What it is
E621 is MSG, the sodium salt of glutamic acid (E620). Modern commercial production is by microbial fermentation of plant sugars.
Why it matters
The iconic umami booster. In instant noodles, savoury snacks, restaurant chinese-food (sometimes), stock cubes, ready meals.
What to do
Always vegan in modern production. The E622 monopotassium glutamate and E623 calcium glutamate are likewise vegan.