Changes in antioxidant system and expression of genes encoding antioxidant enzymes in hot water-treated 'Hom Thong' banana fruit during storage

Abstract

The effects of hot water treatment on antioxidant system were investigated in ‘Hom Thong’ banana (Musa acuminata, AAA group) fruits by immersing fruits in hot water (50 °C) for 10 min, before storage at 25 °C for 10 days or 14 °C for 8 days followed by storage at 25 °C for 8 days. Fruit ripening occurred on day 8 and up to day 10 of storage at 25 °C. According to L value, hue value and pulp firmness, hot water treatment showed a delaying of ripening during day 8 to day 10. The level of H₂O₂ were reduced in hot water treated banana during storage and ripening, although there was a tendency towards an increase of H₂O₂ in hot water treatment after treatment on day 0 and day 2. MDA contents in hot water treatment were also reduced throughout storage time. For electrolyte leakage, hot water treatment had minimal effect on the increase in electrolyte leakage after treatment and during early period of storage. However, the levels of electrolyte leakage in treated fruits decreased during ripening, compared with control. Regarding antioxidant compounds, hot water treatment showed slight effects on AA contents at early period of storage, while it tended to activate caroteniods and GSH contents, and the ratio of GSH/GSSG during fruits approaching ripening. According to phenolics, flavonoids and DPPH radical scavenging activity, the most active compounds were presented in free extracts. Hot water treatment affected phenolics at the beginning to storage but had no significant effect during fruit ripening. Furthermore, flavonoids increased in hot water treated fruit before ripening. The DPPH radical scavenging activity decreased during storage. Hot water treated fruits tended to have higher levels than those of control, although there was no significant difference between the treatments. Hot water treatment can activate CAT after treating (0 h and 1 h), during early periods of storage and also during fruit ripening. However, there was no effect on SOD expression after treatment. The activation of SOD can be observed on day 4 and day 6 of storage, and then it dropped during fruit ripening. Therefore, CAT seems to be an important gene that was induced by hot water treatment during storage at 25 °C. For banana fruit stored at low temperature (14 °C) before being transferred to 25 °C, the ripening was delayed in both control and hot water treatment. Ripening could be observed on day 14 up to day 16 of storage, which delayed ripening was found in hot water treated fruits. H₂O₂ contents in hot water treated fruits were slightly increased before fruit ripening, while the control fruit showed a sharp increase. However, MDA contents and electrolyte leakage in hot water treated fruit tended to be higher than those in control after being removed from 14 °C. Hot water treatment had minor effect on caroteniods, AA and glutathione contents. The increase of free phenolics and flavonoids in hot water treated fruits were higher than those of control during storage, although there was no significant difference during fruit ripening. Moreover, hot water treatment had no effect on antioxidant capacity throughout storage. CAT expression in hot water treatment was reduced after being removed from 14 °C, and then increased after 2 day of storage at 25 °C. When fruit began to ripen, hot water treated bananas exhibited higher CAT expression than in control fruits. SOD expression was induced when fruit was removed from 14 °C and showed continuously higher expression in hot water treated fruits up to day 12 of storage. However, lower SOD expression was found in hot water treatment when fruit ripened. These results suggest that hot water treatment can trigger antioxidant system as indicated by an increase of antioxidant compounds, including glutathione, phenolics and flavonoids, at early period of storage. Also, CAT tended to be activated immediately after treatment and during fruit