## 3 Results and Discussion
### Preparation and Characterization of Sea Buckthorn Leaves Kombucha
The results show that fermentation led to a decrease in proanthocyanidin content, with the lowest concentration observed in the sea buckthorn leaf kombucha sample L4-100. Proanthocyanidins are complex polyphenic compounds composed of flavanol monomers such as catechin and epicatechin linked together. Therefore, during fermentation, microorganisms, especially yeasts, and bacteria, can secrete enzymes such as tannases or glycosidases that degrade proanthocyanidin polymers into smaller phenolic compounds or flavonoids [49; 50]. For this reason, it can be observed that the TPC increases in proportion to the decreasing concentration of proanthocyanidins. In this study, the TPC content of the tea samples ranged from 0.89 g\({}_{\text{GAE}}\)/L to 1.21 g\({}_{\text{GAE}}\)/L, depending on the ratio of sea buckthorn leaves to green tea (Table 3). Scientific publications have found that the introduction of new substrates into kombucha fermentation generally increases the content of bioactive molecules such as phenolic compounds [48; 51]. [PERSON] et al. [52], the TPC of unfermented beverages made from green tea and wax mallow (_Malvaviscus arboreus_ Cav.) were 568.4 u\({}_{\text{GAE}}\)/mL and 50.9 u\({}_{\text{GAE}}\)/mL, respectively. After fermentation into kombucha beverages at 24 \({}^{\circ}\)C for 14 days, the phenolic content of the wax mallow kombucha increased to 124.6 u\({}_{\text{GAE}}\)/mL, whereas the phenolic content of the green tea kombucha did not differ significantly from that of the unfermented green tea beverage. In another work [46], the TPC of unfermented beverages made from hawthorn leaves, nettle leaves, and black tea were 163.33 mg\({}_{\text{GAE}}\)/L, 663.83 mg\({}_{\text{GAE}}\)/L, and 1896.83 mg\({}_{\text{GAE}}\)/L, respectively. After fermentation into kombucha beverages at 25 \({}^{\circ}\)C for 15 days, the phenolic content of the kombucha from hawthorn leaves, nettle leaves, and black tea were 995.33 mg\({}_{\text{GAE}}\)/L, 1479.83 mg\({}_{\text{GAE}}\)/L, and 2452.67 mg\({}_{\text{GAE}}\)/L, respectively. The biggest change was observed in hawthorn leaves and nettle leaves, while in black tea, the change was insignificant. Microbial metabolism converts certain compounds into more active isomers or smaller molecules with enhanced biological activity, thereby enhancing or diversifying kombucha's physiological functions, properties, and sensory attributes [45]. [PERSON] et al. [44] presented that the DPPH values of the different sea buckthorn leaf tea beverages showed significant variation, ranging from 20.69 \(\pm\) 0.61 to 50.21 \(\pm\) 2.71 mmol TE/g. The findings indicated that sea buckthorn leaf tea exhibited a greater capacity to scavenge DPPH free radicals compared to the other two analysis types. In the present study, assays such as DPPH (which measures the removal of DPPH free radicals) were used to evaluate the antioxidant activities of the five sea buckthorn leaf kombucha samples (Table 3). The lowest antioxidant changes during fermentation were observed in sea buckthorn leaf kombucha L4-100. It was also observed that the antioxidant potency of the samples decreased with increasing fermentation time. Similar results have been found in the work of other authors. For example, the free radical-binding capacity of black tea kombucha decreased from 36.4 to 35.4 during the fermentation of 15 days [53]. Fermentation can also produce other compounds, such as organic acids, which can reduce the overall antioxidant potential of the beverage, even if the amount of phenolic compounds increases. It was also noted that antioxidant efficiency decreased with longer fermentation duration. This observation is supported by changes in the pH values of the kombucha and the measured titratable acidity (TA). It was observed that as the fermentation time increased, the pH values decreased, and the total acetic acid content increased in all five samples (see Figure S1 in the Supplementary Material S1).