Fish soups and stews

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

• 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.
• Cook the vegetables and potatoes in the soup first and add the fish when they are almost tender.
• Cut the fish into larger pieces, add it to gently bubbling liquid and don't stir; just shake the pot gently.
• 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.
• Add cream or milk at the latest together with the fish and don't let the soup boil hard afterwards.
• 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).

If it goes wrong

Broken-up fish tastes just the same; serve it as a thicker fish soup and next time add it later and don't stir. The only way to get small bones out of finished soup is to lift out the pieces of fish, feel through them and return them, then strain the broth. Dilute bitter or gluey stock with water or vegetable stock and season with lemon. Don't eat fish that smells bad; if it tastes unusually sharp, peppery or tingles on the tongue, it may contain histamine. A normal taste is no guarantee of safety, though – only keeping it cold is.