## 3 Results and Discussion
### Volatile Composition of Beverages Obtained through Static Discontinuous Fermentations I and II
#### 3.3.3 Aldehydes  
Aldehydes are also an important group of volatile compounds that were identified as responsible for the formation of the characteristic aromatic profile of dairy products by [PERSON] et al. [68]. These researchers observed that the aldehydes developed during milk fermentation with encapsulated strains of _Lb. casei_, a bacterium present in kefir grains, improved the aroma of the final product. In this regard, the researchers [68] highlighted the production of 2-nonenal (8.4 ug/kg) and nonanal (16.8 ug/kg), two compounds also detected in the fermented samples of the second and third subcultures of the repeated 24-h fermentation, respectively. However, the production of both aldehydes was not detected in the repeated 48-h fermentation (Table 1). In this study, no aldehydes were detected in the milk (Table 1). However, the total number of aldehydes increased from the first to the third subculture and then decreased in the fourth subculture of fermentations I and II. The concentrations of aldehydes increased from the first to the third subculture (although not always significantly) and exhibited a significant decrease (\(p<0.05\)) in the fourth subculture of fermentation I. In fermentation II, the concentration of aldehydes increased significantly (\(p<0.05\)) from the first to the third subculture, but the slight decrease in the fourth subculture compared to the third was not significant (\(p>0.05\)). The decrease in the production of volatile aldehydes in the fourth subcultures of fermentation I and II could be related to a reduction in the metabolic activity of the microorganisms of the kefir grains, possibly linked to the decreased consumption of nutrients (L, TN, and Pr) and the production of fermentation metabolites (LA, AA, EtOH, and TAA) (Figures 1 and 2). A comparison between fermentations I and II shows that the total number of aldehydes in the first three subcultures of fermentation I was higher than in the corresponding subcultures of fermentation II. However, in the fourth subculture of fermentations I and II, two and three aldehydes were detected, respectively. Additionally, the concentrations of aldehydes in all subcultures of fermentation I were always significantly higher (\(p<0.05\)) than in fermentation II (Table 1). In addition to 2-nonenal and nonanal, eight other aldehydes were identified in the fermented samples from fermentation I (Table 1). These include (Z)-14-methyl-8-hexadecenal (24-48 h, 48-72 h, and 72-96 h subcultures), 10-undecenal (0-24 h subculture), 2,6-dimethylbenzaldehyde (0-24 h subculture), 2-ethyl hexanal (24-48 h and 48-72 h subcultures), 2-methyl-undecanal (24-48 h, and 48-72 h subcultures), benzaldehyde (0-24 h, 24-48 h, and 48-72 h subcultures), dodecanal (48-72 h and 72-96 h subcultures), and phenylacetaldehyde (24-48 h and 48-72 h subcultures). In this study, only the following two aldehydes were detected at concentrations higher than their corresponding OPT: (E)-2-monenal and nonanal. The first aldehyde was detected in the second subculture at a mean concentration of 0.84 ug/L (Table 1), above its perception threshold in water (0.40 ug/L) [45], suggesting that this compound would contribute directly to the aroma of the beverage. (E)-2-nonenal, described as fatty and green [60], was also detected (3.93 ug/L) in a milk fermented with strains of _Lb. delbrueckii_ subsp. _bulgaricus_ and _S. thermophilus_ (at a ratio of 1:1) [86]. Nonanal was detected in the third subculture at a concentration of 1.61 \(\pm\) 0.81 ug/L (Table 1). Since this concentration is higher than its corresponding OPT in water (1.10 ug/L) [45], it can be concluded that this compound would contribute directly to the aroma of the beverage imparting green, fresh [50], citrus-like, soapy [53], and fruity notes [68] (Table 2). In the fermented samples from the 48-h subcultures, only four aldehydes were detected. 2-Ethyl hexanal and phenylacetaldehyde were present in all subcultures, while 2-methyl-undecanal and (Z)-14-methyl-8-hexadecenal were found in two subcultures (Table 1).