## 3 Results and Discussion
### Dynamic Changes in Volatile Flavor Compounds
Comparing the data before and after fermentation, 23 aldehyde compounds exhibited notable changes. The content of 10-Undecenal increased from 1.15 \(\upmu\)g/L before fermentation to 15.20 \(\upmu\)g/L after fermentation, (E,E)-2,4-Nonadienal increased from 10.71 \(\upmu\)g/L to 59.63 \(\upmu\)g/L, and 2,5-Dimethylbenzaldehyde increased from 21.22 \(\upmu\)g/L to 118.21 \(\upmu\)g/L. These significant increases indicate the microbial transformation and utilization of precursor substances in Jerusalem artichoke juice during fermentation. In contrast, Benzeneacetaldehyde decreased from 98.40 \(\upmu\)g/L to 36.35 \(\upmu\)g/L, suggesting a different metabolic pathway or consumption during the fermentation process. The fermentation process led to an overall increase in aldehyde content from 865.39 \(\upmu\)g/L to 1172.78 \(\upmu\)g/L, highlighting its role in enhancing the flavor complexity and aroma of Jerusalem artichoke juice. During the spontaneous fermentation of Jerusalem artichoke juice, acid compounds also showed significant changes, demonstrating the impact of microbial metabolic activity and diversity on flavor substances. Analyzing the changes in five major acid compounds provides deeper insights into their contributions to post-fermutation flavor. Hexanoic acid increased from 0.23 \(\upmu\)g/L before fermentation to 177.36 \(\upmu\)g/L after fermentation, with its rOAV increasing from 0 to 0.06. This significant increase could be due to yeast breaking down fatty acids during fermentation. Hexanoic acid, a common fatty acid with strong fatty and fruity aromas, is often found in fermented beverages. 9-Decenoic acid increased from 0.64 \(\upmu\)g/L to 43.76 \(\upmu\)g/L. This acid, known for its unique spicy and fruity aromas, may result from the microbial conversion of unsaturated fatty acids. 4-Aminobutanoic acid (\(\gamma\)-aminobutyric acid) increased from 17.57 \(\upmu\)g/L to 78.55 \(\upmu\)g/L. \(\gamma\)-Aminobutyric acid, an important neurotransmitter with multiple health benefits, is possibly produced through microbial amino acid metabolism, significantly enhancing the product's health value and flavor. The total acid content increased from 33.27 \(\upmu\)g/L to 382.24 \(\upmu\)g/L, not only enhancing the sourness and overall flavor, but also potentially improving the product's antioxidant activity and health benefits. Ketone compounds showed notable changes during fermentation, highlighting microbial metabolism's role in flavor development. Analyzing the changes in 24 ketone compounds, we found that Acetophenone increased from 1.28 \(\upmu\)g/L before fermentation to 86.86 \(\upmu\)g/L after fermentation, with its contribution to flavor becoming more significant. 5-Ethyl-3-Hydroxy-4-Methyl-2(5H)-Furanone increased significantly from 4.13 \(\upmu\)g/L to 303.74 \(\upmu\)g/L, a highly significant increase, with strong caramel and fruity aromas forming an important aroma substance during fermentation. 2-Methylcyclohexanone increased from 3.56 \(\upmu\)g/L to 41.88 \(\upmu\)g/L, reflecting its role in the post-fermutation flavor profile. 1-Nonen-3-one increased from 8.76 \(\upmu\)g/L to 216.16 \(\upmu\)g/L, contributing its unique metallic and sweet aromas, which are commonly found in fruits and vegetables. 4-Undecanone increased from 28.48 \(\upmu\)g/L to 170.27 \(\upmu\)g/L, further enhancing the flavor profile. In contrast, 3-Methyl-4-Heptanone and 6-Methyl-3.5-Heptadien-2-one decreased from 170.38 \(\upmu\)g/L to 72.88 \(\upmu\)g/L and from 214.76 \(\upmu\)g/L to 167.15 \(\upmu\)g/L, respectively. Overall, most ketone compounds significantly increased during fermentation, with the total ketone content rising from 926.95 \(\upmu\)g/L to 2195.82 \(\upmu\)g/L. This significant increase mainly results from the microbial conversion and metabolic activity of precursor substances in Jerusalem artichoke juice during fermentation.