Plant oils: the future of food preservation?

From: “Insecticidal Activity of Four Plant Essential Oils against Two Stored Product Beetles”, K. Saeidi and H. Pezhman (2018), Entomology, Ornithology & Herpetology 7:3

https://doi.org/10.4172/2161-0983.1000213

Storage pests are a real problem in feeding the planet, and can result in huge losses of stored crops that have taken many months to grow. Insects, especially beetles such as Bruchus lentis and Callosobruchus maculatus, are a major cause of such losses. One of the most commonly used methods to prevent insects from destroying food stores is fumigating them by pumping them full of synthetic insecticides. This has its downsides, especially as there’s a limit to the amount of pesticides you can use on a food before people become unwilling to eat it.

Karim Saeidi and Hossein Pezhman are among those looking at plant oils as an alternative to traditional pesticides in protecting stored food. They therefore set out to discover how toxic the fumes from the oils of four plants were to the two beetles I mentioned earlier – Bruchus lentis, a lentil pest, and Callosobruchus maculatus, the cowpea beetle. The four plants they chose were all easy to obtain in their region of Iran – peppermint (Mentha piperita), pennyroyal (Mentha pulegium), shirazi thyme (Zataria multiflora), and another local thyme, Thymus daenensis.

But why test plant oils as insecticides in the first place? What made these scientists think that they might be useful in protecting food against insects? The first reason is logical – insects are a major threat to most species of plants, so it would be advantageous for most plants to have some sort of chemical defence. In many cases the insects actually evolve their own counter-defences against their food’s toxicity, often leading to ‘arms-races’ where the plants get more and more toxic and the insects get more and more resistant, right up to the point where there’s no more advantage to be gained from toxicity, and the plants start producing less toxin. This means that there is no longer much advantage, (and there’s usually a lot of disadvantages) in being resistant, so the insects lose their resistance and the cycle starts up again. For this reason, concentrations of insecticidal chemicals can vary massively between different populations of plants – each local population is at a different stage of the cycle. The genius of using plant oils on storage-pest insects is that they come from plants, and parts of the plants, that the pests, being adapted principally to eating seeds, would not normally eat. This means that they are unlikely to have any resistance to them. The researchers also point out that plant oils are unlikely to be harmful to humans, and may even be beneficial, which is a definite advantage.

Saeidi and Pezhman found that fumigating using any of their four oils could kill at least some of the beetles. But the different oils were differently toxic to the beetles. Peppermint was the most toxic oil: a dose of 14 microlitres* per litre of air was enough to kill every beetle after 72 hours of exposure. Oil from the thyme Thymus daenensis was also able to achieve this, but only at a much higher concentration – 80 microlitres (μL**) per litre of air. That is, it took over 5 times as much thyme oil to kill the same amount of beetles in the same time. Neither of the other plants was able to achieve this – at 80μL per litre of air, Zataria multiflora thyme killed all of the Bruchus lentis beetles but only 83% of the Callosobruchus maculatus. Similarly, pennyroyal only killed 85% of the B. lentis and 89% of the C. maculatus at 100μL per litre of air.

The researchers were also able to calculate a toxicity measure known as the LC₅₀ – the concentration which kills 50% of a sample.*** The lower this figure is, the more toxic the substance is. So, the LC₅₀ of peppermint for B. lentis was only 14.62μL per L of air, whereas that of pennyroyal for the same insect was 92.32μL per litre of air. Interestingly, the researcher’s calculations were not able to find any certain difference in the LC₅₀s of pennyroyal and Thymus daenensis, but Zataria multiflorensis seems to be slightly more toxic than both on B. lentis. However, this does not seem to apply to C. maculatus. Peppermint oil was by far the most toxic to both species of beetle. But why? It’s not yet clear, but the active insecticidal compounds in peppermint are thought to be menthone and menthol, compared to thymol and 1,8-cinole in thyme. The researchers speculated that different species may have different concentrations of these chemicals, leading to their differing (and also not-differing) toxicity levels.

So – is peppermint oil fumigation the future of food preservation? Quite possibly, but is remains to be seen if it affects the taste of these foods.

*A microlitre is a millionth of a litre, so that’s 0.0000014 litres of peppermint oil per litre of air

** ‘μ’ is the Greek equivalent of ‘m’, and ‘mL’ was already taken by millilitres. I don’t make the rules.

*** so, Lethal Concentration 50

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