3-MA, a new potential naturally-derived post-harvest fungicide

Garlic is a staple food in cultures and cuisines such as that of the mediterranean basin. However, it is vulnerable to spoilage and the growth of pathogens during its post-harvest life, particularly if stored at high temperatures and humidity. One of the pathogens that poses a particular threat to garlic is Aspergillus niger, a pathogen that not only jeopardises the physical integrity of the garlic but also poses a risk to those who consume it, due to its ability to produce mycotoxins, which are toxic to humans and very difficult to remove. This presents the option of using 3-MA as a naturally derived post-harvest treatment, without having to resort to synthetic compounds.


Researchers at Northwest Normal University, Lanzhou, China, have conducted a study to evaluate the antimicrobial effectiveness of compounds derived from the microorganism Bacillus cereus, applied to garlic during its post-harvest life. Strains of Bacillus cereus were isolated from samples of Pinella ternata, a medicinal plant native to the Chinese province of Gansu. It was found that Bacillus cereus produced several metabolites, including 3-methylvaleric acid (3-MA), which inhibit the growth of Aspergillus niger.


3-MA is a volatile organic compound naturally produced by Bacillus cereus, which was shown to possess antifungal activity not only against Aspergillus niger, but also against other pathogenic fungi of major importance in the post-harvest life of foodstuffs: F. oxysporum, C. capsici, B. cinerea, F. graminearum, and P. capsici. At a very low concentration (100 μL/L) of 3-MA and after 5 days of treatment, 100 per cent inhibition of fungal growth was achieved for the various species.  


The research team identified the mechanisms that enabled 3-MA to interfere so effectively with fungal development. It was found that the presence of 3-MA affected the morphology of the hyphae (filamentous appendages that enable fungi to feed and grow), as treatment with 3-MA resulted in the collapse of cell walls, as well as a disruption in the structural integrity of the conidia, the reproductive structures of fungi.
Measurements were also taken of malondialdehyde levels; as malondialdehyde is a product of lipid peroxidation, its concentration is a good indicator of the integrity of the plasma membrane of fungal cells, which are composed of lipids.  It was observed that, as the concentration of 3-MA in the treatment increased, so too did the concentration of malondialdehyde; in other words, the cell membranes became more damaged.

Given these promising results as an effective fungicide against A. niger and other pathogens relevant to the post-harvest life of vegetables such as garlic, the study proposes the use of 3-MA as an organically derived fungicide. The authors emphasise that further research is needed to optimise formulation, storage and food safety.