Why do some fish go milky when they are cooked?

I have had over the years a few fish fillets that go all mushy and milky when cooked. What is the reason for this milky appearance?

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The “milky” problem sometimes observed in cooked Yellowtail Kingfish and other species is due to a microscopic parasite (from the class Myxosporea), which forms spores in the flesh of the fish while it is still alive. The parasite occurs in fish caught in warmer waters.

Symptoms aren’t evident until the fish is cooked – in its raw state it is perfectly fine to eat. Once cooked however, the flesh collapses into a mushy mass, which looks and tastes unappetizing – although it is not harmful in its raw or cooked state.

The collapse in the flesh is due to the action of an autolytic enzyme called protease. It becomes active at certain pH levels and certain temperatures, and then breaks down the protein and elastin in the flesh, causing it to “melt” into a mushy mass. It is most active at low pH (around 3) and at cooking temperatures. Ironically the fish’s taste and texture is unaffected if eaten raw.

Similar microscopic spore-forming parasites are seen in other species of fish including Swordfish, Barracouta, Lemon Sole, Hake and Flounder. In raw Yabbies and Prawns the presence of this parasite is indicated by a white milky-coloured flesh instead of the usual translucent flesh; it is not evident in cooked crustaceans, as the flesh turns white anyway when cooked.

If you encounter this problem, inform your fishmonger immediately and return the fish for an exchange or refund.

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