Effect of sodium nitrite on nitrosomyoglobin content, visible spectrum of myoglobin and nitrosylation in yak meat. (a) nitrosomyoglobin content. (b) Visible spectrum of myoglobin at 510–610 nm. (c-g) LC-MS/MS primary mass spectra of nitrosylation. c: 0d, d: control 3d, e: NaNO2 3d, f: control 7d, g: NaNO2: 7d. The small letters indicated the significant difference of the control group and the capital letters indicated the significant difference of the treatment group (P < 0.05). The * indicates the significant difference between groups, *: (P < 0.05); **: (P < 0.01).