Phase transition of pectin with sorbed water

The phase-transition behaviour of pectin having various degrees of methyl esterification (DE), in the presence of sorbed water, was investigated by differential scanning calorimetry (DSC). In the first run DSC curves of pectin in the dry state, a melting endothermic peak (T-m) was observed at about 152 degrees C. In the second run DSC curves, glass transition (T-g) was observed at about 35 degrees C. The above facts indicate that the original pectin formed a regular molecular arrangement. However, amorphous glass is formed when the crystalline pectin is melted once. In pectin-water systems, the T-m of pectin decreases with increasing water content (W-c = mass of sorbed water/mass of dry pectin (g/g)). When W-c is greater than 0.4 g/g, melting of free water and the glass transition of the pectin-water system were observed. The T-g of the pectin-water system decreased with increasing W-c in the bound water region. After reaching a minimum value, T-g slightly increased and approached a constant value. Water content at the minimum T-g (W-cmin) decreased with increasing DE. This suggests that the number of water molecules required to break the hydrogen bonds between -OH and -COOH groups decreases with increasing DE. (C) 2000 Elsevier Science Ltd. All rights reserved.