Surströmming was a basic ration of the Swedish army in the seventeenth century during the fifty years of sporadic armed conflict that is known as the Thirty Years War. It is still regulated by a medieval royal ordinance and must be made from herring caught in April and May just before spawning. The head and entrails are removed, but the roe is kept in the herring, which is put into light brine in barrels holding 200 pounds of fish. The fish are left to ferment in the barrels for ten to twelve weeks at a temperature between fifty-four and sixty-four degrees Fahrenheit.
