Ultrasound-assisted extraction of polyphenols and anthocyanins from starch extracted sweet potato peels: optimisation and comparative study of deep eutectic solvent with methanol-based extraction

Abstract Ultrasound-assisted extraction (UAE) enhances extraction rates while preserving the structural integrity and functionality of thermosensitive compounds, such as polyphenols and anthocyanins. Additionally, deep eutectic solvents (DES) are emerging green alternatives to organic solvents, like methanol, for polyphenols and anthocyanins extraction. However, from a green engineering perspective, optimisation of ultrasound-assisted deep eutectic solvent (UA-DES) extraction conditions is necessary to enhance yield and extraction efficiency beyond the conventional methods, because DES type and concentration, temperature, time, and solid-to-liquid ratio can significantly influence the extraction process. In this study, the three-level Box-Behnken experimental design was applied to optimise UA-DES processing time, DES (choline chloride and lactic acid) concentration, and liquid-to-solid ratio for extracting polyphenols and anthocyanins from starch-extracted sweet potato peels (SSPP). The UA-DES extraction efficiencies for total phenolic content (TPC) and total anthocyanin content (TAC) were compared to those of ultrasound-assisted methanol (UA-methanol). In both extractions, ultrasound-assisted water (UA-water) extraction was used as a control. The yield was calculated based on neochlorogenic acid. At the optimal extraction conditions (22.5% v/v water/ DES, 20 mL/g liquid: solid ratio, and 10 min processing time), the UA-DES yield, TPC, and TAC were about 75 µg/g dw, 10.54 mg gallic acid equivalent (GAE)/g dw, and 17.83 g/g dw, respectively, compared to 40.24 µg/g dw, 7.90 mg GAE/g dw, and 13.78 g/g dw for UA-methanol. Therefore, UA-DES is a potentially efficient green method for integration in the starch production process to obtain high-value-added products from SSPP. However, a comprehensive sustainability analysis is necessary for scale-up production.