## 3 Results and Discussion
### Analysis of the Aromatic Profile of the Fermented Milks  
In UHT whole milk, the volatile compounds that had an odor activity value (OAV) \(>\) 1.0 were 2-heptanone (OAV = 5.79) and 2-nonanone (OAV = 2.85), so they will contribute a greater impact to the odor of this substrate. Regarding fermentations I and II, it could be noted that first fermentation provided the fermented beverages with the highest OAV values due to the aforementioned decrease in the concentrations of volatile compounds in fermentation II. Ethyl hexanoate was the volatile compound with the highest OAV in the fermented beverages (except the fourth beverage from fermentation II). Thus, the OAV calculated for this compound in the four fermented beverages from the first, second, third, and fourth subcultures of fermentation I were 36.47, 34.07, 19.74, and 64.05, respectively. The OAV calculated for the four fermented beverages from the first, second, and third subcultures of fermentation II were 8.10, 2.36, and 2.52, respectively. The volatile compounds that presented an OAV \(>\) 1.0 in different subcultures were isoamyl acetate, in beverages from the first (1.29), third (1.01), and fourth (1.89) subcultures, and ethyl octanoate in beverages from the first (1.59) and second (1.56) passages in fermentation I. In contrast, other compounds only exhibited OAV \(>\) 1.0 in one subculture: 2-heptanone (OAV = 1.10) in the first subculture, E-2-nonenal (OAV = 2.01) in the second subculture, and nonanal (OAV = 1.46) in the third subculture of fermentation I. Another compound with an OAV \(>\) 1.0 was ethyl isobutanoate (OAV = 2.60), which was detected in the second subculture of fermentation II (Tables 1 and 2). Figures 4 and 5 show the OAVs calculated for whole milk and the four subcultures of fermentations I and II (separated for better analysis) based on their classification into seven aromatic series (microbiological, fruity, floral, herbaceous, solvent, animal fatty, and sweet). Figure 4: Spider plots of the total OAVs of the odor descriptors of UHT whole milk and kefir beverages from the four consecutive static subcultures of fermentation I (I-24 h, I-48 h, I-72 h, and I-96 h). The aromatic series which made the greatest contribution to whole milk were the fruity, floral, and microbiological series. In the first subculture of fermentation I, the fruity was the most intense series (with an intensity 4.7 times greater than in whole milk), followed by the microbiological and floral series, but both with a lower magnitude than in whole milk (upper left part of Figure 4). In the second, third, and fourth beverages from fermentation I, the fruity series predominated with an intensity of 4.2, 2.6, and 7.6 times greater, respectively, than in whole milk. In the second and third subcultures, the fruity series was followed by the microbiological and herbaceous series, but in both cases, with less intensity than the fruity series. In the beverage from the fourth subculture, only the fruity series predominated (lower right part of Figure 4). The results obtained for fermentation II (Figure 5) indicate that the fruity series predominated in the beverage from the first subculture with an intensity slightly higher than in whole milk (upper left part of Figure 5), while in the subsequent subcultures (48-96, 96-144, and 144-192 h), this series progressively lost intensity with the increase in the number of passages. However, in the second subculture (96 h), the herbaceous series was also important, although with a smaller contribution than the fruity series. In contrast, in the last subculture, the intensity of the seven aromatic series was considerably lower than in unfermented milk, as the OAV of all volatile compounds in this subculture was below 1.0. These observations suggest, once again, that fermentation time significantly influences the sensory profile of kefir beverages.