## 3 Results and Discussion
### Volatile Composition of Beverages Obtained through Static Discontinuous Fermentations I and II
#### 3.3.4 Ketones  
Ketones are well known for their impact on the aroma of most dairy products due to their low perception thresholds [70]. The formation of these compounds is mainly due to the breakdown of fatty acids, sugars, and amino acids, and not solely due to microbial activity [94]. In this study, three ketones (2,6-dimethyl-4-heptanone, 2-heptanone, and 2-nonanone) were identified in UHT whole milk. In the fermented samples from fermentation I, four, two, and four ketones were identified in the first, second, third, and fourth subcultures, respectively. During the repeated 48-h fermentation, three volatile compounds were found in the first subculture, while in the subsequent subcultures, only two ketones were detected. Among the ketones detected, 2,6-dimethyl-4-heptanone was the only compound found in both UHT whole milk and all fermented samples. The mean concentration of this ketone increased from 2.18 ug/L in milk to 4.98, 2.23, 2.52, and 3.19 ug/L in the fermented samples obtained after 24, 48, 72, and 96 h of incubation, respectively (Table 1). However, in the 48-h subcultures, relatively low concentrations (between 0.06 and 0.42 ug/L) of this compound were obtained (Table 1). In any case, the concentrations of 2,6-dimethyl-4-heptanone did not exceed its OPT of 110 ug/L [69] (Table 2). The mean concentration of 2-heptanone in UHT whole milk was 28.96 ug/L but it decreased during fermentation to mean levels of 5.52, 4.05, and 2.39 ug/L in the first, second, and fourth subcultures from fermentation I. The fermented beverages from the repeated 48-h subcultures had lower concentrations of 2-heptanone, ranging from 0.40 to 2.14 ug/L. However, only the unfermented substrate and the fermented beverage from the first subculture of fermentation I had concentrations of 2-heptanone higher than its OPT determined in milk (5 ug/L) [70], thus directly contributing to the aroma of both samples (Table 2). 2-Heptanone was also detected in milk fermented with encapsulated _Lb. casei_ cells [68], with _Lb. delbrueckii_ subsp. _bulgaricus_ and _S. thermophilus_[86], and in traditional yogurt [55]. This ketone was found in the volatile composition of commercial UHT whole and skimmed mills [90], and in mills fermented with 1:1, 1:10, 1:50, 1:100, 1:1000, and 1:10,000 mixtures of _Lb. delbrueckii_ subsp. _bulgaricus_ and _S. thermophilus_ strains [86]. The highest concentration of 2-heptanone (27.48 ug/L) was found in the milk fermented with the 1:10,000 mixture of LAB. Regarding 2-nonanone, its maximum concentration was found in the substrate (UHT whole milk) at a level of 14.26 ug/L, which is higher than its OPT determined in milk (5 ug/L) [70], making it one of the compounds contributing to the sensory profile of milk (Table 2). 2-Heptanone and 2-nonanone were found in yak milk fermented in five eco-regions of the Qinghai-Tibetan plateau [91]. When present at appropriate concentrations, these compounds are considered responsible for the natural and pleasant flavor of milk [94]. These observations indicate that 2-heptanone and 2-nonanone are commonly present in both milk and fermented dairy products [86]. However, fermentation of milk in repeated 24- or 48-h subcultures resulted in a drastic reduction in the concentrations of both 2-heptanone and 2-nonanone (Table 1). 2-Methyl-4-octanone was found in the first (1.06 ug/L), third (1.30 ug/L), and fourth (1.92 ug/L) subcultures, and 8-hydroxy-2-octanone was only detected in the fourth passage (2.06 ug/L) of the repeated 24-h fermentation. 2-Methyl-4-octanone was not detected in the second subculture, probably due to its consumption at the end of the subculture by the microorganisms of the kefir grains for the synthesis of alcohols [95]. In contrast, 5-methyl-3-heptanone was only detected in the first subculture of the repeated 48-h fermentation (Table 1), at a concentration of 0.16 ug/L, considerably lower than its OPT of 41 ug/L [45]. According to [PERSON] et al. [86], 5-methyl-3-heptanone was found (1.31 ug/L) in milk fermented with a 1:1 mixture of _Lb. delbrueckii_ subsp.