What it is
E631 (disodium 5'-inosinate) is a flavour enhancer commonly produced from sardines, anchovies, dried fish, or meat. Synthetic plant-fermented versions exist but are rare in mass-market production.
Why it matters
Common in instant noodles, savoury snacks, crisps, processed meats, and stock cubes. Often appears alongside E627 (disodium guanylate) and E621 (MSG); the trio together is the "umami" boost in cheap savoury foods.
What to do
Treat as not vegan unless the manufacturer specifically states the source. Some brands (notably some Indomie and Maruchan ranges) have confirmed fish origin; a small number of speciality vegan brands source the additive from yeast or bacterial fermentation and state so on the label.