Tea ( Camellia sinensis) is widely consumed and is recognized as a rich dietary source of phenolic compounds and flavonoids, which contribute to antioxidant and antimicrobial properties and have been associated with various health-related benefits ( Nibir et al., 2017; Wang and Kang, 2020). The antioxidant and antibacterial effects of tea are mainly attributed to catechins, including epigallocatechin gallate (EGCG), epicatechin (EC), and epicatechin gallate (ECG) ( Zhao et al., 2014). These compounds work by neutralizing free radicals through metabolic processes in the body, thereby preventing oxidative stress associated with various degenerative diseases, including cancer, diabetes, and cardiovascular disorders ( Wang and Kang, 2020). Beyond the brewed beverage, tea processing and consumption also generate substantial quantities of spent tea residues. Although these residues are often discarded, they may retain appreciable amounts of bioactive compounds and therefore represent potential resources for the development of functional ingredients or natural antioxidants for food applications ( Çakmak et al., 2024).

Green, white, oolong, and black teas originate from the same plant, i.e., Camellia sinensis, but differ markedly in chemical composition because they undergo different degrees of enzymatic oxidation and processing ( Alves et al., 2025). These differences influence the profile and extractability of phenolic compounds, which can translate into variation in antioxidant capacity and antibacterial activity across tea types ( Tanaka & Matsuo, 2020; Esposito et al., 2023). Likewise, converting dried leaves into a spent residue by brewing can remove a portion of water-soluble phenolics, often lowering the functional potency of residue extracts compared with leaf extracts ( Saklar et al., 2015). Furthermore, prolonged storage of these teas may reduce their bioactive compound content ( Hazra et al., 2020), thereby decreasing their antimicrobial and antioxidant activities.

Fermentation is a food-relevant bioprocessing approach that can stabilize wet plant matrices and modulate phytochemical composition through microbial and enzymatic transformations. In sealed anaerobic systems, fermentation may alter the concentration, molecular forms, and extractability of tea phenolics; for example, changes in catechins and antioxidant activity during anaerobic storage have been reported ( Nishino et al., 2007). From a food-processing perspective, a simple sealed-pouch anaerobic fermentation step may be particularly attractive for wet tea residues because it can serve as a low-input stabilization and valorization strategy. However, evidence remains limited regarding how anaerobic fermentation affects the bioactive potential of both tea leaves and spent residues across different processed tea types. Therefore, this study aimed to evaluate the effects of anaerobic fermentation, material fraction (leaf vs. spent residue), and processed tea type on total phenolic and flavonoid contents, antioxidant capacity, and antibacterial activity against Staphylococcus aureus and Escherichia coli.

Across tea types, oolong and white teas showed higher TPC than black tea (p < 0.01), whereas black tea had the highest mean TFC (p < 0.01; Tables 1– 2). Tea leaves contained substantially higher TPC than spent residues. The effects of anaerobic fermentation on TPC and TFC depended on both material fraction and tea type. Fermented leaves showed a slight increase in TPC across all tea types (p < 0.01), whereas fermentation increased TFC only in green tea and black tea leaves (p < 0.01). In the residue fraction, fermentation consistently increased TPC and TFC in black tea residues, whereas the same treatment led to a consistent decline in green tea residues (p < 0.01).

The DPPH IC ₅₀ values indicated stronger antioxidant activity, reflected by lower IC ₅₀ values, in oolong and white teas than in black tea ( Table 3). Tea leaves exhibited substantially lower IC ₅₀ values than spent residues across all tea types and fermentation statuses (p < 0.01). Fermentation increased IC ₅₀ values in the leaf fraction in almost all tea types (p < 0.01), with white tea leaf being the only exception. In contrast, the residue fraction showed a consistent reduction in IC ₅₀ after fermentation across all tea types (p < 0.01), indicating improved antioxidant capacity following fermentation.

Regarding antibacterial activity, white tea showed the lowest MIC ₅₀ values against S. aureus and E. coli (p < 0.01), while oolong tea showed a lower MIC ₅₀ only against E. coli (p < 0.01; Tables 4– 5). Fermentation increased the MIC ₅₀ against S. aureus in the leaf fraction across all tea types (p < 0.01). A similar trend was observed in the residue fraction, in which fermentation increased the MIC ₅₀ of green tea, white tea, and oolong tea (p < 0.01). For antimicrobial activity against E. coli, green tea and oolong tea leaves showed higher MIC ₅₀ values after fermentation (p < 0.01). In the residue fraction, fermentation consistently increased MIC ₅₀ values across all tea types (p < 0.01), indicating reduced antibacterial effectiveness.

The higher TPC observed in oolong and white teas than in black tea may reflect differences in leaf maturity and processing intensity. In particular, the high TPC in white tea may be attributed to the use of young leaves, which are naturally rich in epigallocatechin gallate (EGCG) and epicatechin gallate (ECG) ( Esposito et al., 2023). By contrast, the substantially higher TPC in tea leaves than in spent residues is consistent with the removal of water-soluble phenolics during brewing ( Saklar et al., 2015). The slight increase in TPC after fermentation, especially in the leaf fraction, may reflect degradation of native phenolic compounds such as catechins into smaller molecules that still retain phenolic properties rather than being converted into non-phenolic acids ( Nishino et al., 2007). These breakdown products may continue to react with the Folin–Ciocalteu reagent, thereby contributing to a slight apparent increase in measured total phenols.

The stronger antioxidant activity of oolong and white teas, as indicated by lower IC ₅₀ values, is in agreement with their higher phenolic concentrations. Dudonné et al. (2009) reported a strong correlation between total phenolic content and radical-scavenging activity (R = 0.939 for DPPH), and this activity is mediated through mechanisms such as hydrogen atom transfer, single-electron transfer, sequential proton loss electron transfer, and transition metal chelation ( Zeb, 2020). Although black tea contained more flavonoids than the other tea types, its antioxidant activity was weaker, as shown by its higher IC ₅₀ values. This may be explained by the transformation of catechins into theaflavins and other polymeric compounds during full oxidation in black tea processing ( Tanaka & Matsuo, 2020).

The lower IC ₅₀ values in the leaf fraction than in the residue fraction are also consistent with the substantially higher TPC and TFC of the leaves. Conversely, the higher IC ₅₀ values in tea residues likely resulted from the brewing process, which extracted most of the water-soluble phenolic compounds ( Saklar et al., 2015). Interestingly, fermentation had contrasting effects on antioxidant capacity depending on the material fraction. In leaves, fermentation generally increased IC ₅₀ values and thus reduced antioxidant capacity. In residues, however, fermentation consistently reduced IC ₅₀ values across tea types, indicating improved antioxidant activity. This contrasting response may be related to the presence of bioactive compounds that remain bound to the fibrous matrix of tea residues ( Çakmak et al., 2024). Fermentation may facilitate the release of these bound phenolics, thereby increasing the availability of antioxidant compounds in the residue fraction.

With respect to antibacterial activity, the lower MIC ₅₀ values of white tea against both S. aureus and E. coli, and of oolong tea against E. coli, indicate a stronger antibacterial potential of these tea types. This finding is consistent with their higher phenolic content, as phenolic compounds can disrupt microbial cell membranes, inhibit metabolic enzymes, and interfere with biofilm formation in pathogenic bacteria such as E. coli and S. aureus ( Huang et al., 2022). Nevertheless, fermentation generally increased MIC ₅₀ values in both leaf and residue fractions, indicating reduced antibacterial effectiveness after fermentation. Although TPC appeared to increase after fermentation, antimicrobial activity against both S. aureus and E. coli declined. This reduction may be attributed to polymerization of phenolic compounds during fermentation or to their degradation by microbial activity, producing derivatives with weaker antibacterial effects ( Romero et al., 2025).

This study was conducted without utilizing any human or animal resources.