I will be avoiding products that are not explicitly labeled to use L-cysteine from vegetarian sources which is more expensive. Cheaper source of L-cysteine is from human hair or duck feather.
According to Vegetarian Resouce Group
While researching L-cysteine, The VRG asked several fast food chains and a major vegetarian food company about the sources of L-cysteine in their products. McDonald’s told us that L-cysteine derived from duck feathers is in their Honey Wheat Roll, the Deluxe Warm Cinnamon Roll, and the Baked Apple Pie. The L-cysteine in several items offered at Dunkin’ Donuts is also derived from duck feathers. Burger King told us in June 2007 that it “could not guarantee” the source of L-cysteine in its products.
On the other hand, Subway recently announced in March 2007 that it has removed the L-cysteine from its otherwise animal product-free Carb Conscious Wrap. When asked about the source of L-cysteine in several of its products, Domino’s Pizza told us that L-cysteine is “microbially derived” in its Hand-Tossed Crust and informed us that the L-cysteine in Domino’s Breadsticks, Cheesy Bread, and Cinna Stix is “vegetable-derived.”
The public relations firm for Morningstar Farms told us that the L-cysteine in their Veggie Bites Country Scramble, Veggie Bites Spinach Artichoke, and Veggie Bites Eggs Florentine was a “microbial fermentation product.”