## 4 Discussion
plantarum_ 299v slightly reduced syneresis in dairy beverages, likely due to its interaction with the dairy matrix [28; 29; 30]. However, the highest syneresis levels were observed in dairy-oat samples fermented with the yogurt culture and _L. plantarum_ 299v, possibly due to a weaker structural network or lower content of gel-stabilizing proteins. These results suggest that while _L. plantarum_ 299v may reduce syneresis in dairy products, its effect depends on formulation-specific factors, highlighting the complex interactions within fermented beverages [30]. The fermentation process influenced the level of sugars, especially galactose, sucrose, and lactose, confirming the activity of lactic acid bacteria [31; 32]. These bacteria, including lactobacilli and streptococci, efficiently ferment lactose and sucrose, producing lactic acid and other metabolites [33; 34]. Our study showed that glucose levels remained stable, suggesting rapid consumption at the beginning of fermentation, which aligns with its role as a primary energy source for lactic acid bacteria [35; 36]. However, excessive glucose depletion can lead to metabolic shifts, potentially affecting fermentation efficiency. The most pronounced changes in galactose levels were observed in cow's milk fermented with the yogurt culture and _L. plantarum_ 299v, confirming that specific bacterial strains influence carbohydrate metabolism. The variability in galactose content may result from differences in bacterial metabolic pathways, such as the Leloir and Tagatose-6P pathways, which regulate lactose and galactose metabolism [37; 38]. _L. plantarum_ has been shown to alter the milk metabolite profile, improving fermentation stability [39]. Sucrose hydrolysis in dairy-oat beverages further confirmed the enzymatic activity of lactic acid bacteria, which plays a role in flavor development and nutritional enhancement [21]. Conversely, the minimal changes in raffinose and stachyose levels suggest a limited ability of the tested bacterial cultures to metabolize these oligosaccharides. This is consistent with previous findings that lactic acid bacteria lack the \(\alpha\)-galactosidase enzyme required for the efficient degradation of raffinose and stachyose [18; 21; 40; 41; 42; 43]. In our study, the initial count of _S. thermophilus_ was 7.8-8.9 log CFU/g, and it remained stable after 14 days in fermented beverages. After 28 days, a decrease was observed in milk beverages fermented with the yogurt culture and _L. acidophilus_ La-5, as well as oat beverages fermented with the same combination. The stability of _S. thermophilus_ is crucial for probiotic benefits and sensory quality [44]. While this species typically maintains viability during storage, factors such as temperature, pH fluctuations, and microbial interactions can contribute to its decline [45; 46]. The presence of _L. acidophilus_ can also influence fermentation and bacterial survival [47]. The highest initial lactobacillus count was recorded in samples fermented with the yogurt culture and _L. acidophilus_ La-5, suggesting a synergistic interaction that enhanced bacterial stability [48]. This positive effect is likely due to complementary metabolic activities that support fermentation and improve probiotic viability, as well as the sensory and textural properties of the product [25; 48; 49; 50]. _L. plantarum_ 299v showed better survival in cow's milk and oat beverages, whereas, in dairy-oat mixtures, the count of this bacterium decreased to 5.0 log CFU/g after 28 days. This highlights the importance of selecting appropriate food matrices to maintain probiotic viability. While cow's milk provides a nutrient-rich environment, oat beverages have also been shown to support _L. plantarum_ survival, making them a suitable non-dairy alternative [51; 52]. However, dairy-oat combinations may introduce inhibitory factors such as pH shifts, nutrient competition, and the accumulation of metabolic by-products, reducing bacterial viability [12; 53; 52].