Changes of essential oil content and composition during convective drying of lemon balm (Melissa officinalis L.)

• Glandular trichomes in M. officinalis are distributed on the lower epidermis of leaves.
• Essential oil loss depends on drying temperature and moisture content level.
• Course of oil changes is adequately described by a first-order reaction kinetics model.
• Shrinkage and subsequent cell damage affects the loss of essential oil.
• Neral, geranial and citronellal decreased whereas citronellol increased at 60 °C.

The impact of hot-air drying on essential oil content and composition of lemon balm leaves (Melissa officinalis L.) was investigated at different temperatures within the range of 30 and 90 °C, constant specific humidity of 10 g kg−1 dry air and uniform air flow of 0.2 m s−1. Essential-oil reduction was determined by hydrodistillation of samples during drying and the experimental data was fitted to a first-order reaction kinetics model. The dependence of temperature on the rate constant was expressed by the Arrhenius-type relationship. The volatile compounds of the essential oil were analyzed by GC/FID. The multifunctional surface structures of leaves were also viewed by a scanning electron microscope. In all cases, most of the oil loss was observed at the beginning of the drying process and it was proportional to drying temperature. Convective drying of leaves at 30 and 45 °C resulted in 16% and 23% loss in essential oil respectively, whereas drying at higher temperatures caused significant essential oil losses, for instance 65% at 60 °C. Pronounced changes in the major essential oil components occurred at 60 °C: neral, geranial and citronellal were decreased, while citronellol indicated an increasing tendency. Micrographs showed that the oil glands are sparsely distributed on the lower epidermis of the leaf. Apart from the temperature sensitivity of the oil constituents, the loss of essential oil can also be attributed to the structural modifications caused by drying.