## 3 Results and Discussion
### Evolution of VOC Profiles During Successive Kefir Grain Passages
Conversely, Bev7-72 h and Bev1-72 h, which contain thirteen and eleven volatile compounds with an OAV greater than 1.0 (Table 4), had the lowest calorie content due to their reduced TS concentrations (31.12 and 34.13 g/L, respectively). This indicates that 74.81% and 72.38% of the initial sugars in the substrate were consumed during fermentation. However, in both beverages, only the yeast count exceeded 10\({}^{6}\) CFU/mL [11]. Although two beverages (Bev5-24 h and Bev9-24 h) contained counts of all three microbial groups exceeding 10\({}^{6}\) CFU/mL, their sugar content was above 71 g/L. In contrast, nine beverages (Bev8-24 h, Bev3-48 h, Bev4-48 h, Bev6-48 h, Bev8-48 h, Bev9-48 h, Bev3-72 h, and Bev6-72 h) had LAB and yeast counts, which are associated with recognized probiotic effects [7; 8], exceeding 10\({}^{6}\) CFU/mL. Among these, Bev7-48 h and Bev3-72 h had the lowest sugar concentrations (<57 g/L). Notably, Bev3-72 h exhibited a broader aromatic profile, with 14 volatile compounds having OAV \(>\) 1.0, compared to Bev7-48 h, which contained 8 volatile compounds with OAV \(>\) 1.0 (Tables 3 and 4). Both beverages were subsequently identified as suitable functional foods. Microbiological and Chemical Analysis of Beverages Bev7-48 h and Bev3-72 h During 7, 14, 21, and 28 Days of Refrigerated Storage  
After selecting the most suitable beverages based on their chemical, microbiological, and aromatic characteristics, the subsequent study focused on evaluating their stability under storage conditions. Table 5 illustrates the changes in the microbiological and chemical compositions of the two selected beverages during refrigerated storage at 4 \({}^{\circ}\)C over 7, 14, 21, and 28 days. As shown, the pH values of beverage Bev7-48 h remained largely unchanged (\(p\)\(>\) 0.05) during refrigerated storage, while only minor changes (\(p\)\(<\) 0.05) were observed in Bev3-72 h.  
LAB counts remained relatively stable in both beverages, consistently staying above 10\({}^{6}\) CFU/mL throughout the storage period. The final counts were 2.1 \(\times\) 10\({}^{6}\) CFU/mL for Bev7-48 h and 1.4 \(\times\) 10\({}^{7}\) CFU/mL for Bev3-72 h. In contrast, AAB counts exhibited a declining trend, particularly after day 7 in Bev7-48 h and progressively throughout the entire storage period in Bev3-72 h (\(p\)\(<\) 0.05). Yeast counts, on the other hand, showed a slight increase (\(p\)\(<\) 0.05), with growth rates of 0.015 and 0.006 log CFU/mL/day, reaching final levels of 5.2 \(\times\) 10\({}^{6}\) and 1.6 \(\times\) 10\({}^{7}\) CFU/mL, respectively, by the end of the storage period (Table 5). These findings suggest that storage at 4 \({}^{\circ}\)C, coupled with the acidic pH of the beverages, did not significantly affect LAB viability, but it gradually reduced the viability of AAB cells and promoted yeast growth. Regarding organic acids, both Bev7-48 h and Bev3-72 h exhibited a slight but significant decrease (\(p\) < 0.05) in the concentrations of citric acid (0.11 and 0.10 g/L/day, respectively) and quintic acid (0.05 g/L/day in both cases). In contrast, the levels of lactic and acetic acids increased slightly (\(p\) < 0.05) from day 0 to day 28. Lactic acid rose at rates of 0.005 g/L/day in Bev7-48 h and 0.010 g/L/day in Bev3-72 h, while acetic acid increased at a rate of 0.010 g/L/day in both beverages.