What is L-cysteine, or How Did Duck Feathers Get in My Bread??? L-cysteine (sometimes shown as E920 on food labels) is a food additive often derived from duck feathers or human hair. It is used as a dough conditioner and strengthener, meaning the dough can be stretched out to make a pizza crust, for example. L-cysteine can also help extend the shelf life of commercial breads.