Monitoring of essential oils during storage by selected quality parameters
Essential oils are complex mixtures derived from a considerable number of aromatic plants . While it is well accepted that essential oils are susceptible to chemical alterations during storage , comprehensive data on the physico-chemical alterations are still scattered. Although HPLC has been pronounced as a suitable method to detect less volatile and thermolabile substances in essential oils [3,4] extension to stability studies is lacking. Therefore, the aim of the present study was to establish a set of appropriate quality parameters including HPLC to monitor essential oil alterations upon storage. Aliquots of essential oils from lavender, thyme, rosemary and pine were exposed to stress conditions for up to 14 weeks at 40°C and cool white light in the presence of air. To stop further alterations treated samples were immediately stored frozen at –80°C until analysis. HPLC profiles showed distinct alterations for all essential oils during storage. Changes were also detectable by TLC. All essential oils exhibited a considerable conductivity increase accompanied by a pH-value reduction except for thyme oil. A remarkable elevation of the peroxide value was also monitored. Hence, in addition to peroxide value, conductivity and pH-value can be suggested as appropriate quality parameters to reveal chemical changes in essential oils during storage. Complementary to GC, HPLC appears to be a suitable tool to shed light on the underlying chemical alterations. Further investigations are in progress to confirm the presented concept by extension to other essential oils.
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