Investigations into the stability of essential oils

In recent years, essential oils have reached great popularity and their possible applications are numerous. Biosynthesized by a number of aromatic plants, they represent naturally occurring mixtures of a huge variety of secondary metabolites. It is generally known that essential oils are prone to chemical alterations upon aging, triggered by air, light, and heat. Thus, quality monitoring of essential oils during storage to reveal chemical changes is not only mandatory due to sensory reasons but also to ensure pharmacological safety. Still, only little has been known about underlying processes that essential oils may go through after production and systematic approaches investigating the impact of differing extrinsic factors on the stability of essential oils are rare to be found. Furthermore, only a limited number of analytical parameters were hitherto taken into account that, however, have been reported not to fully assess possible changes upon storage. Therefore, an HPLC method combining diode array detection as well as mass spectrometry to comprehensively assess both genuine essential oil volatiles of different chemical classes as well as storage-induced chemical alterations such as the formation of polymeric oxidation products or thermolabile substances was implemented in the course of the present thesis (CHAPTER 1). In addition, a range of suitable physicochemical parameters complementing HPLC was established in the present work (CHAPTER 2) assessing both primary as well as secondary oxidation processes to comprehensively reveal alterations in essential oils. In the course of storage, refractive index, color, peroxide value (POV), pH, conductivity, as well as HPLC analyses were applied on essential oils and individual parameters were discussed with respect to their capability to track aging processes. The hitherto new approach of combined pH and conductivity assessment in essential oils turned out to be an appropriate quality parameter complementing the POV, as the compounds captured by these methods seemed to be built-up in later stages of storage-induced aging than peroxides. Moreover, several essential oils were kept at 23 °C in the dark as well as at 23 °C and 38 °C under imitated daylight in the presence of atmospheric oxygen for up to 72 weeks, respectively (CHAPTER 3). Storage-induced alterations were monitored by conductivity, pH, and POV as well as HPLC in comparison to gas chromatography. Subjected to varying storage conditions, essential oils displayed individual responses towards the divergent extrinsic factors, depending on their specific substance pattern. The complementary assessment of quality parameters allowed disclosing the respective changes in chemical composition and physicochemical properties in order to gain a holistic view on essential oil alterations upon storage. The impact of storage temperature and the availability of oxygen on the stability of essential oils was further investigated by means of storage experiments at 5 and 23 °C under varying sampleto-air volume ratios (CHAPTER 4). In doing so, the individual character of the chosen oils became again clear and the individual responses to storage temperature and oxygen availability are discussed. It can be concluded that quality parameters established in this thesis represent an appropriate tool to evaluate essential oil quality and stability and to shed light on the specifically different response of essential oils during storage.