Surströmming is an ostensible “delicacy” common to northern Sweden. Referred to as “fermented* or “soured” herring, it is made by putting fresh caught fish in barrels to sit for a couple months, with just enough salt added to suppress the more nasty varieties of bacteria that would propagate in the slurry, otherwise. After two months, the fish is transferred to cans where the “fermentation” process continues, often causing the can to swell (which we in the U.S. would equate with the presence of botulism).
The swelling results from the production of carbon dioxide gas through the action of Haloanaerobium , a species of bacterium which feeds upon the fish.
The fish has such a foul odor that it is often opened and consumed out-of-doors. The smell results from the following compounds, produced during the “fermentation” period, which also add to the “complex” flavor of the product:
- propionic acid: pungent/acrid quality
- butyric acid: rancid-butter
- hydrogen sulfide: rotten-eggs
- acetic acid: vinegar-like
*Not accurate. The process of fermentation refers specifically to the biological action of organisms breaking down carbohydrates (as in grains, fruits, etc.). The processes which occur in animal products (which contain almost no carbohydrate) are properly called “putrefaction” and “rancidification.” It may be that purveyors of putrid, rancid flesh products adopted the term “fermentation” because 1.) The process superficially resembles the process of fermenting carbohydrates 2.) Because “fermented” sounds less noxious than “putrid” and “rancid.”