## 3 Results and Discussion
### Changes in the Free Amino Acid Fraction of Millet Huangjiu
In contrast, His, Tyr, Met, Cys, and Phe exhibited at least a two-fold increase in the millet Huangjiu sample from the Glu-2 group. Previous research has suggested that the production of bitter peptides in sake is highly dependent on the gluten content of rice grains [8], highlighting the significance of \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Amino Acids (mg/L)** & **CK** & **Glu-1** & **Glu-2** & **Alb-1** & **Alb-2** & **Glu + Alb** \\ \hline
**Umami amino acids** & & & & & & \\ Aspartic acid & 90.4 \(\pm\) 5.4 d & 107.9 \(\pm\) 2 2 b & 135.4 \(\pm\) 5.9 a & 90.7 \(\pm\) 0.5 d & 92.1 \(\pm\) 1.2 d & 100.2 \(\pm\) 3.0 c \\ Glutamic acid & 268.2 \(\pm\) 15.9 c & 299.7 \(\pm\) 12.2 b & 332.3 \(\pm\) 18.7 a & 283.1 \(\pm\) 31.4 c & 290.3 \(\pm\) 23.8 c & 310.0 \(\pm\) 8.7 b \\ \hline
**Sweet amino acids** & & & & & & \\ Serine & 75.3 \(\pm\) 7.6 c & 67.2 \(\pm\) 0.2 c & 79.8 \(\pm\) 0.4 c & 85.1 \(\pm\) 4.5 ab & 89.2 \(\pm\) 0.4 a & 82.9 \(\pm\) 0.2 b \\ Glycine & 165.3 \(\pm\) 2.4 c & 174.7 \(\pm\) 8.5 c & 160.5 \(\pm\) 33 c & 181.6 \(\pm\) 6.8 b & 197.6 \(\pm\) 0.8 a & 193.4 \(\pm\) 2.8 a \\ Threonine & 93.1 \(\pm\) 4.6 c & 96.0 \(\pm\) 1.6 c & 91.7 \(\pm\) 1.1 c & 102.9 \(\pm\) 2.2 ab & 112.8 \(\pm\) 0.6 a & 110.9 \(\pm\) 1.2 a \\ Alanine & 195.6 \(\pm\) 11.1 c & 363.2 \(\pm\) 20.2 a & 381.2 \(\pm\) 19.0 a & 209.2 \(\pm\) 22.3 c & 251.3 \(\pm\) 2.9 b & 351.7 \(\pm\) 6.3 a \\ Proline & 155.5 \(\pm\) 9.3 d & 204.2 \(\pm\) 15.0 c & 240.7 \(\pm\) 9.7 b & 231.4 \(\pm\) 9.8 bc & 275.6 \(\pm\) 5.0 a & 217.9 \(\pm\) 4.6 c \\ Lysine & 53.2 \(\pm\) 0.9 e & 63.3 \(\pm\) 0.4 d & 82.2 \(\pm\) 1.0 b & 87.4 \(\pm\) 3.0 a & 71.6 \(\pm\) 1.3 c & 72.3 \(\pm\) 1.9 c \\ \hline
**Bitter amino acids** & & & & & & \\ Histidine & 25.8 \(\pm\) 3.2 b & 28.3 \(\pm\) 0.3 ab & 29.1 \(\pm\) 0.4 a & 22.9 \(\pm\) 0.2 c & 24.7 \(\pm\) 0.7 bc & 21.6 \(\pm\) 0.9 c \\ Arginine & 45.1 \(\pm\) 2.4 c & 54.0 \(\pm\) 10.4 bc & 99.7 \(\pm\) 0.8 a & 49.0 \(\pm\) 5.5 c & 46.6 \(\pm\) 3.1 c & 58.1 \(\pm\) 6.8 b \\ Tyrosine & 61.2 \(\pm\) 3.3 c & 77.3 \(\pm\) 2.7 bc & 87.1 \(\pm\) 1.2 a & 81.4 \(\pm\) 2.7 a & 89.3 \(\pm\) 1.6 a & 85.7 \(\pm\) 3.3 a \\ Valine & 55.5 \(\pm\) 1.7 e & 67.9 \(\pm\) 2.9 c & 95.9 \(\pm\) 1.2 a & 62.6 \(\pm\) 2.3 d & 60.4 \(\pm\) 1.1 d & 81.4 \(\pm\) 1.3 b \\ Methionine & 43.3 \(\pm\) 3.6 d & 40.8 \(\pm\) 0.2 d & 51.6 \(\pm\) 0.2 c & 35.9 \(\pm\) 0.5 e & 58.0 \(\pm\) 0.1 b & 75.6 \(\pm\) 0.4 a \\ Isoleucine & 89.9 \(\pm\) 5.8 d & 98.6 \(\pm\) 5.1 c & 178.4 \(\pm\) 0.2 a & 90.7 \(\pm\) 1.0 d & 98.4 \(\pm\) 8.4 cd & 125.9 \(\pm\) 1.4 b \\ Leucine & 131.4 \(\pm\) 28.6 bc & 154.8 \(\pm\) 3.1 b & 297.0 \(\pm\) 0.5 a & 100.0 \(\pm\) 1.9 d & 127.2 \(\pm\) 2.5 c & 165.6 \(\pm\) 8.2 b \\ Cysteine & 36.8 \(\pm\) 0.9 a & 27.9 \(\pm\) 0.7 a & 31.2 \(\pm\) 0.1 a & 36.7 \(\pm\) 0.8 a & 44.0 \(\pm\) 7.2 a & 37.4 \(\pm\) 0.1 a \\ Phenylalanine & 127.1 \(\pm\) 11.6 e & 202.1 \(\pm\) 2.8 f & 193.6 \(\pm\) 1.0 a & 85.7 \(\pm\) 2.3 d & 89.1 \(\pm\) 1.4 c & 171.7 \(\pm\) 2.7 b \\ Total & 1712.7 \(\pm\) 33.4 e & 2127.9 \(\pm\) 40.8 c & 2567.4 \(\pm\) 49.6 a & 1836.3 \(\pm\) 27.7 d & 2018.2 \(\pm\) 50.1 c & 2262.3 \(\pm\) 20.2 b \\ \hline \hline \end{tabular}  
* Values are presented as means \(\pm\) standard deviations. Different letters in the same row indicate significant differences (\(p<\) 0.05).  
\end{table} 
Table 2: The impact of different protein components on the free amino acid fraction of millet Huangjiu. protein metabolism in shaping the flavor profile of fermented beverages. The development of bitterness in Huangjiu is thought to arise from an imbalance in protein hydrolysis and peptide hydrolysis, emphasizing the importance of carefully balancing the glutelin contents in raw materials to optimize wine quality [4; 6; 9]. Taken together, these findings suggest that carefully selecting the appropriate protein type and amount during the fermentation process is essential to achieving the desired taste profile in millet Huangjiu.