Effects of Sodium Hypochlorite Bleaching on the Quality and Safety of Basa (<i>Pangasius bocourti</i>) Fish Maw: Texture, Collagen Structure, and Semicarbazide Formation

This study aimed to investigate the effects of sodium hypochlorite (NaClO) bleaching on the quality of Basa (<i>Pangasius bocourti</i>) fish maw (BFM) and the formation of semicarbazide (SEM). Production of SEM increased (<i>p</i> < 0.05) when NaClO concentration, soaking temperature, or duration were increased. Notably, increasing NaClO solution pH also enhanced SEM formation. Soaking BFM in NaClO with available chlorine concentrations of 500, 700, and 1000 mg/L generated 0.05, 0.07, and 0.09 μg/kg SEM at pH 3 compared to 0.70, 1.19, and 2.34 μg/kg SEM at pH 11, respectively. NaClO improved BFM texture by creating a tight, fibrous structure, but also damaged the secondary structure and α-chains of collagen. Untargeted metabolomics showed that NaClO treatment significantly upregulated lipid metabolism pathways (biosynthesis of unsaturated fatty acids, linoleic acid metabolism, and glycerophospholipid metabolism) and elevated degradation of arginine, proline, and urocanic acid. This was associated with the accumulation of nitrogen-containing precursors in the urea cycle, which then reacted with NaClO, generating substantial SEM. Controlled SEM-generating reactions experiments confirmed that SEM was produced from reaction of urea and NaClO. This study elucidates the mechanism of SEM formation and identifies key factors influencing SEM levels, thereby providing a theoretical foundation for safe processing and quality control of fish maw.