Microwave-assisted deep eutectic solvent extraction of phenolics from defatted date seeds and its effect on solubilization of carbohydrates

In this study, for the first time, polyphenols are extracted using microwave-assisted deep eutectic solvents (MW-DES) from supercritical CO2 defatted date seeds under variable operating conditions. The best conditions consisted of choline chloride: formic acid (1:2 molar ratio), a solid to solvent ratio of 1/20 (g/mL) (0.75 g of dry biomass in 15 mL solvent), an extraction time of 50 s, and a microwave power of 400 W. The maximum polyphenols extraction yield was 128 g gallic acid equivalent/kg dry biomass (g GAE/kg) (which is the highest value ever reported in literature for date seeds extracts). The antioxidant activity of the optimum extracts was assessed with the DPPH and ABTS quenching capacities being equal to 27 and 145 mg trolox equivalent/g dry biomass, respectively. Mainly mannose and manno-oligosaccharides were solubilized during extraction. The highest yields were (obtained at 200 W, 120 s) 35.07 and 32.18 mg/g of dry biomass for mannose and manno-oligosaccharides, respectively. It was also observed that the total mannose (monomer and oligomer) release during the extraction increased linearly (r2 = 0.9951) with respect to microwave energy applied per gram of date seeds (J/g).