Hannes Bitterling

Investigations into Ligt-indced Terpene Modifications in Essential Oils - Influence of Furocoumarins

PhD Thesis, Faculty of Natural Sciences, University of Hohenheim

Essential oils belong to the volatile secondary plant metabolites, having a lipophilic character and molecular weights below 300 Da. Essential oils are composed of compounds belonging to different chemical substance classes, whereby mono- and sesquiterpenes usually make up the main part of the oil. Due to their low boiling point, they were normally gained by hydro-distillation, except for citrus essential oils which are produced by mechanical peel extraction. In this latter process, further non-volatile plant compounds such as furocoumarins are also transferred into the essential oil fraction. Due to their pleasant smell and fine aroma, essential oils are widely used such as in food and beverages, as well as in cosmetics. Because of their distinct pharmacological properties, a considerable number of essential oils are also used in phytotherapy.

Essential oils are known to undergo several chemical changes during processing and storage, thereby losing their characteristic olfactory and medicinal properties. Essential oils tend to easily degrade in the presence of oxygen. Various factors including elevated temperature, light, and catalytic agents may accelerate degradation rates. The formation of oxidative reaction products, such as hydroperoxides, have been described in the scientific literature. In hydroperoxide quantitation, the commonly used iodometric titration poses several problems such as the requirement of high sample amounts and the impact of accompanying compounds in the respective sample.

Therefore, the first goal was to establish sensitive instrumental analytical methods for the compound-specific detection of terpene hydroperoxides and further oxidation products. These methods should be applied in several studies to gain more information on the alteration processes of terpenes and their corresponding essential oils on a molecular level. For this purpose, samples should be stored under real-time conditions including known essential oil plants such as caraway (Carum carvi L.) and lemon (Citrus limon (L.) Osbeck). Furthermore, a closer look at the impact of the surrounding matrix on terpene degradation should be taken. Not only the plant-like cell matrix but also non-terpenoid substances in the essential oil are to be examined more closely. The focus should be on furocoumarins that were removed and again added into expressed agrumen oils. Hereby, the differences in autoxidation and photo-oxidation should also be considered.