## 3 Results and Discussion
### Changes in VOCs in millet Huangjiu
#### 3.4.2 Alcohols  
Alcohols are the major compounds found in fermented millet wine, which are synthesized via diverse metabolic routes, including amino acid metabolism, reduction of methyl ketones, glycolysis, and degradation of linoleic and linoleic acid [8]. As shown in Table 3, a total of 13 alcohols were identified in millet Huangjiu, with phenylethyl alcohol, 2-methyl-1-propanol, and 3-methyl-1-butanol being the most prevalent, accounting for more than 80% of the total alcohol content detected (Figure 4). This finding is consistent with a previous study by [PERSON] et al. [10]. The volatile component analysis of fermented rice Huangjiu conducted by [PERSON] et al. also showed that 3-methyl-1-butanol and 2-methyl-1-propanol were the predominant alcohols, accounting for over 60% of the total alcohol content [13]. These three fused alcohols have been demonstrated to be characteristic flavor compounds in fermented alcoholic beverages, contributing to the overall sensory complexity of millet Huangjiu [25,26].  
Figure 4: Changes in the concentrations of the predominant volatiles upon addition of exogenous proteins. Different letters indicate statistically significant differences (\(p<0.05\)).  
Phenylethyl alcohol, derived from the degradation of phenylalanine [27], was identified as the most abundant fused alcohol in millet Huangjiu in the present study. This compound occurs naturally in various fermented beverages, including beer, wine, and whisky, and is well-known for its rose-like odor note [28]. In addition, it has also been identified as an odor-active compound in makgeolli [26]. Our study revealed that the addition of glutelin significantly increased the production of phenylethyl alcohol, with concentrations increasing from 70.4 mg/L (control group) to 91.5 mg/L (Glu-2 group) (Figure 4). This increase is likely due to the observed accumulation of phenylalanine (Table 2), which is the precursor of phenylethyl alcohol during the fermentation process. In line with our findings, recent studies have also reported a positive correlation between the levels of phenylethyl alcohol and phenylalanine ([PERSON] et al.) [29; 30].  
Likewise, the levels of 3-methyl-1-butanol and 2-methyl-1-propanol in the Glu groups were significantly higher, with increases of approximately 24.2-60.2% compared to the control group (Figure 4). These higher alcohols are known to contribute fruity, winey, and whisky-like notes to the wine [9], thereby enhancing the richness of the taste. In contrast, the levels of these alcohols in the Alb-1 and Alb-2 groups did not differ significantly from those in the control group. These findings suggest that fortifying the fermentation substrate with glutelin protein can enhance the accumulation of amino acids during millet Huangjiu production and significantly increase the production of alcohol odorants, ultimately improving the sensory properties of millet Huangjiu. It should be noted, however, that the concentration of these compounds should be carefully considered during production to avoid undesirable flavors, such as solvent-like aromas. While alcohol volatiles can contribute to the desirable aroma of millet Huangjiu, excessive amounts greater than their critical values can negatively affect its flavor profile [31; 32].