From io9 with a tip o’ the pileus to Dorothy King … this is the bit from towards the end:
The process sounds very modern, but MSG has been around for a very long time. It was a common food additive during the time of ancient Rome, added to almost all Roman dishes. The Romans had a lot of technology for their time, but they couldn’t genetically engineer bacteria. So how did they come up with MSG? Believe it or not, they used an even more disgusting process than bacteria excretions. The Romans had a fish paste called garum that they exported everywhere. They made it by filling pots with alternating layers of fish – or just fish guts – and salt and letting those pots lie out in the sun for a while.
As the mixture lay out in the sun, the stomach acids for the fish ate through their bodies. They eventually broke down the entire fish, turning the whole thing into a dark brown oily goo. When protein is broken down, the amino acid chains in the protein are freed up. One of these acid chains contains glutamic acid, which meets up with sodium from the salt and forms MSG. The Romans were such fans of the flavor enhancer that they even put it in sweets like custard. They also died off in droves, so anyone who wishes to recreate garum Roman-style — don’t do it. Try organizing gladiator-style games in the back yard as a safer alternative.
Personally, I’d like to see some definite evidence that the garum process would create the ‘free glutamates’ which are apparently necessary for the MSG ‘effect’ … all I have is the Wikipedia article to go on …