What it is and where it comes from
Ascorbic acid is vitamin C. For food it is made from glucose (from corn or wheat) by fermentation and chemical steps. It stops browning and helps bread dough.
Where you find it
Juices, bread and flour, cured meat, tinned fruit and vitamin drinks.
What to look for on the label
Nothing to check for halal.
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.