Effect of Convective Drying on Quality of Lemon Balm (Melissa Officinalis L.)

Abstract

The effect of the conditions of drying air on the quality of lemon balm (Melissa officinalis L.) leaves such as colour, rosmarinic acid and essential oil content was investigated in this research. Fresh leaves with a moisture content of approximately 80% wet basis were dried to a final moisture content of 10% wet basis. The thin-layer drying experiments were conducted in a high precision through flow laboratory dryer at air temperatures of 30, 35, 40, 45, 50, 60 and 70°C and the corresponding relative humidity by maintaining 10g water per kg of dry air. The effect of the increased humidity of drying air was also investigated at 15, 20, 25 and 30g/kg specific humidity. For all drying trials the air velocity flowing through the sample was kept constant at 0.2 m/s. Although drying of leaves at 30°C preserved their medicinal qualities and colour, the duration of the process was comparably long. On the contrary, high drying air temperatures caused considerable colour degradation, a decrease in rosmarinic acid content and significant essential oil losses. The effect of relative humidity of drying air on the overall quality was found to be insignificant. A temperature limit of 40°C can be imposed for convective drying of lemon balm in order to protect the heat-sensitive active ingredients and maintain the green colour of the leaves.