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

Water content of glutinous rice flour were determined after equilibrium at water activity (a_w) of 0.06-0.98 and temperature of 10, 20 and 30 °C. Distribution of water in different states and its evolution with a_w 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 a_w > ~0.7. Water diffusivity behavior with a_w suggested water clustering at high a_w levels. The Zimm-Lundberg theory, Park model and Brown analysis all revealed that critical a_w of water clustering was of 0.81-0.85, depending on temperature, but gave inconsistent prediction about mean cluster size.