Recovery of Syringic Acid from Industrial Food Waste with Aqueous Solutions of Ionic Liquids

Phenolic acids present in industrial food waste display a broad range of biological activities and related health benefits, among which their strong antioxidant and free-radical scavenger activities are the most investigated. However, food waste is still scarcely considered as an alternative source for these compounds, and volatile organic solvents for their extraction are still the preferred choice. In this work, aqueous solutions of ionic liquids (ILs) with hydrotropic or surfactant character were investigated to improve the solubility and effectively extract syringic acid from Rocha pear peels, a relevant waste of the food industry. The solubility of syringic acid in aqueous solutions of a wide variety of ILs at different concentrations at 30 degrees C was first ascertained. The results obtained show that ILs that behave as cationic hydrotropes are the best option to enhance the solubility of syringic acid in aqueous media, with increases in solubility of up to 84-fold when compared with water. After identifying the most promising IL aqueous solutions, a response surface methodology was used to optimize operational extraction conditions (extraction time, solid-liquid (biomass-solvent) ratio, and temperature), leading to a maximum extraction yield of syringic acid of 1.05 wt % from pear peels. Both the solvent and biomass reuse were additionally investigated, allowing to overcome the biomass-solvent ratio constraints and mass-transfer effects and leading to extraction yields of 2.04 and 2.22 wt %. Although other methods for the recovery of syringic acid can be applied, taking advantage of the hydrotropy phenomenon and the solubility of syringic acid dependency with the IL concentration, water was used as an antisolvent, allowing to obtain 77% of the extracted phenolic acid. A continuous countercurrent process conceptualized for large-scale applications and that further allows the solvent recycling after the recovery of syringic acid is finally proposed.