Effects of sodium metabisulfite treatments on Penaeus vannamei post mortem melanosis formation, sulfite levels and polyphenoloxidase activities

Sodium metabisulfite (SMBS) is the most widely used reagent for effective prevention of melanosis in the shrimp industry. Sulfite levels in shrimp frequently exceed guideline levels due to variations in SMBS application, disadvantaging consumers and causing economic losses. This study evaluates multiple factors that influence post-mortem melanosis development, sulfite residue in shrimp and phenol oxidizing enzyme activities. Factors include SMBS dipping solution parameters (concentration (1.5–2-5%), incubation time (5–20 min), salinity (0 and 20 ppt), condition of shrimp (dead or alive, stressed or unstressed) and repeated SMBS treatment. Based on the results of this study optimal concentration of SMBS dipping solutions in 20 ppt water was 2% with an incubation time of 10 min. Salt water use avoided exceeding 100 ppm (legal limits in US) sulfite concentration in shrimp. No polyphenoloxidase (PPO) activity was detectable in the hemolymph of shrimp treated with SMBS. However, it was possible to induce PPO activity in individual shrimp. No differences were detected in any tested parameter when using dead or alive shrimp for SMBS treatment. Cold stressed shrimp showed significantly increased melanosis development in controls ( p < 0.05). Treatment with SMBS effectively suppressed melanosis formation in stressed and unstressed shrimp with no significant differences between treatments. Repeated application of SMBS did not increase melanosis prevention, nor sulfite levels. PPO activity and endogenous trypsin activities were detected in post-mortem shrimp two days after SMBS treatment. Subsequent SMBS treatment did not inhibit PPO activity. Presumably PPO zymogens are not entirely inhibited by SMBS and likely activated post-mortem through endogenous proteases, including trypsin. • Sodium metabisulfite treatment of shrimp is recommended at a 2% dipping solution for 10 min in salt water. • Direct, post-mortem or repeated application of sodium metabisulfite does not result in different melanosis formation in shrimp. • Sodium metabisulfite treatment effectively inhibits polyphenoloxidases in the hemolymph. • Polyphenoloxidase zymogens remain partly functional in the hemolymph after sodium metabisulfite treatment.