The equilibrium moisture contents (MC) of leaves and stems of lemon balm (Melissa officinalis L.) were
determined separately at temperatures of 25, 35 and 45 C over a stepwise increase of relative humidity
(RH) ranging from 3 to 90% by an automatic, gravimetric analyzer (DVS system). Equilibrium was achieved
within 6 h for most of the target values of relative humidity. The equilibrium moisture content of
leaves was significantly higher than that of stems (p < 0.05). Differences in moisture sorption capacity
between the leaves and stems can be attributed to chemical composition and structure of the tissues.
Five three-parameter moisture sorption models (modifications of ChungePfost, GAB, Halsey, Henderson
and Oswin) were tested for their effectiveness to fit the experimental sorption data. The modified Oswin
equation was found to be the best model to describe the adsorption isotherms of both leaves and stems
of lemon balm. The recommended MC values of leaves and stems for microbial safe storage at 25 C were
0.124 and 0.113 kg water per kg dry solids, respectively. The net isosteric heat of sorption was computed
from the predicted sorption data by applying the integrated form of the ClausiuseClapeyron equation