Effect of salt on the coil-helix transition of gelatin at early stages: Optical rotation, rheology and DSC studies.

The influence of the concentration and type of salt on the physical gelation of gelatin at early stages was explored by polarimeter, rheometer and differential scanning calorimetry (DSC). With the increase of salt concentration, both the triple helix content and the storage modulus of gelatin increased slightly and then decreased in CaCl2 solutions, whereas they decreased monotonically in NaCl and CrCl3 solutions. The addition of CaCl2 at low concentrations facilitated the coil-helix transition of gelatin, which could be significantly inhibited by high salt concentrations. The melting temperature of gels was nearly unchanged with varying salt concentration, but the melting enthalpy decreased monotonically with salt addition. This result indicates that salt additions mainly affect the nucleation of triple helices, but had minor effect on its growth. In addition, the coil-helix transition of gelatin in salt solutions at early stages could be treated successfully by an equation of first order kinetics.