Study on essential oil and free menthol accumulation in 19 cultivars, populations, and clones of peppermint (Mentha x piperita)

Bulgarian peppermint oil, also known as 'Bulgaro-Mitchum', is famous as having the finest quality. Being the main constituent of the oil, menthol is the targeted compound in the selection and breeding process. The menthol in the oil is represented in free form (menthol, isomenthol, and neomenthol), or in associated form (menthyl acetate). A two-year field experiment was conducted to study the essential oil and menthol productivity of 19 cultivars and population of peppermint. The aim of the investigation was to evaluate the available peppermint material obtained through various breeding methods with respect to further selection and breeding by using two indices: (1) intensity of essential oil synthesis (as the necessary dry herba for obtaining of 1 g of essential oil), and (2) intensity of menthol synthesis and accumulation (as the necessary dry herba for obtaining of 1 g of menthol). The highest essential oil content in dry herba was measured in two varieties: Dulgo pole (2.5 %), and Bulgarian population #2 (2.9 %). For these two forms, the necessary dry matter for obtaining 1 g essential oil are 40 and 34 g respectively. The highest menthol content was established in the oils from cultivars Zeflr, Dulgo pole, and Clone 11-6-22 and Clone 80-121-33.