## 4 Discussion
The antimicrobial activity of the obtained bio-ferments may be due to the presence of phenolic compounds (Table 2), especially caffeic acid [58] and gallic acid [59]. Polyphenol caffeic acid is a compound found in many species of plants that has proven antimicrobial activity. Caffeic acid is one of the ingredients in cosmetic (dermatological) preparations that enhances the antimicrobial effect of these preparations, even if they do not have a direct antibacterial and antifungal effect [58]. Caffeic acid has significantly been employed as an alternative strategy to combat microbial pathogenesis and chronic infection induced by microbes such as bacteria, fungi, and viruses [60]. Gallic acid (3,4,5-trihydroxybenzoic acid) is a bioactive phytochemical, and its derivatives are often present in cosmetic formulations and can be considered \"safe\" and \"natural\" in the context of cosmetic production [61]. Studies have found that gallic acid killed _Salmonella_ strains by permeabilizing the outer membrane by chelating divalent cations, leading to subsequent cell lysis. Gallic acid has been shown to have antimicrobial activity against _Escherichia coli_, _Campylobacter jejuni_, and _Staphylococcus aureus_[62; 63]. All the bio-ferments obtained showed high antioxidant activity by the DPPH (from 2.41 \(\pm\) 0.01 to 3.21 \(\pm\) 0.01 mmol Tx/L), ABTS (from 6.52 \(\pm\) 2.06 to 22.43 \(\pm\) 2.01 mmol Tx/L), and FRAP (from 10.77 \(\pm\) 0.59 to 16.68 \(\pm\) 2.50 mmol FeSO\({}_{4}\)/L) methods, and had a high content of polyphenic compounds as assessed by the Folin-Ciocalteu method (from 2306.82 \(\pm\) 0.10 to 2599.43 mg GA/L). However, the bio-fermet derived from milk thistle pomace (B-P) showed the most effective DPPH radical scavenging activity, with a Tx equivalent antioxidant capacity of 3.21 \(\pm\) 0.01 mmol/L. In addition, B-P showed the most potent antioxidant activity at 22.43 \(\pm\) 2.01 mmol Tx/L using the ABTS method and 16.68 \(\pm\) 2.50 mmol FeSO\({}_{4}\)/L using the FRAP method--Table 2. The bio-fermet from pomace (B-P) exhibited the highest ability to chelate Fe\({}^{2+}\) ions in the ferrozine assay and also had the highest ability to reduce Cu\({}^{2+}\) and Fe\({}^{3+}\) ions. However, O-S bio-ferments had a lower capacity to reduce copper II ions than B-E and B-O (Table 3). _Lactobacillus plantarum_ (LP), _Rhodotorula glutinis_ (RG), _Metschnikova pulcherrima_ (MP), _Lactobacillus casei_ (LC), and _Rhodotorula glutinis_ (RG) bacteria were used to ferment mango juice (MJ). Studies have shown that all fermentation cases significantly increased the content of phenolic compounds (od 4.7 mmol GA/L to 7.35 mmol GA/L), DPPH radical scavenging activity (7.5-33%), ABTS radical scavenging activity (7-53%), and copper reducing capacity (7-11 mmol/mL Tx) [34]. Our study revealed almost twice the amount of phenolic compounds (13.56-14.97 mmol GA/L or 2306.82-2599.43 mg GA/L) than the literature reports. The bio-ferment obtained with LC + RG had a lactic acid content of 15.05 g/L. In the case of our study, all bio-ferments obtained had a higher lactic acid content (between 22 and 26 g/L) [34]. The results confirm that the analyzed by-products are a good source of many biological functional substances with a significant content of phenolic compounds [14]. A more than 2-fold increase in activity was observed. The antioxidant capacity of extracts obtained from vegetable oil by-products (flour, meal, and groats) was also confirmed by [PERSON] et al. [64]. The extracts contain significant amounts of phenolic compounds, ranging from 1.54 to 74.85 mg GA/g byproduct. DPPH values ranged from 7.58 to 7182.53 mg Tx/g byproduct. ABTS test values of the analyzed samples ranged from 0 to 3500.52 mg Tx/g byproduct. The highest values for the FRAP method were represented by grape seed flour (4716.75 mg Tx/g). For the CUPRAC test, grape seed flour (5936.76 mg Tx/g) showed the highest antioxidant activity [64].