The states of water in glutinous rice flour characterized by interpreting desorption isotherm.

Abstract

Water content of glutinous rice flour were determined after equilibrium at water activity (aw) of 0.06-0.98 and temperature of 10, 20 and 30°C. Distribution of water in different states and its evolution with aw were characterized using four composite models. Interactions of water molecules with solid matrix and themselves were further evaluated. The Park model was a more realistic and mechanism-based approach for describing water desorption of glutinous rice flour. Increased equilibrium water induced by lowering temperature existed mostly as strongly bound water with only a few parts as weakly bound water. The water-polymer thermodynamic incompatibility predominated the water mobility, and resulted in a rapid decrease of diffusion coefficient at aw>~0.7. Water diffusivity behavior with aw suggested water clustering at high aw levels. The Zimm-Lundberg theory, Park model and Brown analysis all revealed that critical aw of water clustering was of 0.81-0.85, depending on temperature, but gave inconsistent prediction about mean cluster size.