Drying process of pullulan edible films forming solutions studied by ATR-FTIR with two-dimensional correlation spectroscopy

Drying phenomenon of aqueous pullulan solutions at 50 °C was investigated by ATR-FTIR spectroscopy and two-dimensional correlation infrared spectroscopy. Two-dimensional asynchronous spectrum at 1800–1500 cm−1 wavenumber was resolved into three separate bands, which were assigned to aggregated water with strong hydrogen bond (AW-S), aggregated water with moderately hydrogen (AW-M) bond, and free water (FW), respectively. Curve fitting results indicated that the quantities of the three species of water were constant during the first 55 min of drying process. However, the area of the three bands (at around 1047, 1022 and 998 cm−1) varied as drying progressed, implying the formation of more-ordered pullulan chains structure, as well as increased pullulan chain–chain interactions. Further analysis using 2D asynchronous correlation spectroscopy in 1200–950 cm−1 wavenumber regions revealed the sequence of spectral changes during the drying process.