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Fish Safety In Depth

Histamine, ciguatera, parasites, mercury and what never to eat

Scombroid and histamine

Mackerel, tuna, bonito, sardine and anchovy carry a lot of histidine in their flesh. Left warm, bacteria convert it to histamine, which cooking, freezing and canning cannot remove. Eating it gives a flushed face, headache, sweating, racing heart, cramps and a peppery burning taste within minutes to an hour. It is usually treated like an allergic reaction and passes, but it can be severe. Prevention is entirely about temperature: bleed and gut these fish at once, get them onto ice or into a cold box within about half an hour of capture, and keep them below 4 degrees Celsius all the way to the pan.

Ciguatera

Ciguatoxins build up through tropical and subtropical reef food chains and concentrate in large predators such as barracuda, large groupers, amberjack, some snappers and moray eels. The toxin is heat stable, odourless and tasteless, and no home test exists. Symptoms mix stomach upset with strange neurological effects, classically a reversal of hot and cold sensation, tingling lips and hands, and they can persist for months. The only defence is local knowledge: in reef regions ask which species and which reefs are implicated, avoid large predatory reef fish entirely, and never eat the head, liver or roe of any suspect fish, where toxin concentrates.

Parasites and the freezing rule

Anisakis worms are common in cod, hake, herring, mackerel and many others, and they are why sushi is made from frozen fish. Cooking to 63 degrees Celsius in the thickest part kills them. For anything eaten raw, marinated, salted lightly, ceviche style or cold smoked, the food safety rule is to freeze first: minus 20 degrees Celsius for at least 7 days, or minus 35 degrees for about 20 hours, measured in the fish, not the freezer air. Many domestic freezers run warmer than minus 18, so use a thermometer. Gutting quickly helps, because worms migrate from the gut into the flesh after death.

Species and parts never to eat

Pufferfish, and all their relatives in the family Tetraodontidae, carry tetrodotoxin, which has no antidote and kills through paralysis. They are caught accidentally in Portuguese and Mediterranean waters as the silver cheeked toadfish spreads. Never eat one, never eat anything you cannot identify, and discard the whole fish rather than trimming around it. Do not eat the liver of large sharks, polar bears or halibut sized predators due to vitamin A toxicity, or the roe of species where it is known to be toxic. Freshwater eel blood is toxic raw, which is why eel is never served raw. Identify before you cook, always.

Mercury and reading spoilage

Methylmercury concentrates up the food chain, so swordfish, marlin, shark and large tuna carry the most. Food agencies generally advise adults limit these to about once a week, and advise pregnant women and young children to avoid them entirely and eat small oily fish instead. For spoilage, judge a whole fish by three things: the eyes should be clear and domed, not sunken and cloudy; the gills bright red, not brown or slimy; and the smell clean, of the sea, never ammonia or sourness. Flesh should spring back when pressed. Any ammonia smell means the fish is finished, whatever it looks like.

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