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

- At the beginning of drying, two T2 values were observed at about 32.77-31.45 and 2154-1417 ms, respectively.
- During the second and third drying stages, an additional T2 value appeared with a relaxation time of around 3.51 ms.
- Estimated dimension range of pullulan network decreased from 7.69-32.66 to 4.73-18.14 µm as the drying time increased.
- Rate of chemical exchange values remained constant about 0.227 × 103 s−1 with increasing drying time from 1200 to 1440 min.
- A significantly increase in rate of chemical exchange value up to 5.817 × 103 s−1 was observed at 1800 min.

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 2149 ms 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 3.51 ms as the drying process continued (beyond 1080 min 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 1920 min. On the basis of diffusive and chemical exchange model, the dimension range of pullulan network decreased from 7.69-32.66 to 4.73-18.14 µm as the pullulan films solidified. Furthermore, the rate of chemical exchange between water and pullulan significantly increased at the later stage of drying process.