## 3 Results and Discussion
### Tolerance Features of YHM-G  
3-Met not only has outstanding flavor characteristics, which endow specific flavor characteristics to traditionally brewed food products, such as _Baijiu_, but also offers the healthy function of antioxidant activity [7; 37]. Therefore, increasing the content of 3-Met in traditionally brewed foods, such as _Baijiu_, will help to improve their quality. Strain YHM-G has been identified as a yeast with a high yield of 3-Met preliminarily screened from the _Baijiu_ production environment. Whether it can be applied to the _Baijiu_ production to improve its 3-Met content depends on its biological characteristics because _Baijiu_ production takes place in a constantly changing microenvironment. Only the strain YHM-G was found to offer excellent environmental tolerance and survivability for a long time in the _Baijiu_ production process, allowing it to produce 3-Met and improve _Baijiu_ quality. Therefore, we analyzed the adaptability of strain YHM-G to the environment regarding its range of growth temperatures and pH and tolerance to sugar, NaCl, ethanol and 3-Met (Figure 1). The optimum growth temperature of strain YHM-G was 25 \({}^{\circ}\)C, similar to most yeasts found in previous reports, with a maximum growth temperature of 40 \({}^{\circ}\)C (Figure 1a), which was higher than most temperatures (20-40 \({}^{\circ}\)C) during the _Baijiu_ production process [38; 39; 40; 41; 42]. Therefore, strain YHM-G can adapt well to the different temperatures used in _Baijiu_ production. Strain YHM-G was able to grow between pH 2 and pH 12 (Figure 1a), a range wider than previously reported [42]. Its optimum pH for growth was weak acidic, at pH 4-7 (Figure 1a). This range is within the range of pH used in the _Baijiu_-production process [38; 39; 40]. As the glucose concentration, sucrose concentration and NaCl concentration increased the growth rate of strain YHM-G declined. When the concentration of glucose, sucrose and NaCl exceeded 47%, 47% and 17%, respectively, strain YHM-G could not grow (Figure 1b). In general, yeast strains have a high tolerance to sugar with strains, such as _Pichia kudriavzzevii_ YF1702 and _Clavispora lusitaniae_ YX3307 [25; 43; 44; 45; 46]. Additionally, like other microorganisms, yeasts have a limited tolerance to NaCl, a concentration of 15% preventing the growth of most microorganisms [42; 47]. Compared with most other reports on yeast, strain YHM-G had a high tolerance to NaCl, so it could better adapt to different environments [47; 48]. However, there are a few yeasts with a high tolerance of up to 30% NaCl screened from particular environments, possibly related to the activation of antioxidant enzymes in the antagonistic yeast, including catalase, thioredoxin reductase, glutathione peroxidase and glutathione reductase [49; 50]. As the ethanol concentration increased, the biomass concentration of strain YHM-G increased first and then decreased. At a concentration of \(\leq\)6%, ethanol could be used as a carbon source for the growth of strain YHM-G, consistent with the previous physiological and biochemical results (Figure 1b and Supplementary Table S2). At an ethanol concentration of \(\geq\)6%, the growth of strain YHM-G was inhibited because of the influence of ethanol on the cell structure and enzyme activity of the strain (Figure 1b) [25; 51]. The ethanol concentration was 12% at maximum growth (Figure 1b), a value much higher than that produced during _Baijiu_ production processes (2-4%), thus strain YHM-G can be considered a yeast with high ethanol tolerance [51; 52]. The ability of a strain to tolerate 3-Met determines its level of 3-Met production. The results showed that strain YHM-G could grow at a 3-Met level of 40 g/L (Figure 1c), similar to that of _S. cerevisiae_ CEN.PK113-7D, indicating that strain YHM-G has the potential to produce more 3-Met [53]. Overall, strain YHM-G can adapt well to different environments for the traditional brewing foods, such as _Baijiu_.