Rapid measurement of sake mash using differential near-infrared spectra of diffuse transmission and transflectance

Currently, various sake brewers employ data-driven approaches to ensure the stable production and supply of high-quality products. This study aimed to develop a novel direct measurement system using near-infrared (NIR) spectroscopy to monitor the fermentation process of sake mash during brewing. Direct measurement of sake mash has been challenging because of the significant absorption bands of water and the impact of physical properties such as multiple scattering within the mash. To address these challenges, we propose the subtraction of spectra, called differential reflectance, obtained through two measurement methods, namely, diffuse reflection and transflectance, to reduce the impact of physical properties. This approach includes limiting the wavelength range from 400 to 1300 nm, utilizing the second derivative in the Savitzky–Golay method, and applying multiplicative scatter correction (MSC) to the entire wavelength. When partial least squares regression (PLS-R) was applied, the root mean square error for predicting (RMSEP) alcohol concentration in the model sake mash sample (14.65–18.41 % v/v) was 0.33 % v/v. Finally, as a practical experiment, alcohol concentrations in the actual mash (0.00–17.49 % v/v) were predicted with differential reflectance spectra by MSC. The resulting RMSEP value was 0.76 % v/v, a significant improvement of 1.99 % v/v predicted by diffuse reflectance. These findings demonstrate the effectiveness of the proposed spectral subtraction method as a new direct measurement approach for monitoring sake fermentation in sake mash.