## 4 Discussion
[47] supports this finding, showing that the presence of \(\alpha\)-casein reduces the antioxidant activity of tea polyphenols in the ABTS test by about 11-27%. Another possible reason for the decreased antioxidant activity in fruit-added yogurts compared to natural yogurt is the lowering of pH caused by the fruit additives. Low pH can affect the stability and redox reactions of some antioxidants [48], making them less effective in ABTS and DPPH tests [49]. Numerous scientific studies have shown that antioxidant activity is directly proportional to the overall polyphenol content. TRC was found to be higher in all commercial fruit yogurts compared to natural yogurts with the addition of selected fruits, except for yogurts with 20% bilberry addition and comrelian cherry, where the values were similar (Table 4). A similar relationship was noted by [PERSON] et al. [50], who reported that enriching yogurt with mango and guava significantly increased (\(p<0.05\)) the total phenolic content as the fruit pulp content increased. Bioactive polyphenols--such as phenolic acids, flavonoids and anthocyanins--present in bilberry and comrelian cherry play a key role in total antioxidant activity, which largely depends on the presence of free polyphenols [51; 52]. Since interactions between phenolic compounds and proteins affect both their content and antioxidant capacity [53], this may explain the discrepancies between the polyphenol content and free radical neutralization ability in this study. Furthermore, the tested commercial yogurts contained carrot juice concentrate, aronia juice concentrate, beet juice concentrate and grape juice concentrate, which could also influence TRC. It is worth noting that a high content of phenolic compounds was reported for yogurts with the addition of bilberry and comelian cherry. These results particularly highlight the need for more advanced scientific research, as, to the best of our knowledge, no previous studies on yogurt enriched with comelian cherry have been reported in the literature. The increase in the antioxidant activity of yogurts observed in Table 5, corresponding with the increasing percentage of fruit, is most likely due to the presence of bioactive compounds such as polyphenols, vitamins (e.g., vitamin C and E), carotenoids and flavonoids, which are known for their ability to neutralize free radicals [19]. Fruits are recognized as rich sources of these compounds, which explains their positive influence on the antioxidant properties of yogurt. A significant increase in antioxidant activity is observed when fruit is added at a 7% level. At this point, the concentration of bioactive compounds likely reaches a threshold sufficient to produce a measurable effect. Many phenolic compounds attain optimal concentrations in foods when a certain fruit content is achieved. For example, research conducted by [PERSON] et al. [54] suggests that even small amounts of fruits can significantly increase polyphenols levels in dairy products, enhancing their antioxidant activity. However, beyond this threshold (in this case, 7%), further increases in fruit content do not result in significant changes in antioxidant activity (Table 4). [PERSON] et al. [55] showed that excessive fruit content can lead to a decrease in the product's pH, which can affect the stability of certain antioxidant compounds. It is well-documented that pH can influence the antioxidant activity of polyphenols, which may explain the plateau in activity observed at higher fruit concentrations. In our study, we also recorded a significant decrease in pH value with increasing fruit content in the yogurts, which may have influenced the antioxidant results due to the impact of pH on compound stability. Nevertheless, the pH values of the tested yogurts remained within the ranges reported in the literature [54]. When analyzing the antioxidant activity of different flavor variants, particularly bilberry yogurts, interesting results were observed for both commercial and diet versions (Table 6).