The surströmming (translated as “sour herring”) is a typical dish of Swedish cuisine and has been defined the smelliest food in the world! Mistakenly called “rotten fish”, it really isn’t rotten nor putrefied: is obtained from fermentation anaerobic (i.e. without oxygen) of a herring caught in the Baltic Sea, a method developed from the need to preserve large quantities of fish with low amounts of salt. It is sold in jars which over time “swell” due to the continuous fermentation responsible for the smell! Thanks to enzymatic processes and to the action of specific bacteria, acid is formed during fermentation propionicacid butyricacid acetic And hydrogen sulfide, responsible for the strong, acidic smell and taste of the dish, and carbon dioxide. Among the smelliest foods of all we also remember Icelandic Hákarlfermented shark meat with the characteristic smell of ammonia.
What surströmming is and what it smells like: the origins and history of the dish
Surströmming has an extremely pungent odor typical of something that is rotten or rancid, which has similarities to the smell of rancid butter or spoiled eggs. The birth of this dish traditionally dates back over 9000 years ago, due to the difficulty in obtaining salt to preserve food through salting, but it became popular around 1500. It is a fermentation of fish, a technique used and known since ancient times, just think of the garumthe fermented fish of the Romans. The very particular name is formed from the word sur (from the Latin acid) and the word strömming (the Swedish term for herring) and in fact it is also known by the name of herring fermented acidic.
There was a royal decree which prohibited eating surströmming before the third week of August. Obviously today that decree no longer exists, but the tradition of eating it in August has remained, accompanied with typical bread and potatoes, strictly outdoorsdue to the pungent odor released when opening the cans, and it is recommended to open pressurized cans in a jar full of water.

How to do surströmming: the long preparation process
For the production of surströmming, a herring from the Baltic Sea is used Clupea Harengustraditionally fished between May and early July, that is when the fat content in the herring is lower (this prevents theranciditythe process of decomposition of lipids). The herring is pre-salted for 1-2 days, after which the head and viscera are removed (except the gonads and the pyloric caecum, we will see why shortly) and it is transferred into barrels with a low salt brine (17%) where it is kept for 3-4 weeks at 15-18 °C. This is where the process begins fermentation.
Next, herring and brine are transferred from barrels to cans and sent to distributors. However, about a year must pass before the surströmming can be consumed, because, given that the cans are neither sterilized nor pasteurised, the fermentation process continues even inside them and will lead to the formation of the typical odor and flavor of the dish and gas which will cause the cans to swell.
Why the smell of surströmming is so strong: the biochemistry of fermented herring
Chemically, one happens anaerobic fermentationi.e. in the absence of oxygen, and an enzymatic degradation of lipids and proteins. The process of surströmming formation begins in the early post-mortem stages of the fish: production of lactic acid And autolysis.
In the muscle tissues, since no more oxygen arrives, lactic acid is formed, a bit like what happens to us when we lack oxygen and the muscles use lactic fermentation to produce energy.

The presence of lactic acid lowers the pH and makes the environment acidic, activating the autolytic enzymes (caspases, cathepsins and proteasomes) present in the pulp, gonads and pyloric cecum of the fish (this is why they are not removed), which begin to degrade amino acids, proteins and lipids.
Finally, the microbial flora is established and stabilized, controlled by Lactobacilliwhich continue the production of lactic acid and avoid the proliferation of pathogens. Bacteria are also present halophiles (i.e. salt lovers), in particular Haloanaerobium prevalens And Haloanaerobium alcaliphilum and in lower concentration Alkalibacterium gilvum And Tetragenococcus halophila. Where do Lactobacilli and halophilic bacteria come from? The first ones come from barrels, used over and over again without being sterilized: in fact, in an experiment in which new barrels were used, the typical flavor and aroma of surströmming did not develop. Halophiles, however, are present in small quantities in the herring habitat and partly also come from the barrels themselves.
To survive, these bacteria “feed” on the macromolecules of the fish and contribute to their degradation. From bacterial metabolism, combined with the action of autolytic enzymes, the molecules responsible for the acrid, acidic and pungent odor of the dish are obtained: propionic acidacetic acid, butyric acid and hydrogen sulfide (quite recognizable because it is typical of rotten eggs and flatulence).
During the months of fermentation in the can, bacteria also produce CO2carbon dioxide, e hydrogen in gaseous form, leading to swelling of the cans.
But is it safe to eat it?
When we see a swollen can, we are used to thinking that whatever food it contains has gone bad and this is usually synonymous with botulism. But the conditions of salinity and acidic pH (between 6.4 and 6.5) of surströmming and the presence of Lactobacilli guarantee its safety.
First of all, the salinity and the osmotic pressure of the brine prevent the proliferation of anaerobic bacteria responsible for putrefaction (Clostridium, Bacteroides And Pseudomonas). Indeed in surströmming cans there is no trace of cadaverine, skatole, indole and putrescinethe molecules typically associated with putrefaction.
Furthermore, the salt concentration of the brine, although low, is high enough to prevent the proliferation of even the majority of pathogenic bacteria. In fact, in an experiment conducted in the Seventies by the Swedish Food Safety Agency, pathogenic bacteria were added to the brine, but they were not found at the end of the fermentation, therefore a sign of an environment that prevents their proliferation.
The only ones who have not been tested are the Clostridium botulinum And Lysteria monocytogenesbut even in their case, the salinity reached in surströmming would still prevent their survival.
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