Bouillabaisse with rouille and baguette

Why

Fish has little collagen, which dissolves at low temperatures, and its proteins set earlier than in the meat of land animals (Fish: when is it done card), so pieces of fish in soup are done within minutes and break up with hard boiling and stirring. That is why they go into the soup or sauce only towards the end, into gently bubbling liquid, and only need to reach 63 °C in the centre; large firm pieces can take longer, delicate fillets go in last. Stock made from the bones, heads and trimmings of lean white fish simmers far shorter than meat stock, mostly 20–45 minutes, because many cooks find that longer cooking clouds it and turns it bitter; heads go in without the gills, whose blood gives the stock an off taste. Soups with cream or milk are no longer boiled hard once it is added, so the dairy doesn't curdle (Why milk and cream curdle card). In fish such as tuna, mackerel, sardines or herring, histamine forms when they are kept warm; neither boiling nor freezing destroys it and you usually can't smell or taste it, so fish for soup goes into the fridge as soon as you buy it.

How to do it

1. Simmer fish stock from the bones and heads of lean white fish, gills removed, gently for only 20–45 minutes and strain it through a fine sieve.
2. Cook the vegetables and potatoes in the soup first and add the fish when they are almost tender.
3. Cut the fish into larger pieces, add it to gently bubbling liquid and don't stir; just shake the pot gently.
4. Cook the fish for only a few minutes, until it is opaque and flakes, or to 63 °C in the centre; add delicate fillets and seafood last.
5. Add cream or milk at the latest together with the fish and don't let the soup boil hard afterwards.
6. Cool fish soup and refrigerate it within 2 hours; reheat it only once and only as much as you will eat (Cooling and storing leftovers card).

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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

1. Pat the fillet dry with kitchen paper before cooking so it fries rather than steams in the pan.
2. Push the thermometer into the thickest part; for fish that is safe for everyone, aim for 63 °C.
3. Without a thermometer, try a fork: the fish is done when it is opaque and separates easily into flakes.
4. 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.
5. Prawns are done when they are firm and opaque; throw away any mussels that stay closed during cooking.
6. Keep fresh fish in the fridge for 2 days at most, otherwise freeze it, and thaw it overnight in the fridge.

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