What it is and where it comes from
E631 is a flavour enhancer that gives a meaty taste, usually used with MSG. The big producers make it by bacterial fermentation of sugar. It can also be extracted from meat, including pork, or from fish such as sardines.
Where you find it
Crisps, instant noodles, stock cubes, soups, seasonings and snacks.
What to look for on the label
Look for a halal logo or a vegetarian or vegan label: in Europe, products marked vegetarian use the fermented kind. Otherwise ask the maker for the source.
This is a guide, not a fatwa. Recipes change, so check the label, look for halal certification, and ask your local imam where scholars differ.