Evaluation of the state of water at different aging stages of filtered soy sauce by a new method using infrared/far-infrared spectra in the 4 000–100 cm<sup>-1</sup> region

This study evaluated the state of water in filtered soy sauce produced using whole and defatted soybeans at different aging stages through a novel spectroscopic approach using spectra in the 4 000–100 cm-1 region. Two indices, A3 210/(A3 210+AM3 374) and A673/(A673+A403), were developed using absorbance at 3210 and 3374 cm-1 and at 673 and 403 cm-1, respectively. The A3 210/(A3 210+A3 374) values in the middle and last stages were lower than those in the initial stage, and the A673/(A673+A403) value showed a slightly increased trend among stages. These results suggest that the localized hydrogen bonding of water molecules may have been disrupted, which at a macroscopic level may have enhanced hydrogen bonding network formation due to interactions with coexisting components. The two-dimensional plot of A3 210/(A3 210+A3 374) against A673/(A673+A403) demonstrated that filtered soy sauce of different ages could be evaluated through the simultaneous analysis of the local water environment and the macroscopic water structure.