## 3 Discussion
Among them, butanoic acid and hexanoic acid significantly alter the flavor of fermented milk and increase steadily during fermentation, reaching a peak at the end. After storage, there is a temporary decrease followed by a gradual rise. Hexanoic and heptanoic acids are likely released through lipolytic activity [33]. Hexanoic acid imparts a rancid, sweet cheese-like taste, and n-decanoic acid and acetic acid are also detected. Octanoic acid, ranging from 1.57 to 14.34 \(\mathrm{\SIUnitSymbolMicro}\)/L, can contribute a soapy taste [34]. Fermented milk containing hexanoic acid, octanoic acid, decanoic acid,and other fatty acids exhibits potential probiotic properties [35]. These fatty acids are present throughout fermentation and storage, with the highest concentration observed at 0 d (S). Thus, fermented milk of _S. thermophilus_ CICC 6063 and _L. heleticus_ CICC 6064 may have probiotic potential. Aldehydes, with their lower threshold, have a significant impact on the flavor of fermented milk [36]. Among the ten aldehydes identified during fermentation and storage, only acetaldehyde was consistently present. Acetaldehyde is considered the most important flavor compound responsible for the fruity aroma in fermented milk, resembling green apple or nutty flavors [37]. The choice of starter cultures and pH level influence acetaldehyde production, with higher levels observed at 0 d (S) and 14 d (S) due to increased acidity [37; 38]. Other aldehydes, like hexanal, are transient and easily converted into acidic compounds or alcohols in fermented milk [17]. The levels of hexanal detected in this study were minimal at both 4 h (F) and 21 d (S). Straight-chain aldehydes such as hexanal, heptanal, and nonanal, derived from the oxidation of unsaturated fatty acids in milk fat, contribute to the grassy and herbaceous aromas in fermented milk [39]. Ketones are formed through the thermal degradation of amino acids, the oxidation of unsaturated fatty acids, and the Maillard reaction. These compounds have a significant impact on the aroma of dairy products due to their low perception thresholds [40]. In this study, 11 ketones were identified, including acetoin, 2,3-butanedione, 2,3-pentanedione, 2-heptanone, and 2-nononone, which are important flavor components. Acetoin, known for its slightly creamy, slightly sweet, buttery flavor, when combined with diacetyl, gives sour milk a mild and pleasant buttery taste [41]. 2,3-Butanedione (diacetyl), responsible for the flavor of sour butter, is derived from the fermentation of citric acid [42]. It has been detected at levels ranging from 32.44 to 135.28 \(\mathrm{\SIUnitSymbolMicro g}\)/L during fermentation and storage. The production of 2,3-butanedione and 2,3-pentanedione is a result of the chemical decarboxylation of their precursors, 2-acetolate and 2-aceto-2-hydroxybutyrate. Acetoin, 2,3-butanedione, and 2,3-pentanedione, which are considered important odor compounds in yogurt, showed the highest levels in the samples, consistent with the findings of this study [43]. The stability of 2, 3-pentanedione and acetoin remains unchanged throughout fermentation and storage, as shown in Figure 5, indicating a potential enzymatic reduction of acetoin from 2, 3-pentanedione by diacetyl reductase [44]. However, it is important to note that this particular result has not been documented in the current literature. Figure 5: Metabolic pathway prediction and concentration changes in discriminatory volatile metabolites in this study. Alcohols are the final byproducts of amino acid metabolism and glucose degradation in yogurts. The presence of alcohols in fermented milk has been associated with lactose metabolism, reduction of methyl ketones, and amino acid metabolism [45]. A comprehensive analysis found 11 different alcohols in the samples, with 1-heptanol being the only one detected during late storage. The presence of 1-heptanol significantly altered the flavor profile of the sample, as it is considered an important flavor component in fermented milk [46].