Water Activity Measurement Using Shortwave Infrared (SWIR) Spectroscopy

We propose a new noninvasive and in situ method for evaluating water activity (Aw) in hydrated materials using shortwave infrared (SWIR) spectroscopy, enabling high-spatial-resolution analysis. In this approach, the hydrogen-bonding energy of water molecules, derived from SWIR spectra, is used as an enthalpy-related descriptor of the water state. Using homogeneous sugar solutions, the relationship between the hydrogen-bonding energy difference and the chemical potential difference was established and described by linear and quadratic models, reflecting the emergence of nonlinearity at larger hydrogen-bonding energy differences. These empirical relationships were further applied to foodstuffs and hydrogels, enabling estimation of Aw within the experimental uncertainty for approximately Aw > 0.878 (linear) and Aw > 0.675 (quadratic). In contrast, electrolyte solutions exhibited systematic deviations from these relationships, indicating additional thermodynamic contributions not captured by the hydrogen-bonding energy of water alone. This method enables in situ, spatially resolved evaluation of water activity in heterogeneous systems at high spatial resolution, offering a practical approach to characterize water states in complex hydrated materials.