# Changes in Selected Quality Indices in Microbially Fermented Commercial Almond and Oat Drinks
## 3 Results and Discussion
### Aidity Changes after the Fermentation of the Used PB Beverages
bulgarcius_ is an example of an aerobic-to-anaerobic homofermentative bacterium, so lactic acid is the major end product of fermentation produced via the EMP pathway [44; 45]. In turn, _L. plantarum_ is a facultative heteroffermentative bacterium that produces lactic, acetic, and formic acids under certain conditions and selective substrates via the Dickens-Horecker and Entner Doudoroff metabolic pathways [45]. Both LAB species used in this study can ferment sugars found in plant materials, such as glucose, fructose, and xylose. _L. bulgarcius_ is commonly used in the production of yoghurts and some types of ripening cheeses. It has also been successfully used in the production of fermented plant drinks [42]. It is part of the microflora of many food and feed products, including dairy, meat, fish, and fermented vegetable products. Strains of this species are part of the starter and protective cultures used in the food industry [46]. This species is also a natural microbiota of humans and animals [47]. Changes in the tittatable acidity and pH of the fermented-by-yeast beverages were more varied (Table 2). The lowest increases in the tittatable acidity (up to 0.22% of lactic acid equivalent) were found for the strains of _C. antarctica_ and _T. casei_ in both beverages, _K. marxianus_ in an almond drink, and _C. lipolytica_ in an oat drink. For other strains, the acidity increase was significantly higher, reaching an upper limit of 0.79% and 0.50% of lactic acid equivalent in the almond and oat beverages fermented by _Y. lipolytica_, respectively. Moreover, the hydrogen ion concentration in both of these beverages was the highest (oat drink with pH 2.85 and almond drink with pH 3.28). These values were close to the values found in drinks prepared using LAB strains. For other fermented-yeast beverages, the pH values ranged from 4.32 to 6.51. Yeasts, particularly _C. lipolytica_ and _Y. lipolytica_, are efficient producers of lipases and proteases [48; 49], enabling more advanced and faster hydrolysis of proteins and lipids into amino acids and fatty acids, which together increase the tittatable acidity. The activity of lipases is particularly important since the ability to degrade triglycerides is an essential process in ensuring energy homeostasis and the availability of precursors for membrane lipid synthesis [50]. In the case of high acidity (measured as low pH), the production of organic acids that are able to dissociate could also be considered. Recent studies have shown that _Y. lipolytica_ is an efficient producer of citric acid [51; 52; 53] and succinic acid [54]. The balance of dissociated to undissociated acids (e.g., citric) allows the yeast to survive at a low pH (the amount of undissociated citric acid increases and can permeate the cell wall, altering the internal pH of the microorganisms) [33]. In a highly acidic environment (such as in the beverages fermented in the current study by _Y. lipolytica_), amino acid molecules also exist in the form of positive NH\({}_{3}\)\({}^{+}\) ions. Among the analyzed fermented plant drinks, the highest tittatable acidity changes were determined for almond beverages. As mentioned earlier, this type of beverage was characterized by the highest fat content (4.1 g/100 mL) and relatively high protein content (1.7 g/100 mL).