Hydrochloride acid

The determination of total monomeric anthocyanins was based on the pH differential method (Lee and others 2005) . For the test, potatoes were removed from the pouches, they were placed on a paper for 1 min, and then the samples were slightly dried with a paper. For sample preparation 40 g of cooked potato were chopped. After, 2 g of the chopped product was homogenized for 30 seconds with 20 mL of methanol (Panreac, Barcelona, Spain) and 0.1 mL of hydrochloride acid (37% HCl, Panreac, Barcelona, Spain). The homogenate was stored during 24 h at 4 °C in dark conditions, and after, it was centrifuged (10.000 rpm, 10 min, 4 °C) to obtain a supernatant. Aliquots of 0.4 mL were added to 3.6 mL of pH 1.0 buffer and pH 4.5 buffer, prepared as suggested by Lee (2005) . After waiting between 20 min and 50 min, samples were evaluated at λ = 700 and 530 nm in a spectrometer (Helios Zeta UV-VIS, Thermo Fisher Scientific, UK). The anthocyanin pigment concentration was expressed as cyanidin-3-glucoside equivalents per 100 g of cooked samples (molecular weight = 449.2 g/mol for cyanidin-3-glucoside (cyd-3glu); ε=26900 molar extinction coefficient, in L mol -1 cm -1 ). Four repetitions were done for each cooking treatment.

Tetraethylorthosilicate (TEOS) 98% (M = 208.33 g/mol, d = 0.934 g/mL), poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (EO20PO70EO20, Pluronic 123, Mav = 5800 g/mol, d = 1.019 g/mL), cetyltrimethylammonium bromide (CTAB) 98%, (M= 364,46 g/mol), octadecylsilane (OTES) 97% (M = 284.61 g/mol, d = 0.795 g/mL), E1, 17β-E2, E3, P, HEX and DES were purchased from Sigma-Aldrich (St. Louis, MO, USA). AND, EE2, 17α-MT, NAN, PRED and T were purchased from Fluka (Busch, Switzerland). Ethanol absolute was purchased from SDS (Peypin, France ). Hydrochloride acid 35% (M = 36.45 g/mol, d = 1.19 g/mL) was purchased for Panreac (Castellar del Vallès, Barcelona, Spain). HPLC-grade solvents acetonitrile (ACN) and methanol (MeOH) were purchased from Sigma-Aldrich (St. Louis, MO, USA).