## 3 Results and Discussion
### Volatile Composition of Beverages Obtained through Static Discontinuous Fermentations I and II
#### 3.3.1 Organic Acids  
Carboxylic acids in fermented milk, like kefir, generally originate from lipolysis, proteolysis, or lactose fermentation. These compounds are responsible for the acidity and contribute to the flavor and aroma of the kefir beverages [48; 70], and act as precursors for a variety of aromatic compounds including alcohols, esters, methyl ketones, and others [48]. Both hexanoic acid and 2-ethyl hexanoic acid were found in all fermented samples, although at levels lower than their corresponding olfactory perception threshold (OPT) values (3000 and 27,000 ug/L, respectively). Hexanoic acid was the organic acid detected at the highest concentrations, ranging between 7.63 and 23.45 ug/L in the 24-h subcultures, and between 3.85 and 7.82 ug/L in the subcultures of fermentation II. The highest concentration of 2-ethyl hexanoic acid (2.61 \(\pm\) 0.63 ug/L) was found in the third subculture of fermentation II (Table 1). Acetic acid was not detected in the unfermented whole milk or in the fermented beverage from the first 24-h subculture. Considering each fermentation independently, it can be noted that the highest concentrations of this compound were found in the fermented beverages obtained from the third subcultures of fermentations I and II, with values of 2.69 \(\pm\) 0.83 ug/L and 3.38 \(\pm\) 0.23 ug/L, respectively (Table 1). However, these levels are considerably lower than the OPT for this acid (99,000 ug/L) [45] and those (between 7.89 and 24.65 ug/L) detected by [PERSON] et al. [85; 86] in millks fermented with different proportional combinations of _Lb. delbrueckii_ subsp. _bulgarcius_ and _Strep. thermophilus_. This suggests that the fermented milk beverages obtained in this study will not have unpleasant notes of vinegar or sourness [46; 47]. Concentrations of butanoic acid considerably lower than its OPT (1400 ug/L) [45] were obtained in the fermented samples from the first and third subcultures of fermentation I, as well as in all beverages from fermentation II. This indicates that this compound alone will not influence the aroma of the fermented beverages. Four organic acids were exclusively detected in the 24-h subcultures, as follows: pentanoic, octanoic, ethyl boronic, and 8-methyl-6-nonenoic acid. Pentanoic acid and octanoic acid were only found in fermentation I, although at concentrations lower than the OPT (3000 ug/L) for these compounds [45]. Pentanoic acid was present solely in the third (0.76 ug/L) and fourth (0.87 ug/L) subcultures, while octanoic acid was detected in the fourth subculture (1.69 ug/L). Similarly, ethyl boronic acid (0.59 \(\pm\) 0.14 ug/L) and 8-methyl-6-nonenoic acid (0.71 ug/L) were only identified in the third subculture of fermentation I. In contrast, two organic acids were only detected in the repeated batch fermentations of 48 h: 2-methyl-propanoic acid in the last three subcultures at concentrations between 0.64 and 2.02 ug/L (<OPT: 6550.5 ug/L) [45], and trans-2-undecenoic acid (1.54 ug/L) in the second subculture (Table 1). Heptanoic acid was detected in the second subculture of fermentation I at a mean concentration of 0.92 ug/L (second subculture) and gradually decreased to 0.78 ug/L at 96 h (Table 1). This organic acid was also detected in the first, second, and third subcultures of fermentation II at concentrations of 0.20, 0.23, and 0.31 ug/L, respectively, which were lower than those found in the 24-h subcultures. All these concentrations are below its OPT (640 ug/L) [45]. Nonanoic acid was detected in the unfermented substrate (4.89 ug/L < OPT: 8800 ug/L [45]), but its concentration decreased in the second and third subcultures of the repeated batch