Anaerobic fermentation of tea leaves and spent tea residues: Effects on phenolic and flavonoid contents, antioxidant capacity, and antibacterial activity

<ns3:p> Abstract Background Tea ( <ns3:italic>Camellia sinensis</ns3:italic> ) and its brewing by-products are promising sources of phenolic-rich extracts that may serve as natural antioxidants in food systems. However, the extent to which anaerobic fermentation modifies the bioactive properties of different processed teas remains unclear. This study evaluated the effects of anaerobic fermentation on the phenolic content, antioxidant capacity, and antibacterial activity of several tea types cultivated in Indonesia. Methods A 2 × 2 × 4 factorial experiment with five replicates was conducted using three factors: fermentation status (unfermented vs. anaerobically fermented), material fraction (tea leaf vs. spent residue), and processed tea type (green, black, oolong, and white). Methanolic extracts prepared with 70% methanol were analyzed for total phenolic content (TPC), total flavonoid content (TFC), antioxidant capacity based on DPPH IC <ns3:sub>50</ns3:sub> , and antibacterial activity against <ns3:italic>Staphylococcus aureus</ns3:italic> and <ns3:italic>Escherichia coli</ns3:italic> expressed as MIC <ns3:sub>50</ns3:sub> . Results Across tea types, oolong and white teas exhibited higher TPC and stronger antioxidant capacity than black tea (p < 0.05). Tea leaves consistently showed higher TPC and stronger antioxidant activity than spent residues (p < 0.05). Anaerobic fermentation increased TPC in most teas and enhanced TFC mainly in leaf samples. However, improved antioxidant capacity following fermentation was observed only in spent residues, as indicated by lower IC <ns3:sub>50</ns3:sub> values. In contrast, fermentation generally increased MIC <ns3:sub>50</ns3:sub> values, suggesting reduced antibacterial effectiveness. Conclusions Anaerobic fermentation differentially affected the bioactive properties of tea materials depending on tea type and material fraction. While this process was effective in enhancing the antioxidant potential of spent tea residues, it was not beneficial for improving antibacterial activity. These findings suggest that anaerobic fermentation may be a useful strategy for valorizing tea brewing by-products as antioxidant-rich functional ingredients. </ns3:p>