Surströmming (“sour herring”) is Baltic herring that’s lightly salted and left to ferment, then canned while fermentation is still going on – so the cans bulge. It comes from the coast of northern Sweden, is famous for one of the strongest smells of any food, and is traditionally eaten in late summer, outdoors, wrapped in thin flatbread with potatoes, onion and sour cream.
Why Does Surströmming Exist?
Surströmming has been made along Sweden’s northern Baltic coast for centuries. The usual explanation is economic: salt was expensive, and fermenting herring with only a little salt let fishing communities keep their spring catch for months. What started as a way to preserve fish became a regional food tradition. Until 1998 Swedish rules set a date before which new-season surströmming couldn’t be sold, to make sure it was fully ripened; producers still keep the surströmmingspremiär on the third Thursday of August.
How It’s Made
Spring-caught Baltic herring (strömming) are salted and fermented in vats over the summer. The fish’s own enzymes and salt-tolerant bacteria break down protein and fat, then the fish are canned in brine. Fermentation continues inside the can, producing gas – which is why surströmming cans bulge.
Despite the name, surströmming isn’t acidic like sauerkraut. Samples from three Swedish producers had a near-neutral pH of 6.7–7.0, some acetic acid and very little lactic acid. Their microbes were dominated by salt-loving bacteria such as Halanaerobium praevalens, Alkalibacterium gilvum, Carnobacterium and Tetragenococcus halophilus (Belleggia et al., 2020). Strictly anaerobic Haloanaerobium isolated from the cans may contribute to both the intense flavor and the swollen cans (Kobayashi et al., 2000).
Why Does It Smell So Strong?
The same analysis found large amounts of trimethylamine (the classic “fishy” smell) and sulfur compounds, plus 1,2,4-trithiolane, phenols, ketones, aldehydes and esters (Belleggia et al., 2020). Short-chain acids from fermentation add a sour, cheesy note. The gas and brine under pressure spray out when the can is pierced, which is why it’s opened outdoors.
Is Surströmming Healthy?
- It won’t change your gut bacteria. In a Karolinska Institutet study, 19 healthy volunteers ate a single surströmming meal and one adult ate it as their main protein for a week; researchers found no changes in their gut microbiome and couldn’t recover the fish’s dominant Halanaerobium in stool samples (Kallner et al., 2023).
- Dioxins and PCBs. Herring from the Baltic Sea and Gulf of Bothnia can contain high levels of dioxin and PCB. The Swedish Food Agency advises children, young people, pregnant and breastfeeding women and anyone who may want to become pregnant to eat it no more than 2–3 times a year; others can eat it once a week (Livsmedelsverket).
- Salt. It’s a salt-cured fish, so eat it in small portions if you watch your sodium.
- Histamine. Genes for histamine-producing bacteria have been detected in surströmming from all three producers tested (Belleggia et al., 2021), so people sensitive to histamine may want to avoid it.
- Nutrients. Like other herring, it provides protein and omega-3 fats – but as an occasional festive food, not a staple.
How to Eat Surströmming
- Open it outdoors, ideally with the can held under water in a bucket so the pressurized brine doesn’t spray you.
- Rinse and fillet the fish (cans often contain whole fish with roe); rinsing tones down the smell.
- Make a surströmmingsklämma: soft flatbread (tunnbröd) with potato, fish, red onion, sour cream and dill (the recipe card below).
- Drink: traditionally cold beer, snaps or milk.
Buy surströmming from Swedish producers or Scandinavian food importers. Don’t try to make it at home. Leftovers go into a sealed container in the fridge and should be eaten within a day or two.
Other northern European fish ferments: Norway’s rakfisk and Iceland’s hákarl; further south, Italy’s colatura di alici.