Multi-Stage Microwave-Assisted Extraction of Phenolic Compounds from Tunisian Walnut (Juglans regia L.) Bark

This study aimed to optimize the extraction of total phenolic compounds (TPC) from Tunisian walnut bark using microwave treatment. Initially, a preliminary investigation was conducted to establish optimal levels for ethanol concentration, liquid–solid ratio, temperature, and time, which were then applied in subsequent conventional solvent extraction (CSE) experiments. To enhance the extraction yield, multi-stage microwave-assisted extraction (MS MAE) was evaluated using three microwave power settings: 100, 200, and 300 W. The results showed a statistically significant (p < 0.05) effect of microwave irradiation combined with multiple solvent extraction stages. The optimized MS MAE protocol, employing 300 W power, six stages of 10 min each, and a liquid–solid ratio of 10 mL/g, achieved an 86% recovery of TPC. In contrast, extraction involving 10 stages of 30 min each without microwave irradiation recovered only 79% of TPC. UHPLC–MS analysis revealed that the phenolic profile of the extracts was dominated by gallic acid, vanillic acid, and quercetin, and that microwave treatment did not significantly alter the qualitative or quantitative composition of these major phenolic compounds compared to conventional extraction. These findings demonstrate that MS MAE is a time-saving, energy-saving, solvent-reducing, and highly efficient extraction technology for producing bioactive extracts from walnut bark.