Surströmming is fermented Baltic herring, a traditional Swedish food famous for producing one of the most intense smells of any food on Earth. The small herring are caught in spring, lightly salted, and left to ferment for at least six months before being sealed in cans that often bulge from the gases building up inside. What arrives at the table is soft, pungent, sour fish that Swedes in northern regions have been eating for centuries.
How Surströmming Is Made
The process starts in May and June, when Baltic herring are caught just before spawning. At this stage the fish are lean and in prime condition. They’re placed in a strong brine solution for about 20 hours, which draws out the blood. After that, workers remove the heads and innards and transfer the fish into a weaker brine.
This lighter salt concentration is the key to the whole process. There’s just enough salt to prevent the fish from truly rotting, but not enough to stop fermentation. The low-salt environment lets a group of bacteria called Halanaerobium thrive inside the fish. These microbes break down glycogen (the fish’s stored energy) into organic acids, which is what gives surströmming its distinctly sour taste. After fermenting in barrels, the fish are canned, but fermentation doesn’t stop there. The bacteria keep working inside the sealed can, producing carbon dioxide that causes the tin to swell into a rounded, pressurized shape. By the time you open a can, the fish have been fermenting for at least six months.
What Creates the Smell
The smell is not just “strong fish.” It’s a cocktail of compounds that individually smell terrible and together create something truly overwhelming. The Halanaerobium bacteria produce hydrogen sulfide (the rotten-egg smell), butyric acid (rancid butter), propionic acid (sharp and pungent), and acetic acid (vinegar). Chemical analysis using gas chromatography has also found massive amounts of trimethylamine, the compound responsible for the classic “fishy” smell in decomposing seafood, along with sulfur compounds, phenols, ketones, and aldehydes.
This is why people routinely open surströmming outdoors. The pressurized can releases a burst of these gases the moment it’s punctured, and the smell can linger in enclosed spaces for days. Many airlines have banned the cans from flights, citing the pressurized containers as a safety concern. Some landlords in Sweden have even argued that opening surströmming indoors constitutes a lease violation.
How Swedes Actually Eat It
Despite its reputation as an internet challenge food, surströmming has a genuine culinary tradition, especially in northern Sweden. The proper way to eat it looks nothing like the viral videos where people gag over an open can.
The fish is typically served on tunnbröd, a thin Swedish flatbread that comes in both soft and crispy varieties. You take a piece of the bread, add boiled potatoes (small, fresh ones are traditional), sliced red onion, chopped chives, fresh dill, and a dollop of sour cream. A small piece of the fermented fish goes on top of all that. The mild, starchy accompaniments balance the intensity of the fish, and each bite contains far more bread and potato than herring. The meal is traditionally washed down with aquavit (a Scandinavian spirit) and beer.
Eaten this way, surströmming is salty, sour, and umami-rich rather than simply foul. The fermentation breaks down the fish proteins extensively, giving the flesh a soft, almost paste-like texture. Fans describe the flavor as complex and deeply savory, with the sourness cutting through the richness. It’s still an acquired taste, but it’s a genuine one, not a novelty stunt.
The Fermentation Season
Surströmming has a traditional premiere date: the third Thursday of August. This timing traces back to a royal decree from the 18th century that originally set a date to ensure the fish had fermented long enough before sale. The premiere is still observed in parts of Sweden, particularly in the northern coastal regions of Ångermanland and Västerbotten, where surströmming parties mark the late summer. Fresh cans from the year’s production appear in Swedish grocery stores around this time, and it’s common for families to gather outdoors for the occasion.
Regulatory Challenges in Europe
Because Baltic herring accumulate higher levels of dioxins than fish from less polluted waters, the European Union sets strict limits on dioxin content in fatty fish. Sweden (along with Finland) has held a special exemption from these EU rules, allowing the domestic sale of Baltic herring products that exceed the standard limits. This exemption has been periodically reviewed and renewed, with the Swedish government arguing that the cultural and economic importance of products like surströmming justifies the dispensation. Swedish health authorities have recommended that consumers limit their intake of fatty Baltic fish, but production and sale continue under the exemption.
Why It Smells Worse Than It Tastes
The disconnect between surströmming’s smell and its actual flavor is partly biological. Your nose detects the volatile gases, especially hydrogen sulfide and trimethylamine, at extraordinarily low concentrations. These compounds hit you from a distance, before you’ve even tasted anything. But once the fish is on bread with accompaniments, your taste buds register a more balanced profile: sour from the organic acids, salty from the brine, and savory from the broken-down proteins. The smell is genuinely challenging, even for seasoned fans, but the eating experience is a different thing entirely. Most Swedes who enjoy surströmming will still tell you to open the can outdoors, preferably downwind from the table.