Fish: when is it done

Why

Fish flesh is more delicate than the meat of land animals: the muscle fibres are short and held together by thin layers of collagen, which is less heat-stable than mammalian collagen and dissolves at a lower temperature. In fish, the main muscle protein myosin starts to change structure at around 40 °C, so a fillet is done early, and a few degrees more are enough for it to lose its juices, dry out and fall apart. For safety, the US FDA and USDA give 63 °C in the core or, without a thermometer, flesh that turns opaque and separates easily into flakes with a fork; the UK FSA uses the same sign. Cooks often leave salmon or tuna translucent inside, at around 50 °C, which is a culinary choice at your own risk: according to the FDA, pregnant women, young children, older people and anyone with weakened immunity should avoid raw and undercooked fish, and for eating raw or with a translucent centre, choose fish that has been frozen first, because freezing kills parasites (EFSA gives, for example, −20 °C for at least 24 hours), though not all other germs.

How to do it

• Pat the fillet dry with kitchen paper before cooking so it fries rather than steams in the pan.
• Push the thermometer into the thickest part; for fish that is safe for everyone, aim for 63 °C.
• Without a thermometer, try a fork: the fish is done when it is opaque and separates easily into flakes.
• Cook thin fillets for just a few minutes and keep an eye on them; in a hot pan they finish cooking even after the heat is off.
• Prawns are done when they are firm and opaque; throw away any mussels that stay closed during cooking.
• Keep fresh fish in the fridge for 2 days at most, otherwise freeze it, and thaw it overnight in the fridge.

If it goes wrong

Dried-out fish won't turn juicy again; flake it and use it in a spread, fish cakes or a sauce. If it is still raw near the bone, put it back, covered, over a gentle heat or in the oven for a few minutes. Next time take a smaller piece off a little earlier and let carryover heat finish it.