The effects of ethanol extract of red mangrove and eucalyptus leaves on antioxidant capacity, enzyme activity and malondialdehyde of fresh banana fruit

Introduction: Banana (Musa spp.) is one of the most important commercial tropical fruits traded. Fruit is perishable and have a relatively short shelf life due to physiological characteristics at the time of ripening. Due to these characteristics postharvest losses of fresh banana fruits have been estimated to be very high, especially in developing countries, and their postharvest life can be very short (Yahia, 2011). Polyphenol oxidase is considered the enzyme responsible for quality deterioration and browning in fresh banana fruits during postharvest period. Browning reaction of banana fruit results from the enzymatic oxidation of phenolic substrates by polyphenol oxidase leading to the production of black or brown pigments which is the cause of enzymatic browning in fruit (Tapre and Jain, 2016). Antioxidant properties of essential oils and plant extracts were obtained from a reduction of enzymatic browning and fruit shelf life extension (Ranasinghe et al., 2002; Yahia, 2011). Treatment with basil essential oil inhibited anthracnose and crown rot and extending storage life of banana as well as eucalyptus oil-enrichment reduced fruit decay and maintained fruit quality of strawberries. The composite edible coatings of gum arabic enriched with lemongrass showed the synergistic effects and the greatest potential to control anthracnose and improved postharvest quality of fruit banana (Tzortzakis and Economakis, 2007). Thyme essential oil treatment induced the activities of peroxidase, phenylalanine ammonia lyase and chitinase which all play an important role in disease resistance in avocado fruit (Sellamuthu et al., 2013). Ethanol extract of Rhizophora mucronata reduced the growth of Penicillium pupurogenome, Penicillium chrysogenum, Penicillium notatum, Penicillium niger, Penicillium alternata and Penicillium italicum in vitro (Rastegar and Gozari, 2017). Mangrove leaf extracts were inhibited Penicillium digitatum of growth. Mangrove leaf extracts, in 20, 40, 60 and 80 Percent concentration, the mentioned extract were shown inhibition effect on mold pathogen growth (Alizadeh Behbahani et al., 2012). Therefore, the objective of the present experiment was to evaluate doses of red mangrove and eucalyptus ethanolic extracts from leaves on the effective control of the polyphenol oxidase activity and enzymatic browning and maintenance ofantioxidant capacity of fresh banana fruit.
Material and methods: Banana fruits (Musa,genome group AAA and subgroup Cavendish) at the mature green stage were harvested from a commercial orchard in Zarabad (Sistan and Bluchestan Province) and were transferred to the postharvest laboratory of hormozgan university. The selected fruits were randomized before being used for plant extract treatments. Ethanolic extracts of red mangrove (Rhizophora mucronata) and eucalyptus (Eucalyptus spp.) leaves in concentrations (0, 500 and 1000 µg/L) for ten minutes. According to the method used by De León-Zapata et al. (2015), the extracts were prepared by adding 5 g of dried leaf powder to 50 ml ethanol (96%). The extracts were stored in opaque vials at 4 °C before using them on fresh banana fruits. Then, the liquid extracts on the surface layer of the fresh banana fruits were left to evaporate, and the fruits were stored at (25°C ±1 and 80-90% relative humidity) for 0, 7, 14 and 21 days. Several parameters were measured every 7 days during the storage time. These were the antioxidant activity, total phenol content, malondialdehyde, peroxidase and polyphenol oxidase activity. The current study was carried out as a factorial assay and was based on a completely randomized design with three replications. Data were processed by ANOVA using the SAS software version 9.4. Significant differences were identified by using LSD test at 1% probability level.
Results and discussion: The results showed that the processed fruits had better quality compared to the control fruits. The most effective treatment for maintaining the total phenol content in fruits was the eucalyptus leaf extract at a concentration of 1000 μg/L with 10.79 mg gallic acid/100 g fresh weight. Phenolic content was higher in fruits at harvest time (15.85 mg gallic acid/100 g fresh weight). Lowest total phenol content was observed in untreated fruits (7.11 mg gallic acid/100 g fresh weight) after 21 days. With ripening, total phenol content decreased in fresh banana fruit. Phenolic compounds and tannins which are responsible for astringency taste of unripe fruits, decreased with ripening mainly due to polymerization. The highest antioxidant activity (11.9%) and the lowest amount of malondialdehyde (0.29 nmol/g fresh weight) were achieved in fruits treated with 500 μg/L Eucalyptus leaf extract, while the lowest amount of antioxidant activity (5.82%) and the highest amount of malondialdehyde (0.44 nmol/g fresh weight) were achieved in the control sample. Also, the highest enzymatic activity of peroxidase (7.72 unit/mL/min) was observed in fruits treated with 1000 μg/L mangrove leaf extract. The lowest activity of polyphenol oxidase (11.22 unit/mL/min) was observed in fruits treated with 500 μg/L mangrove leaf extract. The results showed that the extending of postharvest life of banana fruits from 0 to 21 days significantly enhanced the activity of polyphenol oxidase and peroxidase enzymes. Enzymatic browning reaction of banana fruit is usually caused by polyphenol oxidase and peroxidase enzymes, following cell damage caused by senescence. The study indicated the beneficial effect of extracts of red mangrove and eucalyptus leaves by postharvest immersion on antioxidant capacity and enzyme activity of fresh banana fruit.
Conclusion: Ethanol extracts of red mangrove and eucalyptus treatments maintained greater total phenol content and antioxidant activity, reduced malondialdehyde, peroxidase and polyphenol oxidase activity in fresh banana fruit during ripening. This study showed that the ethanolic plant extracts have a significant effect on preserving the biochemical properties of banana fruits by reducing the rate of mechanisms involved in ripening. Also, the use of plant materials are more acceptable by the consumer due to their higher health safety compared to chemical agents.