The spatial differences of the Chinese sauce aroma liquor Daqu and identification by FT-MIR spectroscopy

Chinese sauce aroma liquor (CSL) is one of main types of Baijiu. As the fermentation starter and raw material, CSL Daqu (CSDQ) influences the flavor formation of CSL. Owing to the spatial difference during processing, fermented CSDQ has three types and the quality of a CSDQ is inconsistent. Traditional empirical method has limitations in classification precision and standardization. In this study, the subtype was defined by the layers and types of CSDQ. Differences of CSDQs were systematically evaluated by colorimetric, chemical, and Fourier transform mid-infrared spectroscopy (FT-MIR) analysis. Linear discriminant analysis (LDA), partial least squares-discriminant analysis (PLS-DA), and k-nearest neighbor method (KNN) algorithms were used to classify the subtypes of CSDQ based on the FT-MIR spectra. Furthermore, the models were optimized, and the wavenumbers with high discrimination were marked by chemical bonds. FT-MIR LDA model achieved 95.61% accuracy after proper preprocessing. An even sampling method was proposed to represent the overall properties of a CSDQ. These findings provided a better understanding of CSDQ and a theoretical basis for promoting industrial standardization.