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Swedish Surströmming

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Nine-thousand, two hundred years ago — 80 centuries before the age of the Vikings — a group of humans stood by an ancient lake near what’s now the province of Blekinge in southern Sweden, and lowered a bundle into a muddy pit. The bundle contained freshwater fish mixed with pine bark and seal fat and covered in wild boar skin. And this wasn’t the only bundle in the pit. At least 60,000 tons of fish were present. As would be expected, given the date, the fish weren’t salted — they were placed in the ground to ferment.

We know this because of a scientific paper published by researcher Adam Boethius, of Lund University. Boethius and his team of 16 archaeologists spent five months excavating the site, where they unearthed close to 200,000 fish bones. The amount of fish at the site could’ve easily supported a reasonably large population. The sophistication and scale of the endeavor is causing archaeologists to rethink the idea that people in Scandinavia at the time were nomadic hunters and gatherers.

In Sweden today, starting on the third Thursday of August every year and extending into September, many Swedes take part in a festivity — a surströmmingsskiva — that harkens back to the ways of Scandinavians over 9,000 years ago. And during this time, they revel in eating the modern-day equivalent of that ancient fermented fish, surströmming.

Surströmming’s Potent Reputation

Surströmming is prepared by fermenting Baltic herring for at least three months in a brine. The brine is salty, but not so salty as to inhibit the fermenting microorganisms. After the fish has fermented in barrels for a while, it’s canned, where it continues to ferment. As a result, cans of this fish product bulge noticeably — a sign in other foods that the contents aren’t safe.

The resulting fish, which contains around 12 percent protein, 9 percent salt, and 4 percent fat, is sour, salty, and extraordinarily aromatic. In fact, it’s been called the stinkiest food in the world. Air France, British Airways, Finnair, and KLM Royal Dutch Airlines have all banned surströmming on their flights, citing the possibility that a bulging tin may rupture under the reduced air pressure of flight at altitude. Even the most ardent surströmming fans admit it’s an acquired taste; however, served correctly, many regard it as a wonderful delicacy.

Swedes celebrate this delicacy during the annual surströmmingsskiva, an outdoor get-together where surströmming is served, most often with snaps. The word surströmming means “sour herring” in Swedish, while “snaps” is a Swedish term for a small shot of alcohol taken with a meal. It shouldn’t be confused with schnapps, a type of sweet liqueur. Commonly, the type of alcohol taken with surströmming is aquavit, a caraway-flavored spirit.

From Sea to Can

Surströmming production starts with fresh-caught Baltic herring in April and May, just prior to their spawning season. The fish have a relatively low fat content, although not so low that drying them would be easy. The fish are initially brined whole for 1 to 2 days in a saturated salt solution in an open vessel. For the first four hours, they’re constantly stirred. After that, the fish may be beheaded, gutted, and transferred to barrels containing a weaker salt solution, around 17 percent. Alternatively, the fish may only be beheaded, leaving their organs intact while fermenting. The barrels with the weaker brine are sealed and rotated occasionally for 3 to 4 days to mix the interior contents evenly. The fish then spend 3 to 4 weeks in that brine at 59 to 64 degrees Fahrenheit for the initial stage of their fermentation and ripening.

In recent centuries, producers used wooden barrels for this process. Afterward, people sold the barrels, or transferred the contents to smaller wooden kegs and sold them that way. The fermented fish distributed in this fashion were for consumption only in the year they were produced, as wooden barrels aren’t completely airtight. Today, producers use plastic barrels over and over, and then can the fish after the initial fermentation. Since the fish is canned in the brine in which it’s fermented, and the fish and brine aren’t pasteurized, it continues to ferment after packaging. This quickly leads to bulging cans — a sign of problems in most canned foods, but expected (and safe) in surströmming. Canning begins in July and takes about five weeks to complete. The producers then store the cans until 10 days before the third Thursday in August. At that point, they’re shipped to retailers.

Fermentation and Ripening

What happens during the fermentation process is a combination of decomposition and action by microbes. Lactic acid forms in the muscles, as would normally happen in decomposing fish. However, the brine contains just enough salt to inhiSurströmming, fermented Baltic herring, is traditionally paired with thin bread, chives, and small potatoes.
bit many of the ordinary decomposition pathways. We know this because many common molecules associated with decomposition, including cadaverine, indole, putrescine, and skatole, aren’t found in surströmming when food scientists chemically analyze it. (Some popular accounts claim these compounds are present. However, no evidence for this is given beyond the fact that the fish smells “rotten.”)

The absence of those noxious odorants doesn’t mean that surströmming has an inoffensive odor. Bacterial action in the barrels produces acetic acid (which smells like vinegar), butyric acid (which is found in rancid butter and vomit, but is also a component in the aroma of Parmesan cheese), and propionic acid (which is a component of human body odor). Hydrogen sulfide, the main odorant in rotten eggs, is also produced. The bacteria responsible for much of this are two salt-tolerant anaerobes in the genus HaloanaerobiumHaloanaerobiumpraevalens and Haloanaerobiuim alcaliphilum. These bacteria are present in low concentrations in the herrings’ habitat, but are mostly transferred to the surströmming fermentation via the reuse of barrels. Experiments have shown that using sanitized barrels doesn’t result in the typical surströmming flavor and aroma. Bacteria in the genus Lactobacillus are also active (as is the case in most fermented foods), producing lactic acid.

Is It Safe?

Swedish food scientists have examined the safety of surströmming by attempting to culture several common foodborne pathogens — including Staphylococcus aureus, Bacillus cereus, and Clostridium perfringens — in surströmming brine. None of them grew. And the salt content of the brine strongly suggests that others — including Clostridium botulinum and Listeria monocytogenes — can’t grow either.

Surströmming’s reported pH is 7.1 to 7.4. Many food fermentations rely on a pH lower than 4.5 to suppress the growth of pathogens. In surströmming, the salt content does this instead. Interestingly enough, the freshwater fish found buried by the Scandinavian lake were mixed with pine bark and seal fat. As every gardener knows, pine bark is acidic. And, although most mammalian fat has a pH around 7.0 in living animals, it begins to break down and acidify slightly after death. So, the ancient method probably used pH instead of salt to render the food safe.

Even though pathogens can’t grow in surströmming, the Swedish National Food Agency still recommends that pregnant people only consume it 2 to 3 times a year. Why? The Baltic Sea is totally enclosed by land and takes runoff from 14 industrialized countries. The dioxin levels in Baltic herring more than 17 inches long exceed the recommended safe levels established by the European Union. Sweden has granted exemptions to surströmming producers because of the historical and cultural significance of the food. And, of course, the amount of surströmming most modern people eat — even for fans of the food — is small.

Surströmming Is Served

Surströmming, fermented Baltic herring, is traditionally paired with thin bread, chives, and small potatoes
GettyImages/marinakhlybova

Each year’s batch of surströmming becomes available on the third Thursday of August. In the 1940s, there was a law mandating this, both so the fish would have adequate time to ferment, and so it could be eaten in surströmmingsskivas when it was still warm enough for a picnic-like get-together. Each bulging can of surströmming is held underwater in a bucket when it’s opened to ensure any spray is captured by the water. The aroma of the fish can be instantly discerned when the bubbles released from the can reach the surface of the water. For this reason, some people open the can away from the eating area.

I’m of Scandinavian descent and was curious about fish prepared in a way that my ancestors probably ate. I understood it was likely to be a challenging and interesting food, but I was willing to give it the benefit of the doubt and try it. Then, I opened the can.

The smell of surströmming is extremely intense and revolting. It’s not the typical smell of rotten fish, but it contains elements of it. The sample I tasted smelled strongly of propionic and butyric acid, and there was a slight fecal cast to it. I don’t have a weak stomach, but I was trying to breathe as little as I could. It was hard for me to believe that so much odor was emanating from a single can. I set the can yards away from the serving area after fishing the fillets out of the brine.

The traditional way to eat surströmming is to spread some tunnbröd (thin bread, which can be either hard or soft) with butter or crème fraîche (a sour cream with a high amount of milkfat). Small potatoes, either sliced or mashed, and diced red onion are placed on top of the bread, and then small bits of herring are placed on top of that. Some Swedes also add tomatoes, chives, or dill. Västerbotten cheese — an aged hard cheese made from cow’s milk, which is often eaten with pickled herring — may also be added. Some people place a second piece of tunnbröd on top to make a sandwich, or wrap the mixture in soft tunnbröd. It’s customary to take a shot of alcohol — most often aquavit, but sometimes other spirits or beer — before taking the first bite of surströmming. Milk is also sometimes served alongside.

The aromatic qualities of the onions are supposed to mask the smell of the surströmming a bit, but in my experience, they weren’t up to the task. Most people say the flavor is considerably better than the smell. I found that to be true, but not much of a saving grace. After fortifying myself with a shot of aquavit, I could only manage one bite. I was thankful for the opportunity to sample surströmming, but will never seek it out again.

Surströmming is an acquired taste, to put it mildly, yet many Swedes feel it’s part of their culture. Served with its traditional flatbread and trimmings, it could be a fascinating experience for someone with an interest in Scandinavian culture. This fermented delicacy is not for the faint of heart — but then again, neither was ancient Sweden. 


More Fishy Fermented Dishes

Fishing is seasonal in Scandinavia, and in the absence of modern preservation methods, the options for preserving fish were historically narrowed to drying, smoking, or fermenting. Some fatty types of fish, including herring, don’t dry properly and so must be smoked or fermented. (Modern herring, of course, is also pickled.) Aside from surströmming, other Scandinavian populations have their own fermented fish products. In Norway, ratfisk is fermented trout (or sometimes char). Hákarl is an Icelandic food product made from fermented Greenland shark. The Faroe Islands have ræstur fiskur — cod or saithe that’s air-dried and fermented. All of these fermented fish hark back to a time before refrigeration, canning, and cheap, abundant salt.


Aquavit Surströmming’s Drinkning Partner

Aquavit is a distilled spirit — usually around 40 percent alcohol by volume, or 80-proof — made from grains or potatoes and then spiced. The most well-known brands come from Sweden, Norway, Denmark, and Finland, although there are a few U.S. producers. Norwegian aquavit is often darker in color from aging in oak barrels, and is more heavily spiced than other aquavits. It’s almost always served chilled before a meal or with cheese, and it’s closely associated with crayfish parties — and, of course, surströmmingsskivas.

Caraway is the dominant spice in most aquavits, but many other spices may also be present, including anise, cardamom, cumin, dill, fennel, and grains of paradise. Most brands are assertively spiced, and aquavit isn’t used in any common mixed drinks.

The traditional Scandinavian toast is “skål” (pronounced skowl), and in social situations, this word is said before the first shot of aquavit is consumed. Store your bottle of aquavit in the freezer, and it will always be ready if friends drop by, or if you ever want to toast your first bite of surströmming.


Chris Colby is a writer with a background in biology and brewing. He lives with his wife and their cats — including their new kitten with the Scandinavian name Lagertha — in Bastrop, Texas. Chris enjoys gardening and drinking beer while admiring his garden.

Published on Aug 13, 2019

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