## 4 Discussion  
This study aimed to explore the effects of different heat treatment on milk VOCs and to achieve this goal through E-nose, E-tongue, and SPME-GC-MS. Milk is rich in nutrients such as fat, protein, and lactose, which are broken down into fatty acids, amino acids, etc., after different processing methods, and these are the precursor substances of VOCs which determine the flavor of milk [11]. The degree to which milk is heated affects the formation of furfurals [26]. In this study, furfural was found in the 135 \({}^{\circ}\)C-treated milk at a concentration of 2.55 \(\pm\) 0.59 \(\mu\)g/L (Table 4), but not in the 65 \({}^{\circ}\)C-treated milk and raw milk. Many studies have gradually confirmed that furfural is a product of milk after heating, and  
Figure 7: Correlation analysis between thermal parameters and VOCs (\({}^{**}\)\(p<0.01\), \({}^{*}\)\(p<0.05\)). a recent study showed that an amount of 15 mg/kg furfural in pasteurized milk treated at 70 \({}^{\circ}\)C for 15 s [27]. Furfural itself exhibits a burnt, roasted taste. In this study, the formation of furfural also causes the bitterness in milk to become the main taste expression in 135 \({}^{\circ}\)C-treated milk. At the same time, studies have shown that bitterness is related to the hydrolysis of whey protein. YGLF, IPAVF, LLF, and YPFPGPIPN are amino acid sequences of bitter peptides, and a high degree of hydrolysis often has increases bitterness intensity [28; 29; 30]. [PERSON]'s research found that skim milk with a higher serum protein content had stronger sulfur and eggy flavor and higher sulfur compounds [31]. [PERSON]'s research shows that ultra-high-temperature instantaneously sterilized milk may have a longer shelf life than pasteurized milk, but it produces a more sulfurous and eggy taste, which may be unacceptable to consumers [32]. The present study also confirmed this theory that furfural, immunoglobulin G, \(\beta\)-lactoglobulin, lactoferrin, and \(\alpha\)-lactalbumin showed a highly significant negative correlation (Figure 7). The milk's bitterness is more pronounced at higher levels of protein hydrolysis. After heat treatment, lipids will produce a large number of methyl ketones, which are also products of milk treated at high temperatures. Additionally, milk heated at 135 \({}^{\circ}\)C for 5 s undergoes a more thorough Maillard reaction with a higher concentration of Maillard products and methyl ketones than milk heated at 75 \({}^{\circ}\)C for 15 s [15]. [PERSON] showed that differences in flavor formation caused by the Maillard reaction and protein denaturation during hot processing affected the flavor quality of super pasteurized milk and resulted in significant flavor differences between direct steam injection and indirect heating of milk [2]. In this study, the maximum amount of methyl ketones was found in 135 \({}^{\circ}\)C-treated milk, with an amount of 13.75 \(\pm\) 1.79 \(\mu\)g/L, 5.72 \(\pm\) 0.75 \(\mu\)g/L, and 2.43 \(\pm\) 0.42 \(\mu\)g/L 2-heptanone, 2-nonanone, and 2-undecanone, respectively. Although the methyl ketone compounds can reflect the heating degree of milk to some extent, the content of methyl ketone compounds, such as 2-heptanone, will decrease with the extension of milk storage time after high-temperature treatment [33]. Methyl ketones have been shown to have a positive correlation with the amount of harmful lactosyl lysine molecules in heat-treated milk, making it an effective method for determining the quality of milk [16]. Acid compounds comprise a large proportion of milk, with 13 VOCs, and are important in forming milk odor. The increase in milk processing temperature leads to a gradual decrease in the total amount of acid compounds (Figure 4). The content of lactone compounds is also among the important factors affecting milk flavor, and milk flavor is generally negatively correlated with the content of lactone [34].