Effect of Harvesting Time and Drying Methods on the Quantity and Quality of the Essential Oil of Peppermint Medicinal Plant (Mentha piperita)

Introduction   Mint has been used both as a medicinal and aromatic plant since ancient times. Peppermint volatile oil is one of the most important essential oils and has strong antimicrobial activity. The stage at which the maximum essential oil is obtained from the plant is not necessarily the stage at which the plant has the most biomass, therefore, the appropriate time for harvesting the plant should be determined according to the maximum amount of the effective substance and the yield of dry matter. In most of the methods of producing medicinal plants, quick access to the equipment for extracting effective substances is not possible, especially at high levels, and this requires drying a large part of the plants and extracting the effective substance in subsequent times. The quality of dried medicinal plants is defined by the content of their biologically active compounds. This study was aimed to determine the best time to harvest peppermint and the effect of different drying methods on the shoots biochemical compounds. Materials and MethodsIn order to increase the yield of peppermint medicinal plant essential oil, two separate experiments were designed and conducted in Shirvan Higher Education Complex (N 26 37', E 45 57', altitude 1067 m) in 2015. The aim of first experiment was finding the best harvest time at three levels (Before flowering, at flowering 50 and 100%) and the second experiment was finding the best method for shoot drying with nine levels (Microwave radiation at 90, 180, 360, 600 and 900 watts; Oven at 40 and 60°C; Shade; and Sun), were based on a completely randomized block design (CRBD) and a completely randomized design (CRD), respectively. In the spring of 2015, tillage operations were carried out and after preparing the land, 9 plots (2 × 3 meters) were created in it. The distance between the plots was considered 1.5 m. Rizhomes of peppermint plant was obtained from Mashhad Agricultural and Natural Resources Research Center and transplanted at 20 × 30 cm intervals on May 26. Some morpholocial and biochemical traits were measured. Essential oil extraction was done by distillation method with water and essential oil components were determined by GC/Mass method. Results and DiscussionThe effect of harvesting time was significant (p≤0.01) on the plant height, number of leaves per plant, wet and dry yield of peppermint plant. The height of the plant after 50% flowering stage did not show any significant difference, but the other investigated traits include the number of leaves per plant, the wet and dry yield of the plant, the yield of essential oil in terms of wet and dry weight, and the inhibition of free radicals in the branches in shoots harvested at the time of full flowering had the highest amount, which showed a statistically significant difference. The yield of essential oil in terms of dry matter in the full flower stage increased 1.8 times compared to the 50% flowering stage and 4.6 times compared to the pre-flowering stage. The percentage of essential oil components was also affected by the harvest time, and the highest amount was observed in the full flowering stage (98.92 percent), include the highest amount in menthol (42.39 percent) and menthone (19.13 percent). The use of micro rays caused the shortest drying time, so that with the increase of power of micro rays, the drying time decreased and the shoots were dried in 0.16 hours at the power of 900 watts. The oven application at 60 and 40 °C were statistically ranked second and third in terms of drying speed with 10 and 14.6 hours. Sun and shade drying needed 46 and 109.6 hours to reach constant moisture, respectively. The highest amount of essential oil was obtained in the shade drying method (2.13%) and the lowest amount was observed in the microwave application at 900 and 600 watt. (0.01%). After shade drying, the highest amount of essential oil was observed in the oven method at 40 °C (2%) and in the sun drying (1.44%). According to the results of this study, it is recommended to harvest the branches at the time of full flowering and dry them using oven at 40 °C to obtain more essential oil yield of peppermint shoots. ConclusionIn general, the harvesting time has a significant effect on the morphological characteristics and essential oil of the peppermint shoots. Harvesting peppermint in the full flowering stage resulted in the highest amount of essential oil and essential oil yield. The maximum height and number of leaves in the plant were obtained by harvesting at the time of full flowering, but the maximum wet and dry yield of shoots was observed at the harvest of 50% of flowering. The amount of menthol and menthone as the most important compounds of mint essential oil in the full flowering stage was higher than the other two harvesting times. Also, based on the results of this experiment, different methods of drying peppermint plant have various effects on the essential oil amount of peppermint shoots. The shortest time until drying was related to the use of microwave, oven, sun and shade, respectively, and the highest yield of essential oil was obtained in shade, oven at 40 °C, microwave at 90 watt and sun, respectively.