Garum is one of the world's oldest condiments. The recorded history of garum dates back to ancient Greece and Rome. Classical techniques entail packing salt and whole fish in large vessels, and letting those vessels sit out in the sun.
Enzymes in the fishes' guts break down muscle protein into its constituent parts: amino acids. The salt curtails growth of harmful bacteria, and the free sodium ions dissolved in the liquid exuded from the fish bind with the non-essential amino glutamic acid to make monosodium glutamate. Sodium ions also bind with aspartic acid, to make sodium aspartate, a chemical known to enhance perception of salty flavors. These reactions result in rich umami flavor.