Impact of different storage conditions on the quality of selected essential oils

Abstract Four common essential oils were subjected to different storage conditions in order to reveal the impact of light and temperature on the physico-chemical properties as well as on the chemical composition of the respective oil. For this purpose, aliquots of lavender, pine, rosemary, and thyme oil were stored for up to 72 weeks in the presence of atmospheric oxygen at 23 °C in the dark as well as at 23 °C and 38 °C under cool white light, respectively. Alterations were monitored by a set of recently established quality parameters such as peroxide value, pH, and conductivity as well as high-performance liquid chromatography (HPLC) with diode array detection and tandem mass spectrometry. Characteristic changes occurred for each essential oil, revealing individual impacts of extrinsic parameters on the particular sample. Most striking degradation of monoterpenes could be observed in rosemary oil: While α-terpinene was reduced to less than 10% within 3 weeks of storage at 38 °C under daylight but did not alter during the same period at room temperature in the dark, its amount in pine oil decreased to about 40 and 65%, respectively. Moreover, trends of peroxide values were compared to conductivity and pH in the course of storage. This approach allows to shed light on the storage history thereby providing a more complete view on essential oil quality. Additionally, gas chromatography (GC) analyses coupled to electron ionization mass spectrometry were performed in order to evaluate the informative and complementary character of GC and HPLC with regard to their capability to retrace essential oil modifications upon storage, respectively.

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