Investigation of the effect of seasonal variation on the accumulation of secondary metabolites and antimicrobial effect of essential oils from selected plants

Postharvest diseases poses major threats to the food industry worldwide. These diseases are usually controlled by means of cultural practices (e.g. sanitation) and chemical application by means of postharvest washing of freshly harvested produce. Yet, there is a limited availability of agrochemical products that can be applied to fresh produce for the control of postharvest pathogens. On the other hand, consumer demands for the use of natural and less toxic sanitisers in food has led the industry into search for healthier alternative fresh produce sanitisers, as well as preventative methods for postharvest diseases.

The preservation of the “freshness” quality through postharvest treatment of fresh produce is relevant due to their economical impact. As an alternative to synthetic preservatives, natural antimicrobial agents have attracted the attention of modern consumers and the fresh produce industry. In particular, natural antimicrobials (phytochemicals) based on plant essential oils are gaining support. Essential oils isolated from medicinal plants typically contain mixtures of different chemical compounds (anti-inflammatory, antibacterial, antifungal, etc.) that may act individually, additively or in synergy to improve health. Furthermore, medicinal plants generally have a wide therapeutic window (i.e. the effective dose differs significantly from the toxic dose). With most medicinal plant products that are on the market today, it is almost impossible to ingest sufficient material to reach dangerous or life-threatening levels of toxicity. In contrast, pure chemical compounds (synthetic chemicals) are highly concentrated and the desirable dose is easily exceeded.

A study was conducted to investigate the potential efficacy and minimum inhibitory concentration (MIC) of selected essential oils on the potato soft rot bacteria (Pectobacterium carotovorum subsp. carotovorum) through an efficacy contact study in-vitro. The essential oils showed high antibacterial efficacy from dosages as little as 300µl/L.  However, the results of the first experiment differed significantly from the second experiment. I started investigating the reason for the difference in efficacy. I found that these medicinal plants contain special ‘hormones’ which are called chemical compound. These chemical compounds are divided into two groups, called primary and secondary metabolites. The primary metabolites are plant growth hormones, which focus on cell division and enlargement for plant growth and reproduction. Whereas, secondary metabolites are responsible for activities like plant defence mechanism during shock and stress, which can be caused by climatic conditions, pests and disease, etc. The secondary metabolites are therefore responsible for antimicrobial activities. I then figured that different seasons of the year and physiological stage of the plant will have different effect in the production of chemical compounds that are needed for antimicrobial activity. The study had to include another objective, which was to investigate the effect of seasonal variation on the accumulation of secondary metabolites and antimicrobial effect of essential oils from the selected plants.

If we can determine the suitable harvesting season or physiological stage of the medicinal plants, whether for crude extraction, essential oils, medication for traditional or conventional use. We can reduce the exploitation of these useful, natural resources by sharing results from these studies. The results should reach researchers, horticulturalists, phytochemists, and worldwide communities that still rely and believe on traditional medications derived from these plants.

Some of these plants are already available and grown for research purposes at South African research centres, like the Vegetable and Ornamental Plant Institute of the Agricultural Research Council (ARC-VOPI). Some plants are sourced from farms where they grow naturally. Some communities also still grow these plants for health purposes. To ensure sustainable production and supply of these plants we will create awareness on the increasing importance and demand for these plants. This will also create sustainable income for communities and farmers by growing these plants at commercial level based on demand. Essential oils will be isolated from fresh plant material using hydrodistillation method. Plant material will be harvested in the four seasons of the year (summer, autumn, winter and spring) to investigate the effect of seasonal variation on quality and antimicrobial effect of essential oils. The essential oil quality will be analysed by gas chromatography (GC) and gas chromatography mass spectrometry (GC/MS) to identify and determine the quantity of major chemical compound from these plants at different seasons. Antimicrobial activity of the essential oils will be tested in vitro through contact study method for bacteria and toxic media method for fungal pathogens. The application method on various fresh produce will be developed.

Tlangelani Nghondzweni, South Africa

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