Moisture sorption isotherms and glass transition temperature of elecampe (Inula helenium L.) and burdock (Arctium lappa L.) roots at 25°C

Sorption isotherms of elecampe (Inula helenium L.) and burdock (Arctium lappa L.) root samples were obtained at 25 degrees C. Elecampe exhibited hysteresis loop in the range of 0.35-0.90 a(w), whereas burdock roots showed significant differences between adsorption and desorption isotherms from 0.65 to 0.80 a(w). Blahovec-Yanniotis was considered to give the best fit over the whole range of a(w) tested. Various parameters describing the properties of sorbed water derived from GAB, Henderson and Blahovec-Yanniotis models have been discussed. Differential scanning calorimetric method was used to measure the glass transition temperature (T-g) of root samples in relation to water activity. The safe moisture content was determined in 12.01 and 14.96 g/100 g d. b. for burdock and elecampe root samples at 25 degrees C, respectively. Combining the T-g line with sorption isotherm in one plot, it was found that the glass transition temperature concept overestimated the temperature stability for both root samples.