A study of casein hydration by the thermally stimulated depolarisation currents method

The dielectric behaviour of bound water in compressed casein pellets has been studied using the thermally stimulated depolarisation currents method over the temperature range from 77 to 300 K and the hydration range from 1 to 24% w/w. Two distinct relaxation regions are obtained: one in the range of T from 100 to 170 K attributed to the reorientation of loosely bound water molecules, and a second one in the range 190-290 K interpreted mainly in terms of water-assisted relaxation of the protein polar groups. The fact that both relaxations are very markedly influenced by the presence of water for hydration contents, h, higher than 9%, strongly suggests that all water molecules sorbed up to this value are irrotationally bound to the protein macromolecules. Comparison with results obtained by other investigators by a variety of methods, as well as calculation of the primary sorption sites at the molecular level, give further support to the above interpretation.