Nitric acid

Pd (99.95%) was obtained from Silver Gold Bull. β-Nicotinamide adenine dinucleotide hydrate (≥99%), β-Nicotinamide adenine dinucleotide, reduced disodium salt hydrate (≥97%, HPLC), propionic aldehyde (reagent grade, 97%), ammonium chloride, bovine serum albumin, L-lactic dehydrogenase from rabbit muscle, alcohol dehydrogenase from Saccharomyces cerevisiae, L-alanine dehydrogenase from Bacillus subtilis, sulfuric acid (95.0–98.0%), H₂O₂ solution (30 wt.% in H₂O) and dimethylsulfone (quantitative NMR standard, TraceCERT) were purchased from Sigma Aldrich and used as received. Sodium pyruvate (≥99%) was purchased from Fisher Scientific. 68–70% Nitric acid was obtained from VWR. Pt gauze (52 mesh, 99.9%) and Pt wire (0.5 mm, 99.95%) were obtained from Alfa Aesar. Ag/AgCl reference electrodes (RE5B) were purchased from BASi.

Acetonitrile (AcN), methanol (MeOH), and Folin–Ciocalteu reagent were purchased from Merck (Darmstadt, Germany), phosphoric acid from Sigma-Aldrich (St. Louis, MO, USA), and acetic acid, hydrogen peroxide, and nitric acid from VWR International S.A.S. (Fontenay-sous-Bois, France). Apigenin, Apigenin-7-O-glucoside, catechin, hydroxytyrosol, luteolin, luteolin-7-O-glucoside, oleuropein, rutin, tyrosol, and verbascoside were purchased from Extrasynthese (Genay, France). All chemicals were of analytical grade purity and were used without further purification.
Deionised water was obtained using a Hydrolab 10 SP purification system (Hydrolab, Straszyn, Poland).

Mineral analysis (Ca, P, Na, Mg, Fe, Zn, Mn, Cu, and Co) of the formulated biscuits was carried out using a standard method (AOAC, 2010 ). Eight-milliliters of concentrated nitric acid (16.2 mol/L) (VWR Chemicals, Fontenaysous-Bois, France) and 2 mL of 9.8 mol/L hydrogen peroxide (SigmaAldrich, USA) were used to digest 0.5 g of each sample overnight in a fume chamber. This was followed by digestion at the following temperatures: 75 °C for 30 min, 120 °C for 20 min, 180 °C for 20 min, and 200 °C for 10 min in a temperature-controlled block digester (Model TE007-A, TECNAL, São Paulo, SP, Brazil). After cooling, the wet ashes were dissolved in 0.4 mol/L nitric acid and then diluted ten-fold to appropriate concentrations based on the mineral element and the resulting calibration curve. Calibration curves for quantification were obtained by also measuring standard solutions from certified stock solutions (Chem Lab, Zedelgem, Belgium) serially diluted to give calibration standards of 400, 800, 2000, and 4000 g/L. The contents of each investigated element and the standard were determined by inductively coupled plasma optical emission spectrometry (ICP-OES) measurements (Optima 4300™ DV ICP-OES, Perkin Elmer, Wellesley MA, USA). Perkin Elmer Winlab 32 software (Perkin Elmer, USA) was used for external standard calibration and data acquisition. All samples were analyzed in duplicates.

Palm kernel seeds were purchased in the local market. Cobalt(II) chloride hexahydrate (CoCl₂·6H₂O, 98%, Sigma Aldrich, St. Louis, MO, USA), Cobalt(II) nitrate hexahydrate (Co(NO₃)₂·6H₂O, ≥99.0%, Sigma Aldrich, Darmstadt, Germany), Iron(III) nitrate nonahydrate (Fe(NO₃)₃·9H₂O, Sigma Aldrich, St. Louis, MO, USA) potassium hydroxide (KOH, ≥85% Carl Roth, Karlsruhe, Germany), sulfuric acid (95%, VWR, Fontenay sous bois, France), nitric acid (65%, VWR, Darnstadt, Germany) and hexane (97%, VWR, Gliwice, Poland) were used as received, without further purification.

Acetone and methanol of HPLC grade, nitric acid, hydrochloric acid, and 25% (v/v) hydrogen peroxide of purity for trace analysis were purchased from VWR (Darmstadt, Germany) and Sigma-Aldrich (Saint-Quentin-Fallavier, France). A 25% (m/v) tetramethylammonium hydroxide (TMAH) solution in methanol and a second in water were purchased from Alfa Aesar (Kandel, Germany) and JKchemical (Pforzheim, Germany), respectively. Boric acid of analytical purity (SO-press001) was purchased from PD Instruments (Kleve, Germany). Environmental Calibration Standard (5183–4688) containing metal ions at two concentration levels was purchased from Agilent Technologies (Stara Zagora, Bulgaria). Three certified reference materials (CRMs) prepared from human hair (NCS DC73347a and NCS ZC 8100 2b, CISRI, Beijing, China, and ERM-DB001, ERM, Geel, Belgium) were used toward calibration of ED-XRF and methods’ validation. Milli-Q Integral Elix system (Merck Milipore, Darmstadt, Germany) was used to prepare ultrapure water (18.2 MΩ/cm).

Dissolving birch wood pulp was kindly provided by the Enocell Mill (Stora Enso, Uimaharju, Finland). Urea (99.5%) (Acros Organics, Geel, Belgium), Eurodenatured ethanol (>99%), hydrochloric acid (37%), sodium hydroxide and nitric acid (69%) were purchased from VWR (Leuven, Belgium). Carboxymethyl cellulose with a 250 kDa mass and DS of 0.9 was obtained from Acros Organics.