*2.1. Materials*

Roasted hazelnut skin of Tonda Gentile delle Langhe PGI was provided by La Gentile srl (Cortemilia, Italy) and obtained by roasting for 7 min at 190 ◦C. All chemicals used were of analytical grade. Folin–Ciocalteu phenol reagen<sup>t</sup> (2 M), 2,2-diphenyl-1-picrylhydrazyl (DPPH), 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (97%; Trolox), methanol (99.9%), formic acid (98–100%), and ethanol (99.9%) were provided by Sigma-Aldrich (St. Louis, MO, USA). High-performance liquid chromatography (HPLC)-grade standards (gallic acid, protocatechuic acid, catechin, epicatechin, and quercetin) were purchased from Fluka (Buchs, Switzerland). Ultrapure water was prepared using a Milli-Q filter system (Millipore, Billerica, MA, USA). Amberlite macroporous resins (XAD 4, XAD 7, and XAD 16) were supplied by Sigma-Aldrich (Table 1). Before use, the resins (50 g) were soaked in pure ethanol (100 mL) for 3 h at 22 ◦C with constant rotatory agitation on a VDRL 711 orbital shaker (Asal S.r.l, Milan, Italy) at 60 rpm. The ethanol was removed, and the resins were rinsed with excess ultra-pure water.


**Table 1.** Physicochemical characteristics of Amberlite resins used for the recovery of polyphenolic compounds by a roasted hazelnut skin extract.
