## 3 Results and Discussion
### Changes in Organic Acids in Millet Huangjiu  
Organic acids play a crucial role in the formation of the flavor and physiological properties of millet Huangjiu. Their presence in appropriate amounts contributes to a harmonized and stabilized taste and aroma, thereby rendering the Huangjiu more palatable [19]. As depicted in Figure 1, the identified organic acids included oxalic acid, tartaric acid, pyruvate acid, malic acid, lactic acid, citric acid, succinic acid, acetic acid, and fumaric acid. The predominant organic acids detected in the control group were succinic acid (45.9%), lactic acid (25.8%), acetic acid (13.6%), and citric acid (11.1%), which accounted for 96.4% of the total organic acid content. This finding is comparable to the results reported by [PERSON] et al. [10]. Pyruvic acid and fumaric acid exhibited the lowest contents, fluctuating around 8-9 mg/L. The concentrations of pyruvate acid and fumaric acid were observed to be relatively low, with fluctuations within the range of 8-9 mg/L. Tartaric acid and malic acid were present in moderate concentrations, at 153.6 mg/L and 102.4 mg/L, respectively.  
The impacts of glutelin and albumin addition on the contents of the primary organic acids, lactic acid and succinic acid, are illustrated in Figure 1E,G. The results showed that  
Figure 1: Concentrations of organic acids in millet Huangjiu upon addition of exogenous proteins. Different letters indicate statistically significant differences (\(p<0.05\)). (**A**) oxalic acid, (**B**) tartaric acid, (**C**) pyruvate acid, (**D**) malic acid, (**E**) lactic acid, (**F**) citric acid, (**G**) succinic acid, (**H**) acetic acid, and (**I**) fumaric acid.  
the levels of lactic acid and succinic acid were significantly increased after the addition of proteins prior to the fermentation process. The extent of these increases was positively correlated with the amount of proteins added, with the highest levels recorded in the Glu-2 group at 3914.0 mg/L and 5585.2 mg/L, respectively. Interestingly, the impact of glutenlin and albumin on the production of lactic acid and succinic acid was not uniform. The addition of glutenlin resulted in a more significant increase in both amino acids compared to albumin, with higher concentrations recorded in the Glu-1 group at 3130.1 mg/L and 4943.7 mg/L, respectively, compared to 2893.8 mg/L and 4433.3 mg/L in the Alb-1 group. A similar disparity was also noted between the Glu-2 and Alb-2 groups. Succinic acid is not only an intermediate metabolite in the tricarboxylic acid cycle (TCA), but also a precursor to other metabolic pathways, with a distinctive umami taste [20]. These results suggest that adding an appropriate amount of glutenlin and albumin to the fermentation substrate can enhance the umami taste in millet Huangjiu, thereby mitigating the pungent sour taste resulting from an increase in acetic acid levels (Figure 1H) [21].  
In contrast to the marked variations observed in the contents of succinic and citric acids, the levels of minor amino acid components, namely, pyruvate acid and fumaric acid, remained unaltered upon treatment with glutenlin and albumin (Figure 1C,I). It is worth noting that pyruvate and fumaric acids are intermediate metabolites of glycolysis and the citric acid cycle [14]. Under aerobic conditions, pyruvate acid acts as a major energy source for microbial cells via the citric acid cycle. However, under anaerobic conditions, such as alcoholic fermentation, pyruvate acid is converted to ethanol [22]. Since pyruvate acid is not a final product of the fermentation process, the low accumulation of pyruvate acid in the millet Huangjiu samples indicates that the yeast cells have sufficient glycolytic and citric acid cycle activities [18]. This finding suggests that the fortification of protein content in the fermentation substrate with glutenlin and albumin has little adverse impact on the Huangjiu production process.