## 3 Results and Analysis
### Comparison of GC-MS and GC-IMS Results
& 122.93 \(\pm\) 14.0b & 13.00 \(\pm\) 25.6b & 12.78 \(\pm\) 235.8b \\
218 & 110-19-0 & Isobular culture & MS & 17.60 \(\pm\) 28.56. & 980 \(\pm\) 95.9b & 0 & 8.38 \(\pm\) 94.2c \\
219 & 140-91-4 & Isobular culture & MS & 9.62 \(\pm\) 3.12. & 68.20 \(\pm\) 92.4 & 0 & 0 \\
220 & 547-64-8 & 1-Diversity-3-phenyliveness & MS & 19.66 \(\pm\) 22.72. & 0 & 1398.11 \(\pm\) 117.2b & 2.30 \(\pm\) 5.54b \\
221 & 574-64-3 & Empty Intake & MS & 48.43 \(\pm\) 35.55 & 306.21 \(\pm\) 102.2 & 52.18 \(\pm\) 61.10b & 96.82 \(\pm\) 122.3b \\
222 & 5359-65 & ethyl23-phenyl Immune & MS & 30.12 \(\pm\) 30.353 & 104.55 \(\pm\) 17.2 & 6.42 \(\pm\) 115.5b & 6.24 \(\pm\) 115.5b \\
223 & 5310-16-9 & Isobular culture & MS & 27.34 \(\pm\) 30.211 & 6.45 \(\pm\) 135.94 & 7.40 \(\pm\) 69.09b & 1.46 \(\pm\) 2.64c \\
224 & 1034-67-7 & Friendly service & MS & 0 & 730.99 \(\pm\) 95.2b & 320.00 \(\pm\) 844.5b & 1088.00 \(\pm\) 504.4a \\  
4 & S-2 methyl ether acid & MS & 43.80 \(\pm\Sourness is the main component of the flavor of koumiss. The relative content of caprylic acid, capric acid, 9-decenoic acid, capric acid, and acetic acid is relatively high in koumiss samples from different regions. Among them, medium-chain fatty acids caprylic acid, capric acid, 9-decenoic acid, and capric acid account for 97% of the total volatile acids, giving koumiss a pungent, sour taste and a faint odor, similar to the results of [PERSON]'s research [25]. This may be due to the abundant free fatty acids in mare's milk or the breakdown of fats by microorganisms. Compared with cereal wines, which are also fermented by lactic acid bacteria and yeast, the fatty acids in koumiss are mostly oleic acid and linoleic acid [26], different from the metabolism and breakdown of fatty acids in fermented dairy products. The acetic acid in koumiss is sour but not astringent, formed by the metabolism of lactose in mare's milk by some \(\beta\)-galactosidase-producing microorganisms [27]. The total volatile acid content of the YL and ALTe regions is significantly higher than that of the TC and ALTwo regions (see Figure 5). Due to the special process of fermenting koumiss in the YL region, the machine stirring method was used in the fermentation process (Table 1 and Figure 1). Compared with hand-stirring fermentation, mechanical stirring fermentation extends the stirring time, so that microorganisms do not settle at the bottom and obtain more oxygen, which can fully ferment and produce volatile acid compounds. However, the TC region also used mechanical stirring to ferment koumiss. Still, the content of volatile acid compounds is significantly lower than that of other regions, which may be related to ester volatile compounds. The esters of volatile substances are important flavor compounds in koumiss due to their very low threshold. Esters make up an average of 63% of the total content of koumiss, and impart a buttery, fruity, and floral aroma to it, helping to reduce the off-flavors of fatty acids and miscellaneous alcohols in koumiss [28]. Therefore, this class of compounds plays an important role in forming the characteristic aroma of koumiss. Most of the esters in koumiss are produced by two pathways. The first pathway involves the esterification of short-chain and medium-chain fatty acids, free fatty acids, and primary and secondary alcohols produced during lactose fermentation or amino acid metabolism in koumiss, and the second pathway involves the formation of esters by alcohol acyltransferases produced by microorganisms from alcohols and acyl-CoA [29; 30]. As shown in Figure 5, the total content of esters of volatile substances in the YL and TC regions is significantly higher than that in the ALT region, which may be due to the diversity of fermentation agents and the special nature of the process used in the fermentation of koumiss in these regions. The activity and enzymatic activity of yeast in the YL and TC regions are enhanced by the  
Figure 5: Changes in the total content of alcohols, acids, esters, aldehydes, and ketones in koumiss from different regions.