Effect of convective-, vacuum- and freeze drying on sorption behaviour and bioactive compounds of lemon balm (Melissa officinalis L.)

Abstract

The influence of drying method on the moisture sorption isotherms of lemon balm leaves (Melissa officinalis L.) was investigated at 25 and 50°C using the hygrometric method. Equilibrium relative humidity (aw) in the head space of the samples was measured at several moisture contents (0.04≤Xe≤0.45kg/kg dry basis) developed by convective-, vacuum- and freeze drying. The essential oil constituents were determined by GC/FID, while rosmarinic acid was analysed by HPLC. The experimental sorption data was modelled by the modified Halsey equation. The isosteric heat of desorption (Qst) was calculated as a function of moisture content and temperature directly by the model in which the Clausius–Clapeyron equation has been incorporated. The different sorption characteristics were ascribed to modifications in the matrix structure caused by the mode of drying. Sublimation of water vapour during freeze drying resulted in a higher sorption capacity for 0.1≤aw<0.8. Qst increased as moisture content and temperature decreased, increasing sharply for Xe below 0.2kg/kg. Qst of freeze dried leaves at Xe of 0.10kg/kg was significantly higher than that of vacuum- and air dried material. Geranial content was higher after freeze- or convective drying, while vacuum drying led to lower contents of neral and geranial. Freeze- and vacuum dried samples exhibited greater rosmarinic acid content than the hot-air dried leaves of M. officinalis.