Effects of freeze drying on retention of essential oils, changes in glandular trichomes of lemon balm leaves

The effect of chamber pressure of a freeze dryer on essential oil contents, drying kinetics, drying characteristics lemon balm leaves and morphology of lemon balm glandular trichomes (oil reservoirs) were investigated. It was found that overall freeze drying (FD) carried out at high (FD-HP) and low pressure (FD-LP) settings consist of sublimation rate, first falling rate and second falling rate periods. Drying rate of FD-LP dried Lemon Balm leaves are higher than FD-HP dried samples, where the drying rates ranged from 0.063 to 0.449 g H2O/ g DM. s and 0.0365 to 0.395 g H2O/ g DM. s, respectively. 3rd order Polynomial model was found to be the best fit model for both drying kinetics. In terms of product quality, eight (8) major constituents of lemon balm leaves essential oil were quantified. Further to this, electro-microscope was used to observe the peltate glandular hairs structure. Product quality analysis showed that FD-HP retained higher amount of essential oil, shape of glandular hairs, but no positive effect on the freeze drying duration.