food australia Journal, Vol. 74 (1) January - March 2022

Figure 1. Green mould decay in inoculated oranges following dipping in water (left) and oxidised orange essential oil (EO) at 4000 µL L-1 (right).

Essential oils: combating green mould disease in citrus as a natural bio-fungicide

Words by Mohammad Mizanur Rahman

Green mould caused by Penicillium digitatum is a major postharvest disease of citrus fruits causing major losses to the industry. (1) It is currently controlled with the application of commercial chemical fungicides, but there are growing health and environmental concerns with continued use of synthetic chemicals. (1,2,3) There is growing interest in sourcing natural alternatives, such as the application of essential oils (EO) to control postharvest decay. (2,3) This project focused on the development and formulation of bio-fungicides from EO extracted from orange peel waste (e.g. citrus juice pomace).

In these trials, orange EO was mildly oxidised using ultraviolet-C (UV-C) irradiation. The antifungal potential of non-oxidised and oxidised orange EOs was assessed both in vitro by agar diffusion assays and in organic Navel oranges inoculated with P. digitatum spores by dipping in aqueous emulsions.

The results showed that UV-C oxidation of orange EO resulted in 20% decline of the major constituent, limonene, from the EO. Three hydroperoxide oxidation products, namely (2S,4R)-p-mentha-6,8-diene-2- hydroperoxide, (1S,4R)-p-mentha-2,8- diene-1-hydroperoxide and (1R,4R)- p-mentha-2,8-diene-1-hydroperoxide, were identified by GCMS as oxidation products. These hydroperoxides have capability to alkylate DNA and proteins that relates the key concept of survival of microbial species. (4)

In vitro agar diffusion assay studies with P. digitatum showed a strong antifungal response from the oxidised orange EO solution relative to the control and non-oxidised EO. Dipping inoculated oranges in oxidised orange EO significantly inhibited growth of P. digitatum. The optimal treatment concentration was 4,000 µL L-1. Interestingly, P. digitatum growth in oranges dipped in nonoxidised orange EO at 1,000 – 4,000 µL L-1 was not significantly different to the control (water dip only) (Fig.1). The enhanced antimicrobial activity of the oxidised orange EO was attributed to the presence of limonene hydroperoxide derivatives. Fortunately, no phytotoxic injury to the rind was observed.

This study showed that UV-C oxidised orange EO has the potential as an alternative treatment to synthetic fungicides for controllinggreen mould (P. digitatum) decay incitrus. Extraction of orange EO fromwaste flavedo generated from juiceprocessing has the potential to addvalue to a currently unwanted byproduct.

References

1. Ismail, M.; Zhang, J. Post-harvest Citrus Diseases and their control. In Outlooks on Pest Management; Research Information: London, UK, 2004; 15, 29–35.

2. Talibi, I.; Boubaker, H.; Boudyach, E.H.; Ait Ben Aoumar, A. Alternative methods for the control of postharvest citrus diseases. J. Appl. Microbiol. 2014, 117, 1–17.

3. Torres-Alvarez, C.; Núñez González, A.; Rodríguez, J.; Castillo, S.; Leos-Rivas, C.; Báez-González, J.G. Chemical composition, antimicrobial, and antioxidant activities of orange essential oil and its concentrated oils. J. Food 2017, 15, 129–135.

4. Suzan, A.K.; Selva, R. Recent progress in photochemical reaction on main components of some essential oils. J. Saudi Chem. Soc. 2018, 22, 855–875.

Mr Mohammad Mizanur Rahman is a PhD (Food Science) candidate at the University of Newcastle, Australia (Thesis submitted). He had a funding collaboration with Horticulture Innovation “Citrus Postharvest Program”(CT19003) with NSW DPI, Ourimbah. He was the winner of the Judge’s Award at the AIFST 2021 Research Poster Competition.

14 food australia