## 3 Results and Discussion
### Comparison of Volatile Organic Compounds in Regular Raw Milk and DHA-Biofortified Raw Milk  
The VOCs identified and their contents in the two types of raw milk are shown in Table 3. The flavor of raw milk is complex and is determined by a variety of volatile compounds with aroma activity, which predominantly include aldehydes, ketones, esters, acids, and sulfur-containing compounds. The sensory characteristics of raw milk largely depend on the relative balance of flavor compounds from fats, proteins, or carbohydrates [9]. One of the primary mechanisms for generating flavor elements is through the hydrolysis or oxidation of fatty acids in milk fat, thereby making milk fat a key source of raw milk aroma. Lipid oxidation products, such as aldehydes and ketones, can impart a rancid and \"oxidized\" flavor to dairy products [24]. The oxidation process initially targets the polyunsaturatedated phospholipids within the milk lipid globule membrane then progresses to triacylglycerols, where unsaturated fatty acids undergo self-oxidation to form aldehydes. Linoleic and linolenic acids are further oxidized to generate propionaldehyde and hexanal. Hexanal was identified in both types of raw milk, with a higher concentration being observed in the regular raw milk than that in the DHA-biofortified raw milk, which could be due to the lower linolenic acid C18:3 content in DHA-biofortified raw milk than in regular raw milk. Acetone and 2-butanone are typically present in small quantities in raw milk, primarily originating from the metabolic processes of dairy cows. Acetone offers a fruity aroma, while the sensory profile of 2-butanone resembles that of acetone [25]. 1-Octene-3-ol is generated through the degradation of unsaturated fatty acids. The concentration of 1-octene-3-ol in the DHA-biofortified raw milk was higher than that in regular raw milk, which is consistent with the results of fatty acid levels. The total content \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline  & \multicolumn{2}{c}{**16–100 d**} & \multicolumn{2}{c}{**101–200 d**} & \multicolumn{2}{c}{**201–305 d**} \\ \hline \multirow{2}{*}{**Lactation Stage**} & \multirow{2}{*}{**Native Raw Milk**} & \multicolumn{2}{c}{**DHA- Biofortified Raw Milk**} & \multicolumn{2}{c}{**Native Raw Milk**} & \multicolumn{2}{c}{**DHA- Biofortified Raw Milk**} & \multicolumn{2}{c}{**Native Raw Milk**} \\  & & \multicolumn{2}{c}{**Biortified Raw Milk**} & \multicolumn{2}{c}{**Raw Milk**} & \multicolumn{2}{c}{**Biortified Raw Milk**} & \multicolumn{2}{c}{**Raw Milk**} \\ \hline SFA & 73.5 & 72.01 & 72.69 & 73.69 & 71.24 & 72.95 \\ \hline UFA & 26.5 & 27.99 & 27.31 & 26.31 & 28.76 & 27.05 \\ \hline MUFA & 23.1 & 23.81 & 24.21 & 21.43 & 24.51 & 22.3 \\ \hline PUFA & 3.4 & 4.18 & 3.1 & 4.88 & 4.25 & 4.75 \\ \hline n6:n3 & 28.95 & 7.09 & 28.57 & 5.95 & 29.55 & 6.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Proportions of fatty acid contents (%) in regular raw milk and DHA-biofortified raw milk. of unsaturated fatty acids in the DHA-biofortified raw milk was higher than that in regular raw milk. Esters are the principal constituents of neutral volatile compounds found in various types of animal milk. Esters identified in both types of raw milk contribute to the fruity and sweet flavor profile of the milk. The content of esters in the DHA-biofortified raw milk exceeded that in the regular raw milk due to variations in the feeding regimen of the two groups of dairy cattle. Acids are generated through metabolic processes, such as lipolysis and microbial fermentation. Unsaturated fatty acids undergo oxidation to form aldehydes, which are further oxidized into acids. The concentration of unsaturated fatty acids in DHA-biofortified raw milk has been shown to be higher than that in regular raw milk [26]. Sulfur compounds are unique substances found in raw milk, with dimethyl sulfide, carbon disulfide, dimethyl carbon disulfide, and dimethyl sulfone being the main compounds [27].