## 4 Discussion  
The growth of LAB and yeast populations is influenced by their diverse ability to utilize sugars and amino acids, and their tolerance to various stresses, including acidity and high concentrations of ethanol [27]. In the late stage of fermentation, where the ethanol content in fermented grains exceeds 10% and the pH can drop as low as 3.5, strains that are unable to tolerate these conditions will cease to grow or even die. Therefore, ethanol and pH tolerance are critical indicators for screening functional bacteria in liquor fermentation. To assess the strain's adaptability to the fermentation environment, its ethanol and pH tolerance were tested. Light-flavor _Baijiu_ is characterized by the presence of ethyl acetate and ethyl lactate. Yeast strains are capable of producing esters, particularly ethyl acetate. LAB have the ability to produce lactic acid and form ethyl lactate through the action of esterases [13; 28; 29]. In our study, the Lbr8 and Lbr9 isolates showed ethanol tolerance. Some strains of _L. buchneri_ and _L. hilgardii_ (Lbu3 and Lhi46) showed lower ethanol tolerance abilities than _L. brevis_, but still exhibited tolerance to 12% ethanol. Generally, _L. brevis_, _L. buchneri_, and _L. hilgardii_ demonstrated slightly higher ethanol tolerance compared to other LAB strains. This suggests that these three species may be important constituents of the microbial community during the late stage of fermentation. The ethanol tolerance of bacteria is influenced by several factors, including ethanol-induced changes in plasma membrane composition and inactivation of cytosolic enzymes (such as ATPase and glycolytic enzymes) [30]. [PERSON] et al. reported that increased stress response and fatty acid biosynthesis, along with decreased amino acid transport and metabolism, may play important roles in strains' adaptation to environmental ethanol [31]. Some _L. brevis_ strains (Lbr8 and Lbr9) showed a high ethanol tolerance, possibly linked to the formation of cell macro-fibers and structured filamentous growth in response to ethanol stress [32]. LAB and yeasts are regarded as functional microbiota responsible for alcohol and flavor compound production. In our study, 4 out of 50 LAB strains and 5 out of 26 yeasts showed superior environmental tolerance compared to others. Enzyme activity was then determined for these selected LAB and yeast. Only one strain of LAB (Lbr9) out of the four strains with better tolerance exhibited preferable enzyme activity. Since esterase has a crucial role in the formation of flavor compounds, Lbr17 was selected based on its high esterase activity. As a result, two LAB (Lbr9, Lbr17) were chosen for the fermentation test. For yeasts, the activity of enzymes in five strains with good tolerance was poor. Therefore, Sf6, which displayed good activity in three enzymes, was selected as the yeast strain. Considering the important role of esterase in the formation of flavor compounds, Sc12 was chosen for its excellent esterase activity. Finally, two LAB (Lbr9 and Lbr17) and two yeasts (Sf6 and Sc12) were selected for the fermentation test, and the volatile compounds in the final fermented grains were analyzed using HS-SPME-GC-MS.  
PCA was conducted on fourteen main flavor compounds in _Baijiu_ (Figure 5). By combining the score plot (Figure 3A) and the loading plot (Figure 3B), it can be observed that the fortified fermentation group with Lbr17 exhibited the greatest distance from the origin in the direction of ethyl acetate contribution. Compared to other fortified fermentation groups, the fortified fermentation with Lbr17 displayed the closest distance with CK, indicating that the flavor substances of the two groups were the most similar. Therefore, Lbr17 was finally selected to carry out the fortified inoculation fermentation to study the effect of fortified inoculation with Lbr17 on the fermentation process. _L. brevis_ is probably the most important LAB species during fermentation, as it has the highest esterase activity (Figure 1).