Evaluation of optical properties of tofu samples produced with different coagulation temperatures and times using near-infrared transmission spectroscopy

• Absorption and scattering of tofu were evaluated by near-infrared spectroscopy. • Tofu samples were produced with different coagulation temperatures and times. • Scattering significantly increased with coagulation temperature and time. • Viscosity and microstructural properties of tofu had correlations to scattering. • Scattering increase was faster at shorter wavelengths with coagulation time. To investigate the feasibility of using near-infrared (NIR) spectroscopy to evaluate the coagulation extent of tofu, the optical properties of tofu samples obtained with different coagulation temperatures and times were measured in the wavelength range 1300–1800 nm. The apparent absorbance was measured using the NIR transmission spectrum, and then divided into absorption and scattering components by curve fitting. The apparent absorbance increased over the entire wavelength range as the coagulation temperature increased, in which the scattering increase was dominant over that of the absorption. The viscosity was measured as an index of the physical properties of tofu, which increased with the coagulation temperature and had a high correlation ( R 2 = 0.803) with one of the fitting parameters of the scattering. The microstructure of the tofu became finer as the coagulation temperature increased, which suggests that the increase in scattering might be due to changes in the microstructure. Furthermore, the light scattering increased with the coagulation time as well as with the coagulation temperature. The increase in scattering with the coagulation time followed the first-order reaction kinetics. Both scattering intensity and increase speed were larger at shorter wavelengths. This can be explained by the theory of Mie scattering and multiple scattering, indicating that scattering measurements at shorter wavelengths are useful for monitoring of coagulation. Thus, light scattering measurements using NIR spectroscopy are promising for investigating the physical properties of tofu.