Raw fish: ceviche, tartare, sushi

Why

Acid from lime or lemon sets fish proteins, so the flesh turns white and firm, but it does not reliably destroy Anisakis roundworm larvae or bacteria; according to EFSA, most traditional marinades and cold smoking are not enough either. The larvae live in the guts and muscle of sea fish such as cod, herring, mackerel or wild salmon, and EFSA states that no sea fishing grounds can be considered free of this parasite; a live larva eaten causes stomach pain and in some people an allergy. EU Regulation 853/2004 therefore requires fish intended to be eaten raw to be frozen throughout to −20 °C for at least 24 hours or to −35 °C for at least 15 hours, except farmed fish fed on feed without live parasites, typically farmed Atlantic salmon. A home freezer freezes more slowly and often holds only −18 °C, so longer times apply at home: Spain’s AESAN gives at least −15 °C for 5 days from loading, the US FDA −20 °C for 7 days. Freezing does not kill bacteria such as listeria, however, so young children, pregnant women, older people and anyone with weakened immunity should not eat raw fish, and according to the UK NHS pregnant women should avoid it even if previously frozen.

How to do it

• Buy fish for ceviche, tartare or sushi explicitly sold for eating raw, and ask the seller whether it has been frozen against parasites.
• Before preparing fresh sea fish from the counter raw, freeze it for at least 7 days in a freezer that holds −18 °C or colder; check the freezer with a thermometer.
• Thaw the fish in the fridge, cut it chilled on a clean board, and keep it in the fridge at 4 °C or below until serving.
• Leave ceviche in the acid in the fridge for only 15–30 minutes and serve it at once; the acid flavours the fish but does not make it safe.
• Do not eat freshwater fish such as carp or zander raw; they can carry flukes that the prescribed freezing may not kill.
• For young children, pregnant women, older people and anyone with weakened immunity, cook the fish to 63 °C in the centre (Fish: when is it done card).

If it goes wrong

If you do not know whether the fish has been frozen, cook it briefly to 63 °C in the centre instead of making ceviche or tartare; neither acid nor salt replaces freezing. If you see coiled whitish larvae in the fish, do not eat it raw; cut away a generous margin around them and cook the rest. Sharp stomach pain, nausea or vomiting within hours of eating raw fish, or hives, may mean anisakiasis or an allergy: see a doctor, and call emergency services at once if swelling or breathing difficulty appears.