## 3 Results and Discussion
### Analysis of the Aromatic Profile of the Fermented Milks
Comparison of the Chemical Composition of Unfermented Milk and the Beverages obtained in the Repeated 24- and 48-h Subcultures  
Considering the different chemical composition of the nine milk beverages (Figures 1 and 2), it is very difficult to visualize, a priori, the similarities and differences between them. Therefore, principal component analysis (PCA) was used to reduce the initial number of variables (pH, L, Pr, TN, LA, AA, EtOH, GOH, and TAA), transforming them into factors that explain the variability observed in the experimental data. Figure 5: 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 II (II-48 h, II-96 h, II-148 h, and II-192 h). This variability can be conveniently explained by two factors (1 and 2) with eigenvalues greater than 1.0, which accounted for 67.75% and 18.86% of the total variance in the data, respectively. Together, these factors explained 86.61% of the total variance (Figure 6). UHT whole milk was located in the lower right quadrant as the most different beverage. This is attributed to its high pH value (6.70), high nutrient (L, Pr, TN) contents, and low product (LA, AA, EtOH, GOH, and TAA) concentrations, compared to the fermented beverages (Figures 1 and 2). The fermented beverages from the first and second subcultures of fermentation I (I-24 h and I-48 h) were located in the upper right quadrant because both beverages had higher final pH values, higher residual nutrient (L, Pr, TN), EtOH, and GOH concentrations, and lower LA, AA, and TAA levels compared to the other fermented beverages (Figures 1 and 2). The fermented beverage from the third subculture of fermentation I (I-72 h) was located in the upper left quadrant, as this beverage contained the lowest LA concentration (Figures 1 and 2). The I-96 h beverage was located in the lower right quadrant, although far from the unfermented milk (Figure 6), since this fermented beverage contained the highest lactose concentration and the lowest LA level compared to the other fermented beverages (Figures 1 and 2). Regarding the fermentation II, it can be observed that the first fermented beverage (I-48 h) was located in the upper left quadrant since this sample contained the highest ethanol concentration (Figures 1 and 2). The other three fermented samples (II-96 h, II-148 h, and II-192 h) were located in the lower left quadrant (Figure 6) because these three beverages were the most acidic, have the lowest pH values, and the highest levels of organic acids (LA and AA). Additionally, the three beverages contained the lowest concentrations of alcohols (EtOH and GOH) and the highest TAA titers (Figures 1 and 2). Figure 6: Principal component analysis based on the differences between the pH values and contents in lactose (L, in g/L), total nitrogen (TN, in g/L), proteins (Pr, in g/L), lactic acid (LA, in g/L), acetic acid (AA, in g/L), ethanol (EtOH, in g/L), glycerol (GOH, in g/L), and total antibacterial activity (TAA, in AU/mL) in the unfermented UHT milk and fermented beverages from fermentation I (I-24 h, I-48 h, I-72 h, and I-96 h) and II (II-48 h, II-96 h, II-148 h, and II-192 h). Comparison of the Volatile Composition of Unfermented Milk and the Beverages Obtained in the Repeated 24- and 48-h Subcultures  
The high number (107) of volatile compounds detected in the nine milk beverages (Table 1) makes it difficult to determine the relationship between the volatile compositions of the different beverages. Therefore, PCA was also applied to these data to easily interpret the relationships between the different beverages considering the total mean concentrations of each family of volatile compounds (organic acids, alcohols, aldehydes, ketones, esters, hydrocarbons, and other compounds). In this analysis, factors 1 and 2 explained 46.01% and 37.89% of the total variation in the volatile composition of the beverages, respectively, accounting for a combined total of 83.90% of the variance (Figure 7).