Modulation of phenolic extraction from grape seeds by varying the composition of natural deep eutectic solvents

Abstract

Grape by-products, particularly seeds, are of great interest owing to their content of phenolic compounds, which offer significant health benefits. Traditional organic solvents used for phenolic extraction can be toxic and are highly flammable, raising environmental concerns. In contrast, natural deep eutectic solvents (NADES) present a greener alternative, as they are formed through hydrogen bonding between donors like sugars, amines, carboxylic acids, amino acids or alcohols, and acceptors like quaternary amines. The number and strength of these hydrogen bonds influence the physicochemical properties of NADES, and these in turn influence the extraction efficiency and selectivity of phenolic compounds. This study aimed at understanding how the physicochemical properties of NADES, particularly polarity, viscosity, conductivity and pH, affected the extraction efficiency of phenolic compounds from grape seeds, their antioxidant activity and the selectivity toward reducing sugars, compared with a hydroalcoholic solvent. NADES with different physicochemical properties were chosen, namely choline chloride with 1,2-propanediol, urea or malic acid, and at varying water contents (25, 50 and 75\% w/w). The highest extraction efficiencies were obtained with 1,2-propanediol (TPC 5.13\% DW) and malic acid (TPC 5.11\% DW) at 50\% water. Polarity was found to be a key property as matching the polarity of NADES to that of the target molecules led to improved extraction efficiency. In the case of malic acid, a combination of polarity and acidic pH led to high phenolics extraction efficiency and selectivity.