Surströmming is fermented Baltic herring, a traditional Swedish food famous for producing one of the most intense smells of any product sold for human consumption. The fish is lightly salted and left to ferment in cans for months, creating a sour, pungent delicacy that remains a cultural staple in northern Sweden despite its polarizing reputation worldwide.
The Fish and Where It Comes From
The herring used for surströmming comes from the Baltic Sea, where it has been classified as a distinct subspecies (Clupea harengus membras) since the time of Linnaeus. Baltic herring is smaller and leaner than its Atlantic cousin, shaped by roughly 8,000 years of adaptation to the brackish waters of the Baltic. Most of the catch used for surströmming comes from the waters off northern Sweden, particularly around the Bothnian Sea and the Gulf of Bothnia.
The tradition dates to the 16th century, when Sweden faced a salt shortage. Salt was expensive, and preserving fish by heavy salting, as was common across Scandinavia, became impractical for many communities. Fishers discovered that using less salt didn’t necessarily spoil the herring. Instead, it fermented, producing a sour but edible product that could last through the winter.
How Surströmming Is Made
Production starts with freshly caught herring placed into a strong brine for about 20 hours. This initial soak draws out the blood. After that, the heads and guts are removed, and the fish goes into a weaker brine solution. The key to the entire process is using just enough salt to prevent the fish from rotting while still allowing fermentation to proceed.
The salt concentration creates conditions where ordinary spoilage bacteria can’t survive, but a specific group of salt-tolerant, oxygen-free bacteria called Haloanaerobium can thrive. These microbes, first formally identified in surströmming cans in a study published in the International Journal of Systematic and Evolutionary Microbiology, break down the sugars stored in the fish and produce organic acids that give surströmming its characteristic sourness. They also generate carbon dioxide and hydrogen sulfide as byproducts, which is why the cans visibly bulge over time.
The fish is sealed in cans while still fermenting, typically in spring, and continues to ripen through the summer. By late August, when the traditional surströmming premiere takes place on the third Thursday of the month, the cans are swollen and pressurized from months of gas buildup inside.
Why It Smells So Strong
The smell is not a side effect of surströmming. It is, in a sense, the product itself. During fermentation, the bacteria produce a cocktail of compounds that includes hydrogen sulfide (the rotten-egg gas), along with butyric acid, propionic acid, and acetic acid. Natural enzymes already present in the fish flesh and digestive tract break down proteins simultaneously, adding another layer of funky, complex aroma.
The combination is so overpowering that most people open surströmming outdoors, often submerged in a bucket of water to contain the initial burst of gas. For newcomers, the smell is often compared to a mix of rotten eggs, strong cheese, and vinegar. For fans, the actual taste is milder than the smell suggests: salty, acidic, and deeply savory, with a soft, almost creamy texture.
How Swedes Actually Eat It
Surströmming is not eaten straight from the can like sardines. The traditional preparation involves rinsing the fillets and building a small open sandwich called a surströmmingsklämma. You start with tunnbröd, a thin Swedish flatbread, spread with butter at room temperature. On top of that goes sliced almond potato (a small, waxy variety prized in northern Sweden), chopped onion, and a piece of the fermented herring. Some people add a dollop of sour cream, fresh dill, or sliced tomato. Västerbotten cheese, a sharp aged cheese from the same region, is another popular addition.
The accompaniments serve a real purpose. The starchy potato, the fat from butter and sour cream, and the sharpness of the onion all balance the intensity of the fish. Drinking milk or beer alongside the meal is common. Eating surströmming is as much a social ritual as a meal, particularly during late-summer gatherings in northern Sweden where the premiere is celebrated with friends and family outdoors.
Airline Bans and Pressurized Cans
Starting in 2006, several major airlines including British Airways, Air France, Finnair, and KLM banned surströmming from both checked and carry-on luggage. The reason is not primarily the smell, though that certainly doesn’t help. The cans are classified as pressurized goods because the ongoing fermentation generates enough carbon dioxide and hydrogen sulfide to make them swell significantly. At high altitudes, where cabin pressure drops, the risk of a can rupturing increases. Airlines treat them essentially the same way they treat other potentially explosive pressurized containers.
Even on the ground, a bulging can of surströmming requires careful handling. Opening one indoors is widely considered a mistake, and countless viral videos exist of people learning this the hard way. The brine inside is under enough pressure to spray when punctured, and the smell can linger in enclosed spaces for days.
Baltic Herring Supply and Availability
Surströmming production depends on healthy herring populations in the Baltic Sea, which have faced pressure from overfishing and environmental changes in recent decades. The International Council for the Exploration of the Sea sets annual catch limits for Baltic herring. For 2026, the recommended total allowable catch for central Baltic herring is up to roughly 155,000 tonnes, depending on how management plans are applied.
These quotas directly affect how much herring is available for surströmming producers, most of whom are small operations concentrated along Sweden’s northern coast. In years when quotas tighten, prices rise and supply becomes limited. Surströmming is available for purchase online from Swedish suppliers who ship internationally, though the pressurized cans and shipping restrictions make it more expensive outside Scandinavia. A single can typically costs between $10 and $30 depending on the source, with specialty importers charging more.