Surströmming ("soured (Baltic) herring") is a northern Swedish delicacy consisting of fermented Baltic herring. Surströmming is sold in cans, which when opened release a strong smell. Because of the smell, the dish is often eaten outdoors. However, opening the can under water or inside a plastic bag, somewhat lessens the smell, as well as prevents the person opening it from being soaked in brine, as the fermentation often builds up a considerable pressure inside the can.
The herring is caught in spring, when it is in prime condition and just about to spawn. The herring are fermented in barrels for one to two months, then tinned where the fermentation continues. Half a year to a year later, gases have built up sufficiently for the once cylindrical tins to bulge into a more rounded shape. These unusual containers of surströmming can be found in supermarkets all over Sweden. However, certain airlines have banned the tins on their flights, considering the pressurized containers to be potentially dangerous. Species of Haloanaerobium bacteria are responsible for the in-can ripening. These bacteria produce carbon dioxide and a number of compounds that account for the unique odor: pungent propionic acid, rotten-egg hydrogen sulfide, rancid-butter butyric acid, and vinegary acetic acid.
One proposed explanation of the origins of this method of preservation is that it began long ago, when brining food was quite expensive due to the cost of salt. When fermentation was used, just enough salt was required to keep the fish from rotting. The salt raises the osmotic pressure of the brine above the zone where bacteria responsible for rotting (decomposition of proteins) can prosper and prevents decomposition of fish proteins into oligopeptides and amino acids. Instead the osmotic conditions enable the Haloanaerobium bacteria to prosper and decompose the fish glycogen into organic acids, giving it the sour (acidic) properties.<- Per Gessle