Keeps bread from molding. Propionic acid is also made naturally in your colon when bacteria ferment fiber.
Job
Inhibits mold and rope-forming bacteria in baked goods.
Natural link
Propionate is one of the short-chain fatty acids gut bacteria produce from fiber — another reason 'chemical-sounding' doesn't mean foreign.
Evidence & sources
EFSA concluded propionic acid and its salts are of no safety concern at reported use levels and did not need a numerical ADI. Evidence: regulatory. Known: EFSA re-evaluation. Unknown: — Source: European Food Safety Authority, Re-evaluation of propionic acid (E 280) and its salts (E 281–283) (2014).