## 3 Results and Discussion
### Volatile Composition of Beverages Obtained through Static Discontinuous Fermentations I and II
#### 3.3.5 Esters  
According to [PERSON] et al. [72], esters in dairy products are commonly synthesized via two enzyme-catalyzed reactions, esterification or alcoholysis. The first reaction occurs between alcohols and carboxylic acids, while the second can occur in two ways, either between alcohols and acylglycerols, or between alcohols and fatty acyl-CoAs synthesized during the metabolism of fatty acids, amino acids, and/or carbohydrates [72]. Other researchers have indicated that esters are produced during the microbial degradation of lactose or amino acid [96], or due to the lipid metabolism of yeasts [97]. These compounds are desirable in beverages due to their high volatility and fruity aromatic characteristics, which can help mask the undesirable aromas of other compounds, including acids, aldehydes, and amines [48]. It has been reported that esters, even at low levels, can contribute to the formation of fruity and floral aromas and flavors in fermented and non-fermented foods, including fermented dairy products, because most of these compounds have a low taste threshold [68; 86; 97]. In the present study, esters comprised the majority of the volatile compounds identified in the unfermented and fermented milk (Table 1), consistent with previous findings in a beverage obtained by fermenting whey with _K. lactis_, a yeast present in milk kefir grains and known for its efficiency in producing these compounds [53]. Five esters (2-phenylethyl acetate, allyl pentadecyl oxalate, ethyl octanoate, ethyl decanoate, and methyl hippurate) were identified in unfermented milk (Table 1). The fermentation of this substrate with kefir grains led to an increase in the number of esters. Thus, in the fermented samples from the 24-h subcultures, the number of esters increased to 14 (24 h), 11 (48 h), 13 (72 h), and 15 (96 h). Regarding the concentrations of these volatile compounds, it can be noted that repeated 24-h fermentation improved the concentration of esters compared to the unfermented substrate (\(p<0.05\)), suggesting an increase in the fruity aromatic notes in the four fermented beverages. A similar increase in the number and concentrations of esters with fermentation was observed when soy whey was fermented with water kefir grains for 48 h [98]. In the repeated 48-h subcultures, the number of esters was also higher than in whole milk. Thus, 13, 16, 11, and 9 volatile compounds were found in the first, second, third, and fourth subcultures, respectively (Table 1). The fermentation of milk in repeated 48-h subcultures did not always result in an increase in the total concentration of these compounds. Specifically, the first two subcultures provided total ester concentrations higher (\(p<0.05\)) than in unfermented milk. The total concentrations of esters in the third and fourth subcultures were either similar (\(p<0.05\)) or lower (\(p<0.05\)) compared to that found in milk. 2-Phenylethyl acetate, ethyl octanoate, and ethyl decanoate were the only esters compounds identified in both unfermented milk and all subcultures (Table 1). Of these esters, only ethyl octanoate was found at mean concentrations higher than its corresponding OPT of 19.30 \(\upmu\)g/L (Tables 1 and 2) [45]. This was observed in the beverages obtained from the first two subcultures (30.70 and 30.12 \(\upmu\)g/L) of the repeated 24-h fermentation. Additionally, in the beverage from the fourth subculture, the mean concentration of this compound (18.92 \(\upmu\)g/L) was very near to 19.30 \(\upmu\)g/L. Therefore, this compound is one of the key compounds in the aromatic profile of these beverages. Isoamyl acetate was detected in the fermented beverages obtained in three subcultures (0-24, 48-72, and 72-96 h) from fermentation I, at mean concentrations above its OPT of 2 \(\upmu\)g/L [45]. Therefore, this compound will make a direct contribution to the aroma of these beverages, with aromatic notes of banana, fruit, and sweet [50; 74].