Effects of different drying methods (natural method and oven) on drying time and some secondary metabolites of Lemon balm (Melissa officinalis L.)

Background and objectives: Lemon balm is a medicinal herb that its essential oil has a wide range of applications in the pharmaceutical, health and food industries. It also contains Rosmarinic acid (RA) which is presently utilized against HIV1 viruses and treatment of AIDS disease. Drying is one of the oldest methods of preserving agricultural products after harvest. This process involves the removal of moisture by evaporation to reach a certain threshold. The product can be stored for a long time. Actually, this process will stop the enzymatic activity of microorganisms and yeasts which cause decay of plant tissues. The drying process according to the type of active ingredients (alkaloids, essential oils, flavonoids, etc.), an appropriate method must be selected. In addition to great effect during the process of drying the durability of the product, the results of some studies have also shown that the method used for drying influence on yield and secondary metabolite content in herbs and ingredients.

Materials and methods: The experimental design was randomized complete block design with three replications and treatments were: drying in room, shade and under sunlight, seven temperatures: 30ºC, 35ºC, 40ºC, 45ºC, 50ºC, 55ºC and 60ºC. The drying process was continued until the mass of the sample reduced to a moisture content of about 0.10 on a dry weight basis or 10% on a wet weight basis. The dried samples used for extraction of essential oil (Essential oil content) and methanolic extraction applied for the measurement of some secondary metabolites (antioxidant activity, total phenolic, and rosmarinic acid content). The essential oils of dried samples were obtained by hydro-distillation, and the Rosmarinic acid (RA) contents were analyzed by HPLC, while their antioxidant activity was assessed by DPPH free radical scavenging capacity and total phenolic compounds were determined by the Foline-Ciocalteu method.

Results: The results showed that the highest level of essential oil (0.38%) and antioxidant activity (83.19%) was achieved in the treatment of drying with oven at 30°C. The highest total phenol content (58.77 mg/g DW) was obtained in the dry treatment with oven at 40°C. Additionally, highest level of rosmarinic acid (39.78 mg/g DW) was observed in the treatment of room drying. The lowest amount of essential oil (0.22%), antioxidant activity (20.58%) and rosmarinic acid (0.26 mg/g DW) was achieved in the treatment of drying at 60°C. Increase in oven temperature (60°C) and decrease in oven temperature (30-40°C) in different methods of drying, respectively cause the fall and rise in the amount of secondary metabolites of Lemon balm.

Conclusion: According to the results, if the aim of drying of Melissa officinalis L. is achieving higher levels of essential oil, drying at 30°C will be recommended. However, if the aim is achieving high levels of rosmarinic acid, drying at the room and 40-45°C be recommended.