## 3 Results and Discussion
### Comparative Analysis of Volatile Compounds in YCSP, YMMP, and YEMP Samples  
We further classified and statistically analyzed the identified volatile compounds. The results are shown in Figure 2. Venn diagrams can reflect the distribution relationships between different sample data and are often used in the field of food flavor research to compare the distribution of volatile compounds among different samples [23; 29]. According to the Venn diagram results, the numbers of volatile compounds detected in the YCSP, YMMP, and YEMP samples were 36, 30, and 27, respectively. The unique volatile compounds in the YCSP, YMMP, and YEMP samples were six, seven, and four, respectively, while 14 volatile compounds were shared among the three yak milk powder samples. Additionally, excluding the volatile compounds shared by all three samples, 11 volatile compounds were common between the YCSP and YEMP samples, five were shared between the YCSP and YMMP samples, and only one volatile compound was shared between the YEMP and YMMP samples (Figure 2A). This indicated that the types of volatile compounds in the YCSP and YEMP samples were more similar, while there was a greater difference in the types of volatile compounds between these two samples and the YMMP sample. The 48 volatile compounds identified from the three yak milk powder samples were mainly classified into six categories: alcohols, ethers, acids, ketones, esters, and alkanes. As shown in Figure 2B, the main volatile compounds in the three yak milk powder samples were ethers and alkanes. Additionally, it was evident that the types and contents of volatile compounds in the YCSP and YEMP samples were quite similar, while the YMMP sample contained higher levels of acids and esters and lower levels of alkanes and alcohols. This result corroborated the findings from the Venn diagram, suggesting that the compound composition in the YCSP and YEMP samples was more similar, whereas the YMMP sample had a markedly different composition. The variations in volatile compounds in yak milk from different lactation periods have significant biochemical implications. For instance, compared to YCSP, YMMP contains higher concentrations of esters and aldehydes, which may reflect changes in the fat metabolism process of the milk or be linked to microbial activity and enzyme expression. These changes directly influence the flavor and nutritional composition of the milk. Additionally, the changes in volatile compounds are closely related to the nutritional components of yak milk. For example, the increase in aldehydes and alcohols in YMMP may be associated with higher fat content, as these compounds are typically generated during the fat breakdown process. Figure 2: Venn diagram of identified compounds in YCSP, YMMP, and YEMP samples (**A**) and stacked bar chart of the relative content of various compound types (**B**). Hierarchical clustering is a commonly used method in clustering analysis that replaces the relationships between individual samples with the distances between them, gradually clustering samples that are close in distance into a group step-by-step to complete the clustering process [30; 31]. Combining hierarchical clustering with heatmaps allows for a clear visualization of the data distribution between sample datasets [32]. Therefore, we further conducted hierarchical clustering heatmap analysis of the 48 volatile compounds identified from the three yak milk powder samples, and the results are shown in Figure 3.  
The results in Figure 3 indicated that the relative contents of volatile compounds varied among the different yak milk powder samples, with the distributions of volatile compounds in the YCSP and YEMP samples being more similar, while those in the YMMP  
Figure 3: Hierarchical clustering heatmap of volatile compounds in YCSP, YMMP, and YEMP samples. The horizontal axis represents the sample names, and the vertical axis represents the detected volatile compounds.