Qualitative and Quantitative Evaluation of Essential Oil of Catnip (Nepeta cataria L.) Under Different Drying Conditions

Drying is the most common way to preserve quality of aromatic and medicinal plants. Chosen drying methods can affect on the essential oil content and composition of medicinal plants.
This study aimed to assess the changes in essential oil constitutes of catnip aerial part under various drying methods.
The experiment was performed as the randomized complete block design (RCBD) with 10 treatments and 3 replications. The treatments were included the freshly harvested plants, sun-drying, shade-drying, oven-drying at 35, 45, and 55 °C, microwave-drying at 100 and 200 W, and initial sun-drying followed by subsequent shade-drying and oven-drying at 45 °C.
The result indicated that the various drying methods had a significant effect (p ≤ 0.01) on the essential oil content and compositions. The highest amount of essential oil was observed in the oven-drying at 55°C. GC-MS analysis revealed the presence 9 compounds in the essential oil of catnip, which three isomers of nepetalactone with a predominance of 4a-α, 7-α, 7a-β-nepetalactone were major essential oil components. Although, the amount of 4a-α, 7-α, 7a-β-nepetalactone were reduced by oven-drying at 55 °C, but other the isomers of nepetalactone were increased. However, the total content of three isomers of nepetalactone were equal in the fresh catnip with the dried plants by the oven drier at 55°C.
In general, the oven-drying at 55°C is recommended as an effective method to dry the catnip.