Sequential Microwave-Ultrasound-Assisted Extraction and Optimization of Phenolic Compounds from Black Tea Waste

The fibrous waste generated during the processing of black tea presents a significant opportunity for recovering valuable bioactive compounds. Our study focused solely on establishing the optimum process conditions to maximize the yield and antioxidant activity of phenolic compounds from this readily available agro-industrial byproduct. For the extraction, a novel sequential microwave-ultrasound-assisted approach (MUAE) was employed with four key process parameters, including microwave power (300-900 W), irradiation time (50-200 s), ultrasound temperature (20-50 °C), and ultrasound time (15-45 min). The process was optimized using a four-factor, three-level Box-Behnken Design (BBD). Analysis of variance (ANOVA) demonstrated a strong fit of the second-order polynomial models for total phenolic content (TPC, R²=0.92), total flavonoid content (TFC, R²=0.93), and DPPH radical scavenging activity (R²=0.87). The optimized conditions (199.98 s of irradiation time at 899.97 W power followed by 24.09 min of sonication at 36.26 °C temperature) yielded experimental values of 95.74 ± 1.09 mg GAE/g DM for TPC, 37.16 ± 1.23 mg QUE/g DM for TFC, and 78.09 ± 0.68% for DPPH activity. Additionally, the extraction efficacy was found to be superior for the sequential MUAE technique when compared to individual microwave, individual ultrasound, and combined ultrasound--microwave extraction methodologies. The findings of our study signify the utilization of factory tea waste as a source of phenolic compounds and further provide a sustainable and efficient extraction method, opening avenues for its application in the food, pharmaceutical, nutraceutical, and cosmetic industries.