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

• We constructed the state diagram of pomegranate peel water-extract powder.
• We characterized the stability of pomegranate peel water-extract during storage.
• The stable storage temperature of peel-extract was identified.

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 0.950 g/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 (Tm′)u equal to −32.2 °C and (Tg‴)u equal to −33.4 °C, and the characteristic solids content, Xs′ was 0.52 g/g sample (i.e. un-freezable water, Xw′ = 0.48 g/g sample), respectively. Similarly the value of Tg′ (i.e. vertical line passing through (Tm′)u and crossing the glass transition line) was determined as −76.0 °C.

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