## 3 Results and Discussion
### Changes in the Free Amino Acid Fraction of Millet Huangjiu  
Millet Huangjiu is renowned for its distinctive taste and aroma, which can be attributed, in part, to its high amino acid content. Amino acids are not only essential for yeast metabolism and growth during the fermentation process, but also serve as precursors for a range of flavor and aroma compounds [16]. Analysis of millet Huangjiu using a high-speed amino acid analyzer revealed the presence of 17 different free amino acids (Table 2). Among these, Glu was found to be the most abundant in the control group, with a concentration of 268.2 mg/L, followed by Ala and Pro, with levels of 195.6 mg/L and 155.5 mg/L, respectively. In contrast, the lowest concentration of free amino acids was detected for His, with only 25.8 mg/L observed in the control group. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **CK** & **Glu-1** & **Glu-2** & **Alb-1** & **Alb-2** & **Glu + Alb** \\ \hline Total soluble solids (\({}^{\circ}\)Bx) & 11.80 \(\pm\) 0.19 d & 14.10 \(\pm\) 0.33 b & 15.6 \(\pm\) 0.20 a & 13.2 \(\pm\) 0.11 c & 12.5 \(\pm\) 0.08 d & 14.6 \(\pm\) 0.41 b \\ Amino acid nitrogen (g/L) & 0.65 \(\pm\) 0.04 d & 0.85 \(\pm\) 0.03 b & 0.92 \(\pm\) 0.05 a & 0.72 \(\pm\) 0.02 c & 0.76 \(\pm\) 0.06 c & 0.87 \(\pm\) 0.07 b \\ Reducing sugar content (g/L) & 7.62 \(\pm\) 0.15 d & 8.89 \(\pm\) 0.23 c & 9.54 \(\pm\) 0.28 b & 10.10 \(\pm\) 0.17 a & 8.15 \(\pm\) 0.33 c & 8.43 \(\pm\) 0.26 c \\ Ethanol (\% vol) & 12.40 \(\pm\) 0.11 c & 13.50 \(\pm\) 0.08 b & 14.60 \(\pm\) 0.16 a & 12.80 \(\pm\) 0.27 c & 12.50 \(\pm\) 0.04 c & 13.30 \(\pm\) 0.22 b \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The impact of different protein components on the enological parameters of millet Huangjiu. Table 2 revealed that the 17 identified amino acids in millet Huangjiu were classified based on their taste presentation characteristics into umami, sweet, and bitter tasting. The results showed that Glu, Leu, Phe, Gly, Ala, and Pro were the most abundant amino acids, with concentrations exceeding 100 mg/L, accounting for over 60% of the total amino acid contents. The predominance of these amino acids in Huangjiu may contribute to its unique taste and flavor. The production of free amino acids during fermentation occurs mainly through enzymatic degradation of proteins in the raw material by microorganisms that secrete proteases, as well as by intracellular proteases following the autolysis of microorganisms [17]. This process can lead to the release of amino acids from proteins, resulting in their accumulation in the fermented product [18]. The results indicate that the Glu-1, Glu-2, Alb-1, Alb-2, and Glu + Alb groups showed a significant increase in total amino acid content, ranging from 7.2% to 49.9%, with the highest level reaching up to 2567.4 mg/L in the Glu-2 group. Specifically, the umami amino acids Asp and Glu exhibited the most pronounced increase, with contents rising to 135.4 mg/L and 332.3 mg/L, respectively. In contrast, the addition of albumin (Alb-1 and Alb-2) showed no effect on the umami amino acid contents. The most abundant sweet-tasting amino acid, Ala, was almost twice as high in the Glu-1 compared to the control group. Interestingly, the extra addition of gluten (Glu-2) did not further increase the concentrations of Ala in millet Huangjiu, indicating a saturation effect. In contrast, the levels of the sweet amino acids Ser, Gly, and Thr showed a dissimilar variation tendency, with the addition of gluten having no effect on their contents, whereas a slight increase in their contents, ranging from 9.9% to 21.1%, was observed in the Alb-1, Alb-2, and Glu + Alb groups. These results demonstrate that variation in the contents of sweet amino acids in millet Huangjiu depends heavily on the type and amount of protein added to the fermentation substrate. The addition of albumin to millet Huangjiu resulted in minor changes in the levels of bitter-tasting amino acids, including Arg, Val, Ile, and Leu.