*2.2. Sample Preparation*

Before digestion, the cheese samples were incinerated at 500 ◦C for 24 h. For each cheese sample, the ash was weighed (0.5–1.5 g) and rinsed with nitric acid and three times with deionised water to remove acidic debris. For digestion, samples were transferred to TFM®PTFE vessels with 6 mL of concentrated 65% HNO3 (14.33 mol/L) and 1 mL of 30% H2O2. The samples were submitted to digestion in a microwave digestion apparatus (MW-AD, Ethos EZ microwave digester, Mileston, Shelton, CT, USA). The heating program for digestion consists of 4 steps: step 1 (90 ◦C for 7 min), step 2 (170 ◦C for 5 min), step 3 (210 ◦C for 5 min), and step 4 (210 ◦C for 20 min). In all steps the power was set to 1000 W. The final solutions were diluted up to 25 mL with doubly distilled water for analysis by Graphite Furnace Atomic Absorption Spectrometer (GFAAS).

#### *2.3. Graphite Furnace Atomic Absorption Spectrometer Analysis*

Analyses for Cr, Mn, Se, and Zn were performed by Atomic Absorption Spectroscopy (AAS) according to the AOAC International method, 1995. An AA-6300 atomic absorption spectrophotometer (Shimadzu, Columbia, MD, USA) was used, equipped with an ASC-6100 autosampler (Shimadzu, Columbia, MD, USA) and GFA-EX7i graphite furnace atomizer (Shimadzu, Columbia, MD, USA). The control of instruments and analysis of data were performed using Multi-Element Program Software (WizAArd software, Shimadzu, Columbia, MD, USA). Argon was used as the internal and external gas. The AAS instrument was equipped with a hollow cathode lamp for Cr, Se, Zn and Mn line sources. A

deuterium lamp was used as a background corrector. Graphite pyrolytically coated tubes with a L'vov platform were employed. To optimize the analytical signal, various tests were performed with different lamp intensities, sample injection volumes and temperature ranges (1600–1800 ◦C for atomization).

#### *2.4. Reagent and Calibration Curves*

The water used was 18 megohm water, purified with a Milli-Q® Integral 10 system (Merck, Darmstadt, Germany).

A selenium standard solution of 1000 mg/L, and a multi-element standard solution IV (at the concentration of 1000 mg/L): Ag, Al, Ba, Bi, Ca, Cd, Co, Cr, Cu, Fe, Ga, In, K, Mg, Mn, Na, Ni, Pb, Sr, Tl, Zn, were purchased from Merck KGaA (Darmstadt, Germany).

Calibration blank (Cal Blk) and calibration standard (Cal Std) solutions were prepared at concentrations of 0 μg/L (Cal Blk), 4 μg/L (Cal Std 1), 12 μg/L (Cal Std 2), and 20 μg/L (Cal Std 3), using the selenium standard stock solution, or multi-element standard stock solution, into separate 50 mL DigiTUBE® tubes (SCP SCIENCE, Baie-D'Urfe, QC, Canada) with the addition of 0.5 mL internal standard stock solution, 0.5 mL methanol and 5 mL concentrated nitric acid, and then diluted to the final volume with water.
