A buyer opens a box of shrimp and sees dark spots forming near the shell. The instinct is to assume something went wrong — spoilage, contamination, mishandling. In almost every case, that instinct is wrong. What’s happening is called melanosis, and it’s one of the most misunderstood quality issues in the entire shrimp trade.
What melanosis actually is
Melanosis — known in the trade as “black spot” — is a natural enzymatic reaction, not decay. The moment a shrimp is harvested and dies, an enzyme called polyphenol oxidase (PPO, sometimes referred to as tyrosinase in shrimp) starts reacting with oxygen and naturally occurring phenolic compounds in the tissue. That reaction produces melanin pigments — the same basic chemistry behind why a cut apple turns brown. The black spots are cosmetic. The shrimp is not spoiled, and it’s safe to eat. But cosmetic doesn’t mean irrelevant: a buyer’s own customers judge quality by appearance first, and black spot tanks a shipment’s commercial value even when nothing is biologically wrong with it.
Why it can’t just be prevented by keeping shrimp cold
Cold storage slows melanosis — it doesn’t stop it. The PPO enzyme is still active at freezer temperatures; freezing only reduces the rate of the reaction, and melanosis can resume once the product thaws. That’s the detail most buyers miss: a perfectly frozen, perfectly cold-chain-managed shipment can still develop black spot if nothing was done to interrupt the enzyme itself before freezing.
How sodium metabisulfite actually stops it
Sodium metabisulfite (SMBS) works through more than one mechanism at once, which is part of why it’s remained the industry’s default treatment for decades despite newer alternatives:
- It acts as a reducing agent, converting the reactive intermediate compounds (quinones) back into their original, non-pigmented phenolic form before they can polymerize into visible melanin.
- It binds to the PPO enzyme’s active sites, directly reducing the enzyme’s ability to catalyze the reaction in the first place.
- It scavenges dissolved oxygen, removing one of the core ingredients the reaction needs to proceed.
Timing matters more than concentration. Research on Penaeus vannamei specifically found that metabisulfite treatment effectively suppresses melanosis for roughly three to six days — after that window, the reaction can begin again regardless of how the shrimp was treated initially. The treatment has to happen before the enzymatic process gets underway; once melanosis has visibly started, no treatment reverses it, only slows further spread.
Why this isn’t a niche technical detail for buyers
This is directly connected to the export compliance conversation the industry is having right now. Regulatory bodies — including China’s GACC — scrutinize metabisulfite residue levels precisely because it’s such a widely used compound, and testing methodology changes on their end have already affected export declarations from multiple Ecuadorian processors in 2026. A buyer who understands why metabisulfite is used, not just that it’s used, is better positioned to ask their supplier the right questions: what concentration was applied, when relative to harvest, and whether residual levels are documented within the regulatory thresholds that matter for their destination market.
What KATUNA verifies
KATUNA tracks metabisulfite application and residual-level documentation as a standard part of pre-shipment quality control — not as a reaction to a specific customer’s question. If traceability on this specific point matters for your import documentation, ask us for it directly; it’s part of what we already check before a lot ships.
Read more about KATUNA’s shrimp sourcing, formats and compliance practice on the shrimp product page →