Spent coffee grounds (SCG) were supplied by NovaDelta-Comércio e Indústria de Cafés S.A. (Campo Maior, Portugal). The material
was dried in an oven at 60C until 5% moisture content and stored
for further extractions. Phenolic standards including gallic acid,
chlorogenic acid, protocatechuic acid, and quercetin; methanol
and Folin–Ciocalteu phenol reagent were purchased from Sigma
(Sigma–Aldrich GmbH, Sternheim, Germany). All other chemicals
used were of analytical grade and obtained from either Sigma–Aldrich or Merck (Darmstadt, Germany)