Effect of acidity/alkalinity of deep eutectic solvents on the extraction profiles of phenolics and biomolecules in defatted rice bran extract

This study investigated the influence of deep eutectic solvent (DES) acidity/alkalinity on the extraction profiles of phenolics and other biomolecules (phytic acid, reducing sugar, and protein) in defatted rice bran (DFRB). The DES with varying pH levels were prepared using different hydrogen bond acceptors (choline chloride (ChCl) and potassium carbonate (K2CO3)) and hydrogen bond donors (lactic acid, urea, and glycerol). The results reveal that the acidic DES (ChCl-lactic acid; pH 0.42) demonstrated superior extraction efficiency for total phenolic acids (4.33 mg/g), phytic acid (50.30 mg/g), and reducing sugar (57.05 mg/g) while having the lowest protein content (5.96 mg/g). The alkaline DES (K2CO3-glycerol; pH 11.21) showed the highest levels of total phenolic acid (5.49 mg/g) and protein content (12.81 mg/g), with lower quantities of phytic acid (1.04 mg/g) and reducing sugar (2.28 mg/g). The weakly acidic DES (ChCl-glycerol; pH 4.72) exhibited predominantly total phenolics (3.46 mg/g) with lower content of protein (6.22 mg/g), reducing sugar (1.68 mg/g) and phytic acid (0.20 mg/g). The weakly alkaline DES (ChCl-urea; pH 8.41) resulted in lower extraction yields for total phenolics (2.81 mg/g), protein (7.45 mg/g), phytic acid (0.10 mg/g), and reducing sugar (7.36 mg/g). The study also explored the distribution of phenolics among various DESs, with the alkaline DES (K2CO3-glycerol) containing the highest concentration of free phenolics. Notably, ChCl-based DESs predominantly contained soluble esterified bound phenolics and soluble glycosylated bound phenolics. Furthermore, a significant correlation between antioxidant activities and phenolic contents was observed. In conclusion, this study has revealed that the acidity and alkalinity of a DES significantly impact the extraction of phenolics and other value-added biomolecules in DFRB. These findings highlight the potential for manipulating the properties of DESs through pH variation, making them versatile solvents for extracting and isolating valuable compounds from agricultural by-products like DFRB and offering opportunities for sustainable utilization and value addition in various industries.