Black tea processing waste as a source of antioxidant and antimicrobial phenolic compounds

The objective of this study was to evaluate the value-added potential of black tea processing waste (BTPW) as a source of antioxidant and antimicrobial phenolic compounds. The effects of extraction solvent (water, 50 % ethanol and 80 % ethanol) and sample [different BTPW streams and black tea (BT)] and their interaction on the yield of phenolics (catechins, theaflavins and gallic acid), antioxidant and antimicrobial activity of the extracts were investigated. Total catechin (EC, EGCG, ECG) and theaflavin contents of BTPW samples were in the range of 5.2–6.0 and 11.5–16.0 mg/g DW, respectively. While catechins and theaflavins were recovered quantitatively using aqueous ethanol solvents, only 25–28 % of catechins and 6–7 % of theaflavins could be recovered in water extracts. Antioxidant activities of BTPW extracts, which were in the range of 2.23–3.59 (for DPPH), 0.59–0.92 (for FRAP) and 1.59–2.43 µmol TE/mg extract (for ABTS assay), were comparable to those of BT extracts. BTPW and BT extracts exhibited antimicrobial activity against S. aureus (1.26–3.65 mm), S. flexneri (1.33–3.89 mm) and B. cereus (1.87–3.90 mm); however, inhibition of C. albicans was not observed. BTPW can be used as a raw material for the development of value-added products with antioxidant and antimicrobial properties for applications in food, pharmaceutical, cosmetic and agricultural sectors. Aqueous ethanol solvents offer low-cost, non-toxic, green alternatives for the recovery of antioxidant and antibacterial phenolics from BTPW.