## 3 Results and Discussion
### Analysis of Characteristic Volatile Compounds  
In the OPLS-DA model analysis, we obtained the variable importance in projection (VIP) scores for each volatile compound, as shown in Figure 6. From the figure, it can be observed that 26 volatile compounds have VIP values greater than 1 (indicated by the red bars in the Figure 6). VIP provides a significant reference value for identifying characteristic volatile compounds in samples. Generally, compounds with VIP > 1 are considered characteristic volatile compounds [40; 41]. We selected 21 volatile compounds with VIP > 1 and \(p\) < 0.05, as shown in Table 2. These compounds include Pentadecane, 2-methyl-; Tetradecane, 6,9-dimethyl-; Pentadecane, 2,6,10-trimethyl-; 1-Decanol; Heptadecane; Hexadecane; Ethanol, 2-phenoxy-; Phenylmethanol; Caproic acid; Sulfurous acid, dodecyl 2-ethylhexyl ester; Butyric acid; Dodecane; Undecane, 2,6-dimethyl-; Adipic acid, di(but-2-en-1-yl) ester; Pentadecane, 3-methyl-; Dimethyl sulphone; Pentadecane; Dimethyl ether; Acetic acid; Propanoic acid; 2-Hydroxy-2-methyl-; ethyl ester; and 2-Hydroxy-2-methyl-4-pentanone. Figure 6: Variable importance in projection (VIP) results for 18 volatile compounds. Red bars highlight volatile compounds with VIP > 1. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Number**} & **Retention Time** & **Volatile Compounds** & **CAS ID** & **Yak Colostrum** & **Yak Mature** & **Yak Ending** & \(p\) & **VIP** \\  & **(min)** & & & **Powder** & **Milk Powder** & **Milk Powder** & **Value** & **Value** \\ \hline
1 & 20.659 & Pentadecane, 2-methyl- & 1560-93-6 & 221 \(\pm\) 96 a & ND & ND & 0.003 & 1.281 \\
2 & 20.284 & Tetradecane, 6,9-dimethyl- & 55045-13-1 & 326 \(\pm\) 119 a & ND & ND & 0.002 & 1.298 \\
3 & 22.278 & Pentadecane, 2,6,10-trimethyl- & 3892-00-0 & 327 \(\pm\) 123 a & ND & \(329\pm 11\) a & 0.002 & 1.256 \\
4 & 29.899 & 1-Decanol & 112-30-1 & 22.5 \(\pm\) 7.9 a & ND & \(309.67\) a & 0.002 & 1.021 \\
5 & 23.939 & Heptadecane & 629-7-37 & 392 \(\pm\) 107 a & \(23.84\) b & \(433\pm 3\) a & 0.001 & 1.354 \\
6 & 21.709 & Hexadecane & 544-76-3 & 1841 \(\pm\) 518 a & \(220\pm 28\) a & \(1766\pm 80\) a & 0.001 & 1.3 \\
7 & 33.497 & Ethanol, 2-phenoxy- & 1229-96 & 122 \(\pm\) 167 a & \(102\pm 11\) b & \(1117\pm 155\) a & 0 & 1.36 \\
8 & 28.331 & phenylmethanol & 100-51-6 & 402 \(\pm\) 8.4 a & ND & ND & 0 & 1.333 \\
9 & 27.683 & capric acid & 142-62-1 & ND & \(234\pm 13\) a & ND & 0 & 1.305 \\
10 & 22.243 & Sulfutrous acid, dodecyl 2-methyl-eyerl ester & 100309-19-5 & ND & \(36.55\pm 31\) a & ND & 0 & 1.304 \\
11 & 22.997 & bultry acid & 107-92-6 & ND & \(302\pm 23\) a & ND & 0 & 1.304 \\
12 & 10.962 & dodecane & 112-40-3 & ND & \(159\pm 10\) a & ND & 0 & 1.303 \\
13 & 22.249 & Undencane, 2,6-dimethyl- & 17301-24-3 & ND & \(352\pm 26\) a & ND & 0 & 1.303 \\
14 & 35.313 & Adipic acid, dibut-2-in-\(1\)/\(\dagger\)) ester & 1003024-71-1 & ND & \(14.62\pm 21\) a & ND & 0 & 1.298 \\
15 & 20.896 & Pentadecane, 3-methyl- & 2882-86-4 & 201 \(\pm\) 46 a & \(35.44\pm 07\) b & \(213\pm 17\) a & 0 & 1.23 \\
16 & 29.134 & dimethyl sulphone & 67-71-0 & 355 \(\pm\) 33 r b & \(271\pm 22\) a & ND & 0 & 1.201 \\
17 & 19.281 & pentadecane & 629-62-9 & 2330 \(\pm\) 513 a & \(416\pm 38\) b & \(2715\pm 173\) a & 0 & 1.149 \\
18 & 4.267 & Dimethyl ether & 115-16-6 & 12,703 \(\pm\) 1558 a & \(3614\pm 259\) b & \(15599\pm 334\) a & 0 & 1.087 \\
19 & 18.83 & acetic acid & 64-19-7 & 133 \(\pm\) 39 b & ND & \(182\pm 8\) a & 0 & 1.045 \\
20 & 11.645 & Propanoic acid, 2-hydroxy-2-methyl-, ethyl & 80-55-7 & 227 \(\pm\) 61 b & ND & \(321\pm 56\) a & 0 & 1.012 \\
21 & 16.279 & 2-hydroxy-2-methyl-4-pentanone & 123-42-2 & 156 \(\pm\) 27 a & \(12.3\pm 13\) b & \(220\pm 4\) a & 0 & 1.01 \\ \hline \hline \end{tabular} ND = not detected. The values labeled with different lowercase letters indicate significant differences (\(p<0.05\))  
\end{table} 
Table 2: Information on characteristic volatile compounds in YCSP, YMMP, and YEMP samples (VIP \(>1\) and \(p<0.05\)).