The extent of protein nitrosation in the prepared sausages was
assessed by calculating the average number of 3-NT residues using the
method described by Vossen and De Smet (2015) with some modifications.
Briefly, two grams of sample was added to 20 mL of phosphate
buffer (20 mM, pH 7.0) and homogenized. Aliquot of 0.2 mL homogenates
(20 mg/mL) were treated with 0.4 mL of ice-cold TCA (20%). The
supernatant was discarded after centrifugation (3040 ×g, 10 min,
4 °C). Two milliliters of 6 M guanidine hydrochloride were added to dissolve
the pellets. After addition of 2 mL bicarbonate buffer (0.2 M,
pH 10.5), the sample was centrifuged again at 10,000 ×g for 10 min at
4 °C. The absorbance of supernatant was measured at 430 nm in a 96
well plate using 3-nitro-L-tyrosine as standard. The protein concentration
was measured by a bicinchoninic acid (BCA) protein quantification
assay (Thermo Fisher Scientific, Germany) using bovine serum albumin
(BSA) as standard. Results were expressed as nM of 3-nitrotyrosine per
mg of protein.