What it is
E330 is citric acid. Despite the name suggesting citrus fruit, commercial production is by fermenting sugar (corn, sugar beet) with the mould Aspergillus niger.
Why it matters
The most widely-used food additive in the world after salt and sugar. Acidity regulator, preservative, flavour enhancer, chelating agent. In nearly every processed food.
What to do
Always vegan. The related citrate salts (E331 sodium, E332 potassium, E333 calcium) are likewise vegan.