What it is
E951 is aspartame, a synthetic dipeptide of aspartic acid and phenylalanine. Both amino acids are produced by microbial fermentation of plant sugars; the methyl ester is chemically synthesised.
Why it matters
Diet soft drinks (the canonical example: Diet Coke), sugar-free chewing gum, tabletop sweeteners. Mandatory EU label warning for phenylketonuria sufferers.
What to do
Always vegan.