Diurnal Effects on Mentha canadensis Oil Concentration and Composition at Two Different Harvests

Japanese cornmint, also known as menthol mint, (Mentha canadensis L. syn M. arvensis var canadensis L.), is an essential oil crop cultivated in several countries in Asia and South America. The plant is currently the only commercially viable source for natural menthol due to the high concentration of menthol in the oil. The hypothesis of this study was that harvesting time within a 24 h period will have an effect on essential oil yield and composition of Japanese cornmint grown at high altitude in Northern Wyoming. Flowering plants were harvested every 2 h on Aug 7th – 8th (Harvest 1) and again on Aug 14th -15th (Harvest 2) and the essential oil was extracted and analyzed on gas chromatograph-mass spectroscopy (GC-MS). Harvest date (Harvest 1 and Harvest 2) and harvest time (12 times within 24-h period) were significant on the oil yield (content), on the yield of menthol, whereas harvest date alone was significant on the concentration of menthol in the oil. The interaction effect of oil harvest date and harvest time were significant on moisture content, on the concentrations of menthol and menthofuran in the oil, as well as on the yield on limonene, menthol, and menthofuran. Overall, the oil yield in g per 100 g dried material (1.26 and 1.45, respectively), the concentration of menthol in the oil (67.2 and 72.9%, respectively) and menthol yield were higher in plants at Harvest 2 than at Harvest 1. The oil yield in dried Japanese cornmint was higher in plants harvested at 11:00 AM and at 1:00 AM and lower in the plants harvested at 3:00 AM. Menthol yield was the highest in plants harvested at 1:00 PM and lower in the plants harvested at 7:00 AM, 7:00 PM and 3:00 AM. This study demonstrated that harvesting time may affect essential oil yield and composition of Japanese cornmint grown at high altitude in Northern Wyoming.