Surströmming smells so bad because bacterial fermentation floods the fish with sulfur compounds, trimethylamine, and a cocktail of other volatile chemicals that individually rank among the most offensive odors humans can detect. The fermentation never fully stops, even after canning, which means the smell intensifies over time and builds enough gas pressure to visibly bulge the tin.
The Chemistry Behind the Smell
When researchers at the University of Turin analyzed surströmming using gas chromatography, they found a “massive presence” of two dominant odor categories: trimethylamine and sulfur compounds. Trimethylamine is the chemical most responsible for the smell of rotting fish. Your nose can detect it at extraordinarily low concentrations, and in surströmming it’s present in quantities far beyond what you’d encounter in ordinary seafood.
The sulfur compounds are what push surströmming from “bad fish” into its own category. Hydrogen sulfide, the gas that gives rotten eggs their smell, is one contributor. But the analysis also identified 1,2,4-trithiolane, a ring-shaped molecule built from three sulfur atoms that amplifies the rotten, savory stench. On top of these, the researchers detected phenols, ketones, aldehydes, alcohols, and esters, each adding its own layer to the overall odor profile. The result is a smell so complex that your brain struggles to categorize it, which is part of why many people find it so overwhelming.
Why Fermentation Creates These Compounds
Surströmming starts as Baltic herring caught in the spring, gutted but not fully cleaned, then packed in barrels with just enough salt to prevent full preservation. The salt concentration is deliberately kept low, typically around 10 to 12 percent, so that bacteria can survive and begin breaking down the fish from the inside. In a fully salted or pickled product, bacterial activity would be suppressed. In surströmming, it’s the whole point.
The bacteria feed on the proteins and fats in the herring, dismantling amino acids and releasing gases as byproducts. Sulfur-containing amino acids like cysteine and methionine get broken apart, producing hydrogen sulfide and other sulfur volatiles. Meanwhile, a compound called trimethylamine oxide, which is naturally present in all ocean fish, gets converted into trimethylamine. In fresh fish this conversion happens slowly and signals spoilage. In surströmming, it happens aggressively and by design over months of fermentation.
After roughly two months in barrels, the herring is transferred into tin cans. But the bacteria aren’t killed by heat or pressure the way they would be in normal canned food. Surströmming is never cooked or pasteurized. So fermentation continues inside the sealed can, and the chemistry keeps intensifying on the shelf.
The Bulging Can Problem
One of surströmming’s most recognizable features is its swollen, dome-shaped can. That bulge comes from carbon dioxide produced by ongoing fermentation after canning. Hydrogen sulfide and other gases contribute too. The pressure inside a can of surströmming is substantially higher than in a normal tin of fish, which is why the cans require special packaging designed to flex rather than burst.
This pressure is also why several major airlines banned surströmming from flights starting in 2006. British Airways, Air France, Finnair, and KLM all classified the cans as pressurized goods. At high altitudes, where cabin pressure drops, the already strained cans face an even greater risk of rupturing. The airlines categorized them alongside potentially explosive materials. The official concern was the pneumatic hazard rather than the smell, though it’s fair to say neither factor helped surströmming’s case for carry-on luggage.
How People Actually Eat It
If surströmming is so pungent, why does anyone eat it? The answer is that the smell and the taste are two very different experiences. The flavor is intensely salty, tangy, and umami-rich, more like an aggressive anchovy than anything truly rotten. Swedes who grew up with surströmming treat it as a late-summer tradition, typically eating it in August and September when the new season’s cans are ready.
The classic preparation is a klämma, essentially a wrap made with tunnbröd, a thin Swedish flatbread that can be soft or crisp depending on the regional style. You lay a fillet of surströmming on the bread alongside sliced almond potatoes (small, waxy fingerlings), chopped raw onion, and sometimes sour cream or butter. The mild, starchy sides are there specifically to balance the intensity of the fish. Nobody eats surströmming straight from the can as a main course, despite what YouTube challenge videos might suggest.
Opening the Can Safely
The single most important piece of practical advice about surströmming is to open the can outdoors and submerged in a bucket of water. This serves two purposes. First, it contains the pressurized brine that would otherwise spray outward when you puncture the seal. Getting surströmming liquid on your clothes is a mistake people describe in terms usually reserved for minor disasters. Second, opening underwater traps many of the volatile gases in the water rather than letting them aerosolize into the air around you.
The recommended technique is to tilt the can at a 45-degree angle and puncture the top edge first. Because a small pocket of gas rises to the highest point, this lets the pressure escape without forcing liquid out. Once the hissing stops, you can open the rest of the lid normally, lift the fillets out, and rinse them lightly before serving. Even with these precautions, the smell will carry. Eating indoors is generally considered a bad idea unless you’re very comfortable with the scent lingering in your home for days.