## 3 Results and Discussion
### The Content of Organic Acids, Sugars, Ethanol, and Vitamin C
In the next phase, the formed aldehyde is oxidized by the enzyme aldehyde dehydrogenase (ALDH), which is located on the outer surface of the cytoplasmic membrane, thereby producing the end product. Strains of the Gluconobacter sp. are unable to further process acetic acid due to the non-functional cycle of tricarboxylic acids in this case, and more precisely by two enzymes occurring in it, namely \(\alpha\)-ketoglutarate dehydrogenase and succinate dehydrogenase [39]. In the study of beer beverages, a statistically significant difference was observed in the amount of acetic acid produced by the added _G. oxydans_ H32 strain compared to the samples without the addition of starter AAB (_p_ < 0.05). These amounts were 3-4 times higher than the BJP beer sample and the control \"C\" samples (Table 4). Gluconobacter species are glucophilic and can oxidize glucose via two alternative pathways. The first operates inside the cell, where oxidation takes place through the pentose phosphate pathway, and the second, outside the cell, which involves the formation of gluconic and ketogluconic acid [40]. Moreover, gluconic acid is a metabolite of AAB with a pro-health effect, including a precursor in the biosynthesis of natural vitamin C. As a substrate for the initiation of this reaction, i.e., obtaining 2-oxo-L-gluonic acid, a precursor of vitamin C, glucose is used. Besides glucose, other possible carbohydrates can be fructose, galactose, as well as also arabinose, rhamnose, xylose, sorbose, or sucrose [41]. In this study of beer beverage variants, significant amounts of gluconic acid (_p_ < 0.05) were determined only in the beers with the addition of AAB starter culture (pasteurized beer BPGL = 69.07 mg 100 mL\({}^{-1}\), BPGD = 15.17 mg 100 mL\({}^{-1}\) and unpairedized beer BGL = 44.93 mg 100 mL\({}^{-1}\), BGD = 8.60 mg 100 mL\({}^{-1}\)). Additionally, in these products, the content of vitamin C was the highest (_p_ < 0.05) at BPGL = 2.83 mg 100 mL\({}^{-1}\), BPGD = 2.43 mg 100 mL\({}^{-1}\) i w BGL = 2.03 mg 100 mL\({}^{-1}\), BGD = 2.60 mg 100 mL\({}^{-1}\), which, per 500 mL of the study beverages, provides about 10 mg of vitamin C. According to the EFSA recommendation, the average intake of vitamin C among adults aged 35-64 years old in European countries should be 100 mg per day [42]. Moreover, although developed beer samples may serve as a minor source of vitamin C in the diet, vitamin C addition is significantly higher than in commercial beers. The conscious consumer can therefore choose the more nutritious, pro-health product. Similar conclusions were shown from the [PERSON] et al. [8] study, wherein the authors also highlighted the potential benefits to human health, including, among others, a high content of vitamin C and high antioxidant capacity, pointing to new approaches that can increase the interest in craft beer. The content of malic, fumaric, lactic, and pyruvate acids was at the level characteristic for beers. Their content proves the proper course of fermentation and a lack of contamination with unwanted microorganisms [43]. The addition of fruit juices to beer samples affected the profile of organic acids. Additionally, in this case, the acid content in the products was similar to the literature data on this subject [44, 45].