## 3 Results and Discussion
### Microbiological Quality and Biological Activity of Fermented OBB
In all examined variants (except for control samples), a significant decrease in antioxidant activity was concluded after 20 h of fermentation. In the oat beverage samples with the addition of hawthorn extract and lyophilistate (in the amounts of 1% and 5%), the TPC was significantly higher compared to control samples. The results are presented in Table 5. A higher TPC was observed immediately after application and remained at a high level after 20 h of fermentation. The increase in TPC at 0 h of fermentation was 154.46 and 134.67 mg GAE/L in samples with the 1% addition in comparison with the control, while in samples with the 5% addition, it was 158.72 and 129.2 mg GAE/L for the hawthorn extract and lyophilate, respectively. After 20 h of the fermentation, a significant decrease in TPC was observed in control samples as well as in samples with the 5% extract and 1% lyophilate. Comparing the TPC values in oat drinks depending on the amount of additive as well as on the type of additive (extract/lyophilistate) and fermentation time (0 h and 20 h), significant differences were noticed between the samples with 1 and 5% of hawthorn extract and lyophilicate after fermentation. The antioxidant activity of the prepared oat drinks was significantly higher compared to control samples only in the case of the 5% addition of chokeberry extract (0.743 mM) as well as 1 and 5% additions of chokeberry lyophilistate (0.808 and 0.732 mM, respectively) immediately after application (at 0 h of fermentation). In all examined variants (except for control samples), a significant decrease in antioxidant activity was concluded after 20 h of fermentation. The fortification of dairy or non-dairy beverages with plant material was reported in the literature. Some experiments were conducted with yogurts fortified with hawthorn extracts, where authors observed a dose-dependent linear effect in the amount TPC and antioxidant activity as in the presented paper. [PERSON] et al. [69] prepared yogurt with the addition of plant extracts, including hawthorn, and observed a significant increase in TPC, while [PERSON] et al. [70] prepared yogurts with the addition of hawthorn and strawberry extracts in amounts of 8 and 12 mg/mL, noticing a tendency to increase the TPC value and antioxidant activity; however, the differences were not significant. In the work of [PERSON] et al. [71], chokeberry juice was added to oat fermented beverage, where the values for the antioxidant activity increased with the increase in the concentration of chokeberry juice. Lactic acid fermentation can lead to changes in the amount and profile of biologically active compounds such as phenolic acids, sterols, and vitamins, increasing or decreasing their levels in the product [72,73]. Due to the microbial metabolism, polyphenol compounds may be released from conjugated forms and become available, influencing the antioxidant properties. However, it should be underlined that the metabolism of polyphenols by LAB strongly depend on the composition of the raw material used for fermentation as well as the microbial strain responsible for the process [74,75,76]. Therefore, different variations are described in the literature during the fermentation with plant material or milk with the addition of plant extracts. Similar effects of increasing and