Kinetics of Ultrasound-Assisted Polyphenol Extraction from Spent Filter Coffee Using Aqueous Glycerol

Culinary filter coffee residues were utilized as raw material, with the aim of recovering functional polyphenolic components. The extraction was performed under ultrasonication using aqueous glycerol, and for this reason, the process was initially optimized with regard to glycerol concentration (Cgl) and extraction time (t), by implementing response surface methodology via a central composite (Box-Behnken) experimental design. The optimized conditions determined were Cgl = 3.6% (w/v) and t = 175 min. The comparative assessment, based on a kinetic assay, illustrated that the progression of the extraction was slower in aqueous glycerol than in water, yet aqueous glycerol was proven a more efficient extraction medium, providing 7.4% higher total polyphenol yield. Liquid chromatography-diode array-mass spectrometry analysis revealed that the major antioxidant phytochemicals in the aqueous glycerol extract were chlorogenic acids (caffeic acid derivatives), accompanied by some other polyphenolic metabolites.