Thermal characteristics of a water soluble extract obtained from pomegranate skin: Developing a state diagram for determining stability

A water-soluble extract of pomegranate peel was prepared and freeze-dried to measure its thermal characteristics as a function of moisture and temperature. Moisture contents of the freeze-dried extracts varied from 0.018 to 0.950g/g sample. A state diagram of the extract was developed by measuring the freezing curve, glass transition line and ultimate maximal-freeze-concentration conditions. The freezing point decreased with increasing solids; whereas glass transition decreased as solid levels decreased. The freezing point and glass transition were modelled using Chen's model based on the Clausius-Clapeyron equation, and Gordon-Taylor equation, respectively. The ultimate maximal-freeze-concentration conditions were (T_m')_u equal to -32.2°C and (T_g‴)_u equal to -33.4°C, and the characteristic solids content, X_s' was 0.52g/g sample (i.e. un-freezable water, X_w'=0.48g/g sample), respectively. Similarly the value of T_g' (i.e. vertical line passing through (T_m')_u and crossing the glass transition line) was determined as -76.0°C.