## 3 Results and Discussion
### Volatile Compound Analysis  
Volatile compounds play a crucial role in determining the flavor characteristics and consumer acceptance of fermented sausages. In this study, SPME-GC-MS was employed to analyze the differences in volatile compounds among the groups, and heatmaps of their relative contents, along with stacked bar charts for each compound category, were generated (Figure 4A,B). A total of 54 volatile compounds were detected, including 12 alcohols, 16 aldehydes, 6 acids, 5 ketones, 8 esters, 6 pyrazines, and 1 furan compound. Specifically, 45 compounds were detected in the control group, 44 in the FCC group, and 38 in the FNP group. Significant differences in the composition of volatile flavor compounds were observed among the groups (\(p<0.05\)), suggesting that the addition of carbon and nitrogen sources significantly affected the accumulation of volatile compounds in \(D\). _hansenit-_ fermented sausages. The relative contents of alcohols and acids were similar among the three groups, with alcohols being the most abundant volatile compounds in each group. These compounds accumulated in large quantities during the early stages of fermentation and served as precursors for key flavor compounds, such as esters and aldehydes [44]. Alcohols and acids primarily originated from yeast lipid oxidation, amino acid metabolism, and carbohydrate metabolism, with typically high thresholds, contributing to alcoholic, grassy, acidic, and cheesy aromas in fermented meat products [45]. The differences in alcohol content among the treatment groups may be attributed to the metabolic regulation characteristics of \(D\). _hansenii_. In the FCC group, due to the lack of exogenous nitrogen sources, the strain may have relied more on sugar metabolism for energy, resulting in the production of alcohols, such as isobutanol [46]. In contrast, the high levels of FAAs in the FNP group may have triggered the Ehrlich pathway, which could convert amino acids into higher alcohols [47]. For instance, phenylethanolamine, a metabolic product of phenylalanine, was significantly increased in the FNP group (\(p<0.05\)), contributing to rose and honey-like aroma characteristics. Figure 4: Abundance heatmap of volatile compounds (**A**) and stacked bar chart of relative content of each category of volatile compounds (**B**). The heatmap is color-coded from red to green, with red indicating relatively high content and green indicating relatively low content. Relatively high levels of aldehydes were observed in the FCC group (27.56%) and control group (27.33%). Aldehydes generally result from the oxidation and degradation of unsaturated fatty acids [1]. The high free fatty acid content in the control group served as a sufficient substrate for aldehyde accumulation. Although the free fatty acid content in the FCC group was lower, aldehyde intermediates might still have been generated through metabolic pathways such as sugar degradation [48]. In contrast, the aldehyde content in the FNP group was significantly lower (4.71%) than that in the other two groups (\(p<0.05\)). The observed effect may be associated with the antioxidant activity of soy protein isolate hydrolysates. According to previous studies, these hydrolysates can contribute to lipid stabilization through mechanisms such as metal ion chelation, hydrogen donation, and free radical scavenging [49]. It is worth noting that aldehydes have a low sensory threshold, and even low concentrations can impart a fresh, herbal aroma to fermented meat products, whereas higher concentrations may result in greasy or rancid odors [50]. Esters are key flavor components in fermented meat products, characterized by low sensory thresholds and playing a crucial role in enhancing aroma. They are typically formed through the esterification of alcohols and acids, and yeasts can also catalyze the formation of ethyl esters via the alcohol acetyltransferase pathway, utilizing acetyl-CoA and higher alcohols [51].