The influence of temperature, light, and storage period on the colour and chemical profile of kunzea essential oil (Kunzea ambigua (Sm.) Druce)

Abstract

Essential oil distilled from the leaves and twigs of Kunzea ambigua is in high demand for its ‘soft, medicinal and soothing aroma’ and claimed muscle relaxant properties. This study investigated the changes in oil colour and chemical composition of kunzea essential oil over eight months of storage in a freezer (−20 °C), a refrigerator (4 °C) and at room temperature (20 °C), with and without ambient light. The oil colour was measured by a colourimeter, and chemical compositions were analysed using Gas Chromatography (GC) Mass Spectrometry (MS) and Flame Ionisation Detection (FID). Samples were analysed at one-, three-, five- and eight-month intervals. The colour difference (Δ E * ) was highest in oils stored in the light at room temperature for eight months (5.60 ± 0.03%), whilst samples kept in the dark were more stable with ΔE * at room temperature of 1.97 ± 0.00%, refrigeration at 1.55 ± 0.02%, and freezing temperatures at 1.08 ± 0.17%. Stored kunzea essential oil became darker in colour, becoming less yellow and decreasing in greenness, with extended storage periods.The monoterpenes, sesquiterpenes and total volatile compounds in oil stored for eight months in the freezer, refrigerator, and room temperature without light were overall generally stable compared to fresh oil. Significant interactions between storage period and temperature/light treatments were recorded for bicyclogermacrene, α-humulene, α-campholenal and linalool, showing a decrease in all oils under all conditions during extended storage periods, though changes were minimal in frozen oil. However, the oils exposed to the light showed a significant decrease in these components, particularly for germacrene D, β-caryophyllene and α-humulene, likely due to thermal isomerization, relative to fresh oil.Overall, kunzea essential oil stored in a freezer or refrigerator was the most stable and consistent in colour and chemical profile whereas there is evidence that some components may be when exposed to light with negative consequences for oil colour and quality.