-boxes with ice to the Key Laboratory of Genetic Improvement and Ecophysiology of Horticultural Crop (Hefei) by car. Normal fruits were selected and stored at room temeperatre (25±2 ℃) . The fruits were stored for 6 days and were sampled once each day from the beginning of the storage. The peel was cut off and then stored in the liquid nitrogen for experiment. The physiological and biochemical parameters were assayed at one-day interval. There were 3 replications for each assay and 10 fruits for each replication.The physiological and biochemistry indicators including browning index, water loss rate, reducing sugar, amino acid nitrogen, 5-hydroxymethylfurfural (5-HMF) , total polyphenols, polyphenol oxidase (PPO) , malonaldehyde (MDA) and 1-diphenyl-2-picrylhydrazyl (DPPH) free radical scavenging ability were measured.【Results】Water loss rate in the peel of pomegranate gradually increased with prolonging of the storage. The highest water loss rate (20.90%) was found on the 6th day of the stroage. Browning index was significantly and gradually increased with progress of the storage (P<0.05) . The browning index on the 6th day of the storage was 0.661. There were significant correlations between the related indicators of non-enzymatic browning such as reducing sugar content, amino acid nitrogen and browning degree.During the storage, the content of reducing sugar decreased at first and then slightly increased. The content of reducing sugar on the 6th day was 1.34 fold higher than that on the 3rd day of the storage. The content of amino acidic nitrogen in the peel gradually decreased and the content of 5-HMF increased with fluctuation during the storage. The phenolic substrate and PPO activity were significantly correlated with browning index of the peel. Decreasing trend was observed in the activity of PPO from the first day to the6 th day during the storage. The maximum PPO activity was recorded on the first day (550 U·g^-1·min^-1) , and then remarkably decreased. The content of total polyphenols decreased at first and then increased as browning. The results indicated that postharvest browning of the pomegranate peel was associated with enzymatic browning and non-enzymatic browning. The MDA content increased gradually during the storage.The 1, 1-diphenyl-2-picrylhydrazyl (DPPH) free radical scavenging activity in pomegranate fruit peel increased at the beginning of storage but decreased sharply during the later stage of the storage. The DPPH free radical scavenging activity of the pomegranate peel ranged from 71.35% to 77.72%. And the highest DPPH free radical scavenging activity was found on the 3rd day of the storage. Correlation analysis showed that there was a significant positive relationship between the browning index and the water loss rate (r=0.896**) , the reducing sugar content (r=0.465*) , the total polyphenols (r=0.808**) and the MDA content (r=0.950**) , and a significantly negative correlation between the amino acid nitrogen (r=-0.860**) and the PPO activity (r=-0.926**) .【Conclusion】This study investigated the browning of the postharvest pomegranate peel during the storage at room temperature. The obvious water loss was observed and the higher MDA content might indicate that lower membrane integrity in the storage period might be the causes for browning of the pomegranate peel during the storage. The postharvest browning of the pomegranate peel could attribute to the enzymatic browning and the non-enzymatic browning. This progress was associated with enzymatic oxidation of phenolics by PPO. The 5-HMF could lead to non-enzymatic browning.