Once the heart stops, digestive enzymes leak out of the hepatopancreas and start taking apart the protein of the crab itself, leaving free amino acids.A live crab's digestive enzymes are held in the hepatopancreas. The moment the heart stops they leak, and they start taking apart the crab's own protein — a pile of free amino acids. This step needs no outside bacteria. It is why dead-crab meat goes mealy and soft.
2 · Bacteria take over a ready table of amino acids
Once the host dies, immunity no longer holds its bacteria down, and they grow on the amino acids from step one, faster the warmer it is.The gut and surface already carry bacteria. While it is alive, immunity holds them down. The host dies, the hold lifts, and step one has set out a ready table of amino acids. The warmer it is, the faster they grow.
3 · You killed the workers who made it, not the thing itself
Bacteria turn the amino acids into biogenic amines; heat kills the bacteria but not these heat-stable small molecules, which stay no matter how long you steam.Bacteria remake the amino acids into biogenic amines. The dangerous carrier has switched from live bacteria to a pile of compounds. High heat denatures bacterial protein and the bacteria do die. A biogenic amine is not a protein. It is a simple, heat-stable small molecule, still there no matter how long you steam.
4 · Heat can stop it continuing; it cannot roll it back
Spoilage cannot be reversed: heat can stop it going further but cannot roll it back, so cook live crab at once and discard a dead one.Spoilage is irreversible. Next time the thought cook it and you are fine comes up, ask first: am I dealing with something still alive, or with what it has already left behind. The rule is hard: kill live, cook now; a dead crab is discarded.