Postharvest control of citrus pathogens using free and encapsulated essential oils.
Bopima, Sinclair Ngemba
Bopima, Sinclair Ngemba
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Abstract
South Africa is one of the major exporters of citrus fruit worldwide. During the season 2015/2016, the country was ranked the second largest exporter of oranges and exported about 180 000 cartons of kumquat (Citrus japonica) to the European Union and the Middle East as major destinations. However, postharvest decay caused by green mould (Penicillium digitatum) and sour rot (Galactomyces citri-aurantii) is a threat to the industry. Kumquat in particular, is susceptible to postharvest rot caused by green mould. Health and environmental concerns with regard to synthetic fungicides have provided opportunities for research into green alternatives. The application of essential oils (EOs) as low risk mycobiocides for the postharvest management of decay on fruit, has gained momentum. However, the volatility of EO compounds and the possible phytochemical effects associated with EO application at high concentration have resulted in novel approaches for better application. The applications of EO as reducing and stabilizing agent in the synthesis of silver nanoparticles (SNPs), as well as the use of coating substances such as chitosan (CHI) and wax used as encapsulating matrices of EOs, have been investigated. In this study, the in vitro and in vivo inhibitory potential of selected EOs in the free and encapsulated form, and of oil-mediated SNPs, were investigated for the control of green mould and sour rot.
Both toxic medium and microtitre assays were used to compare the inhibitory effects of the oils, individually and in combination. At 500 μL/L, cinnamon leaf oil was the most effective against green mould, while lemongrass was the most effective against sour rot in the toxic medium assay. Spearmint oil was the least effective, because of the high concentration required for inhibition. Cinnamon performed best in the microtitre assay at 2000 μL/L. Oil-mediated SNPs were synthesised, characterised and their activities were subsequently assessed against the pathogens. The SNPs retarded the growth, suppressed sporulation and prevented spore germination of green mould, but were ineffective against sour rot. Chitosan alone demonstrated antifungal activity against green mould and sour rot at 7.5 mg/mL. The combination of CHI and EOs, both at their sub-inhibitory concentration, inhibited the growth of green mould and sour rot; and provided 100% inhibition of the mycelial growth after seven days of incubation. Essential oils alone and the mixture of EO with wax were tested in a semi-commercial trial on artificially wound-inoculated (green mould and sour rot) Valencia oranges. The outcome indicated that the inoculation conditions were too harsh, thereby limiting the ability of the EOs applied alone and/or with wax to control decay. The application of CHI alone and encapsulation of EO in CHI coating matrix at high concentrations resulted in poor performance of the substances to control sour rot infection on kumquat.
In vivo trials under commercial conditions indicated that both cinnamon leaf and spearmint oils at 3000 μL/L were able to control decay compared to the conventional treatment. In addition, the SNPs were also effective compared to the control treatments. Applying the EOs at higher concentration (8000 μL/L) and the use of a mixture of EO and wax resulted in a higher incidence of decay under all storage conditions tested. The in vitro and in vivo results did not correspond when using EOs. The use of high concentrations of EO possible affected fruit metabolism on kumquat since the fruit has a large surface to volume ratio. Spearmint and cinnamon leaf oils outperformed lemongrass oil when applied to commercial fruit, despite displaying less potential for pathogen control in vitro.
Potential hot spots for contamination were sampled in the packhouse, but proved to have low spore counts, suggesting that the orchard may be the main source of kumquat contamination. Although there was no difference in the degree of ripening of kumquat at all tested warm water dip temperatures, treatment at 35 °C yielded the most promising results. A lower percentage of fruit rot and firmer rind were recorded at this temperature than for the others.
This study has contributed to the search for greener alternative applications of fungicides to fruit using natural substances. It has revealed that EOs in the free form hold promise for pathogen control and that encapsulated forms do not necessarily perform better. However, due to the volatile nature of EOs and the variable conditions in packhouse settings, it is recommended that future trials using EOs be carried out only in the packhouse, to mimic realistic environmental conditions and thereby guarantee the expected outcome.
Description
Submitted in partial fulfilment of the requirements for the degree, Magister Technologiae: Biotechnology in the Department of Biotechnology and Food Technology, Faculty of Science at the Tshwane University of Technology.
Date
2018-10-01
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Tshwane University of Technology
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Keywords
Essential oils, Citrus fruit, Mycobiocides, Cinnamon, Chitosan