Anti-fungal activity of phenolic sweet orange peel extract for controlling fungi responsible for post-harvest fruit decay
Introduction
Fresh fruits and vegetables are highly perishable during their post-harvest phase, resulting in substantial economic losses of up to 25% in industrialised countries, and around 50% in developing countries, due largely to inadequate storage and transportation facilities (Agrios, 2005, Singh and Sharma, 2007). Infection by fungal pathogens is recognized as the main cause of fruit decay during storage and transport (Serrano et al., 2005). This is mainly caused by moulds species from Penicillium, Botrytis, Monilinia, Cladosporium, Rhizopus, Mucor and Alternaria genera. Monilinia spp. are the causal agent of brown rot, a disease associated with severe production losses of stone fruit worldwide (Martini and Mari, 2014), while Botrytis cinerea, which incites grey mould rot, can infect a wide range of fruit and is ranked among the world’s top-10 fungal plant pathogens of scientific and economic importance (Dean et al., 2012). Substantial post-harvest losses of many fruits and vegetables also derive from black rot, which appears during marketing, due to latent infection by Alternaria alternata during cold storage (Troncoso-Rojas and Tiznado-Hernández, 2014). Post-harvest diseases caused by fungal growth can seriously damage the fruit, contributing unfavourably to the quality, nutrient composition and market value of the fruit (Valero and Serrano, 2010). Besides, fungal proliferation may result in potential mycotoxin contamination, posing risks to human health (da Cruz Cabral et al., 2016, Wu et al., 2014).
Currently, the addition of synthetic fungicides remains the main strategy to control post-harvest decay. However, their use is increasingly limited by the emergence of resistant fungus strains associated with their excessive application (Feliziani et al., 2013). Furthermore, today’s consumers demand high-quality, safe and environmentally-friendly products with low or no chemical residue. For this, fruit producer countries are establishing more restrictive regulations regarding the maximum levels of fungicides allowed and further motivating the design of alternative technologies with less impact on human health and the environment (Sivakumar and Bautista-Baños, 2014). Over the past 20 years, intense efforts have been made to develop useful and practical alternative technologies to control post-harvest losses. These can be broadly classified into four treatments: microbial antagonist agents (Sharma et al., 2009), natural anti-microbial substances (Ribes et al., 2018), disinfecting agents (Guentzel et al., 2010) and physical means (Romanazzi et al., 2012). Among these possibilities, the use of plant extracts with anti-fungal capacity is of particular interest due to their natural origin, decomposability and low-toxicity to human health and the environment (Palou et al., 2016). So far, some have displayed promising results to control many post-harvest phytopathogenic fungi species, and can be considered as suitable alternative. For example, soybean flour phenolic extract inhibited the growth of Monilinia laxa in vitro, and when combined with modified atmosphere, controlled fungal decay in figs (Villalobos et al., 2016a, 2016b). Pomegranate extracts have shown strong fungicidal activity against B. cinerea, Penicillium digitatum and Penicillium expansum and effectively reduced natural rots on both sweet cherries and lemons under semi-commercial conditions (Nicosia et al., 2016). Gatto et al. (2016) controlled the post-harvest rotting of sweet cherry by treatment with extracts from two wild edible plants (Orobanche crenata and Sanguisorba minor).
Sweet orange (Citrus sinensis L.) is highly consumed worldwide as fresh produce or juice, and comprises around 70% of the total citrus production and consumption (Liu et al., 2012, Sharma et al., 2017), generating substantial quantities of orange peel waste. The juice industry is interested in profiting from orange peel and minimising the adverse environmental impact of the large quantity of this by-product produced every year (Rezzadori et al., 2012). Citrus peel could have a high economic value, as it is an important source of various bioactive molecules, such as polyphenolics (flavonoids and phenolic acids), carotenoids, dietary fibre, essential oils and ascorbic acid, and also contains considerable amounts of some trace elements (Okino Delgado and Fleuri, 2016, Sharma et al., 2017). The polyphenolic extract from orange peels can exert diverse beneficial effects, such as antioxidant, anti-carcinogenic, anti-inflammatory and anti-microbial properties (Casquete et al., 2015, Hegazy and Ibrahium, 2012, Ke et al., 2015). The natural flavonoids from citrus species (hesperidin, naringin and neohesperidin) exhibited anti-fungal activity against the growth of Aspergillus spp., Fusarium semitectum and P. expansum, fungi commonly found in food, and also reduced mycotoxin accumulation (Salas et al., 2011, Salas et al., 2012, Salas et al., 2016a, Salas et al., 2016b). However, to our knowledge, the anti-fungal activity of polyphenolic extracts from orange peel against post-harvest fungal pathogens has not been addressed. Therefore, to maximise the reuse of sweet orange peel by-products and minimise their impact on the environment, the present study investigated their anti-fungal potential against three mould species responsible for post-harvest fruit decay, including Monilinia fructicola, B. cinerea and A. alternata, and elucidated the compounds responsible for the inhibitory activity.
Section snippets
Fungal strains
Three post-harvest fungal pathogens, B. cinerea CECT 20518, M. fructicola UEX2 and A. alternata H648 (Villalobos et al., 2017) were used in this study. Mould strains were routinely cultured in potato dextrose agar (PDA; Pronadisa, Madrid, Spain) at pH 3.5 ± 0.1, adjusted with 1% (v/v) of tartaric solution at 10% (w/v). The plates were incubated at 25 °C for around 10 days until the occurrence of sporulation. The conidia of each strain were harvested by adding 15 mL of sterile water containing
Anti-fungal activity of OPE
The capacity of different concentrations of OPE to inhibit the conidial germination and radial growth of M. fructicola, B. cinerea and A. alternata is shown in Table 1, Table 2, respectively. The percentages of inhibition against the three target fungi varied, depending on the OPE concentrations. Overall, at the highest concentration assayed, of 1.5 g/L, the conidial germination and growth of the three fungal pathogens were inhibited completely. However, patterns of inhibition among the target
Discussion
Current agricultural policies are promoting eco-friendly crop management, a scenario that has provoked intense interest in developing alternative, natural-derived anti-fungal products for the sustainable management of pre- and post-harvest fruit diseases (Romanazzi et al., 2016). Numerous literature studies have reported the suitability of plant-derived polyphenolic extracts as a safe alternative to synthetic fungicides for controlling post-harvest fungal diseases. Natural extract from orange
Acknowledgments
This work was funded by a Junta Extremadura Government and European Regional Development Fund (ERDF) grant (GR18165 and GR18196). A.V. Merchán and A.I. Galván were financed by pre-doctoral grants from the Extremadura Regional Government (PD16026), and the Ministry of Science, Innovation and Universities (RTA2017-00032-CO2-01), respectively. The authors are grateful to Ronald J.W. Lambert and Angel Medina for their assistance in carrying out the Bioscreen C raw data treatment.
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