## 3 Results and Discussion  
### Identification and Quantitative Analysis of Volatile Compounds in YCSP, YMMP, and YEMP  
The nutritional content of yak milk powder obtained by freeze-drying yak milk collected during different lactation periods varied, and the differences in nutritional components led to distinct flavors and sensory characteristics of the yak milk powder. The characteristics of volatile compounds in yak milk could also reflect its quality and determine the degree of consumer acceptance and preference [13]. To compare and analyze the differences in volatile compounds among yak milk powders collected at different lactation periods, we used HS-SPME-GC-MS to analyze the volatile compounds in YCSP, YMMP, and YEMP samples. The total ion chromatograms of the three yak milk powder samples are shown in Figure 1.  
Table 1 shows the information on the volatile compounds identified in the YCSP, YMMP, and YEMP samples. A total of 48 volatile compounds were identified across the three yak milk powder samples, and the content of each volatile compound varied significantly among the samples. All three samples contained higher levels of pentadecane, hexadecane, and dimethyl ether. Notably, the YCSP milk powder sample contained large amounts of 6,9-dimethyl-tetradecane, 2-phenoxy-ethanol, and heptadecane. The YMMP milk powder sample had higher levels of butyric acid and caprotic acid, while the YEMP milk powder sample contained more toluene and 3-ethoxy-3-methyl-2-butanone. Previous studies analyzed the flavor changes in yak milk under different storage temperatures and times using HS-SPME-GC-MS and identified 23 volatile compounds (retention match score above 600) from yak milk [13]. Additionally, our previous research used headspace gas chromatography-ion mobility spectrometry (HS-GC-IMS) to determine the changes in volatile organic compounds in yak milk powder under different drying methods, identifying 17 volatile compounds [27]. Similarly, we detected volatile organic compounds in yak milk powder under different sterilization conditions using HS-GC-IMS and identified only 17 volatile compounds [28]. Compared to the HS-GC-IMS method, the majority of the compounds identified in this study were identified for the first time. Clearly, our current study identified a greater variety of volatile compounds, which may be attributed to the more comprehensive compound coverage in the database we used. Figure 1: Total ion chromatograms (TIC) of YCSP, YMMP, and YEMP samples. The X-axis represents the collection time (min), and the Y-axis represents the signal intensity. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Number** & **Retention Time** & **Volatile Compounds** & **Formula** & **CAS ID** & **Yak Colostrum** & **Yak Mature Milk** & **Yak Ending Milk** \\  & **(min)** & & & & & & & \\  & **Volatile Compounds** & **Formula** & **CAS ID** & **Pawder** & **Pawder** & **Pawder** \\ \hline
1 & 4.267 & Dimethyl ether & C\({}_{2}\)H\({}_{\rm O}\) & 115-10-6 & 12,703 \(\pm\) 1538 a & 3614 \(\pm\) 259 b & 15,599 \(\pm\) 334 a \\
2 & 6.307 & toluene & C\({}_{2}\)H\({}_{8}\) & 108-88-3 & 830 \(\pm\) 182 b & ND & 1610 \(\pm\) 34 a \\
3 & 9.419 & butyl propionate & C\({}_{2}\)H\({}_{4}\) & 590-01-2 & ND & ND & 912 \(\pm\) 14.5 a \\
4 & 10.962 & dodecane & C\({}_{2}\)H\({}_{8}\) & 112-40-3 & ND & 159 \(\pm\) 10 a & ND \\
5 & 11.645 & Propanoic acid, 2-hydroxy-2-methyl-, ethyl-, ethyl ester & C\({}_{4}\)H\({}_{2}\)O\({}_{3}\) & 80-55-7 & 227 \(\pm\) 61 b & ND & 321 \(\pm\) 56 a \\
6 & 12.803 & styrene & C\({}_{2}\)H\({}_{8}\) & 100-42-5 & 168 \(\pm\) 38 b & ND & 283 \(\pm\) 8 a \\
7 & 13.565 & 2-Butanone, 3-ethoxy-3-methyl-, C\({}_{2}\)H\({}_{4}\)O\({}_{2}\) & 3668-79-97. & ND & ND & 344 \(\pm\) 11 a \\
8 & 16.279 & 2-hydroxy-2-methyl-4-pentanone & C\({}_{2}\)H\({}_{2}\)O\({}_{2}\) & 123-42-2 & 156 \(\pm\) 27 a & 123 \(\pm\) 13 b & 220 \(\pm\) 4 a \\
9 & 16.662 & tetradeacac & C\({}_{4}\)H\({}_{8}\) & 62-59-4 & 162 \(\pm\) 55 b & 127 \(\pm\) 6 c & 221 \(\pm\) 21 a \\
10 & 17.324 & Propanoic acid, 2-methyl-,