Structure evolution of pullulan-alginate edible films during drying studied by low-field NMR

The evolutions of spatial structure for pullulan, alginate, and pullulan-alginate blend aqueous solutions during drying were investigated by low-field nuclear magnetic resonance. For pullulan solutions, during the first 1,320min of drying, NMR signal intensity in profiles did not vary with position. Thereafter, the apparent shift of the air-pullulan interface began at 1,440min, indicating the onset of progressive shrinkage of pullulan samples. Similar shrinkage phenomena were observed for alginate and pullulan-alginate blend solutions. In contrast, spatial structural heterogeneity was observed for alginate solution during drying time from 1,200 to 1,560min, due to the formation of a skin layer near the air-alginate interface. Based on the change of polymer solutions during drying, two-stage evaporation process was detected in the aqueous pullulan, alginate, and their blend solutions. Moreover, the evaporation rate of water during the second stage of drying was significantly lower than that of the first stage. Practical applicationsEdible films have emerged as an alternative to synthetic petroleum-based polymers for food packaging. The evolutions of spatial structure of pullulan-alginate solutions during drying were studied by monitoring of their NMR profiles. Information shown in this study would provide some scientific basis for studying film-forming mechanism of edible films and their applications in the food field.