Drying process of pullulan edible films forming solutions studied by low-field NMR

The dynamics of water in pullulan film-forming solutions during drying were investigated by low-field nuclear magnetic resonance. At the begin of drying, two transverse relaxation times (T2) at around 32.77 and 2149ms were attributed to bound and free waters in pullulan samples, respectively. An additional T2 value, ascribed to the tightly bound water in entanglement zones of pullulan chains, appeared at around 3.51ms as the drying process continued (beyond 1080min of drying time). The observed three relaxation times revealed the multi-exponential relaxation behavior of water in pullulan. Moreover, the polymer exhibited spatial heterogeneity with increasing drying time from 1200 to 1920min. On the basis of diffusive and chemical exchange model, the dimension range of pullulan network decreased from 7.69-32.66 to 4.73-18.14µm as the pullulan films solidified. Furthermore, the rate of chemical exchange between water and pullulan significantly increased at the later stage of drying process.