**5. Conclusions**

In conclusion, the developed mass spectrometry based analytical methods show to be suitable to notice the illicit treatment of fishery products with hydrogen peroxide. Both the UHPLC-MS method and the HS SPME-GC-MS method are applicable to detect molecules related to the use of hydrogen peroxide solution to whiten and refresh aged fish food.

By LC-MS, low molecular mass amines were detected with high selectivity and good sensitivity. The TMAO and TMA ratio was shown to be reversed by the illicit treatment, simulating an unreal apparent freshness of fishery products foods.

The use of hydrogen peroxide on fish products was confirmed by the measurement of 2-hydroxyanisole with HS SPME-GC-MS after a redox reaction between anisole and residual H2O2 in the extracted solution, by exploiting potassium ferricyanide catalysis. After the optimization of sample preparation for headspace solid-phase microextraction and redox reaction parameters, the method was suitable to quantify the H2O2 residues in fish food matrices. When fishery products, especially squid, were whitened with hydrogen peroxide, its amount was shown to be easily detectable.

**Author Contributions:** Conceptualization, F.D.B., V.G., and C.M.; Methodology, F.D.B., M.Z., and R.A.; Validation, M.Z. and R.A.; Formal analysis, F.D.B.; Data curation, F.D.B.; Writing—original draft preparation, F.D.B.; Writing—review and editing, C.M. and R.A.; Supervision, C.M.; Project administration, C.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
