[MartijnHoutman]

There is a Dutch television programme that does research on ingredients of common food products, to check where the ingredients come from. Sometimes based of speculation, but they tend to back it up with scientific research.

Here is the episode about l-cysteine (in Dutch, parts are in English, not sure if watchable from outside NL): http://keuringsdienstvanwaarde.kro.nl/seizoenen/2014/30-1187...

One of the suppliers actually confirms that human hair is being used in production, and being sold to bakeries around the world. While European bakeries claim to only use the l-cystein from animal sources, this video tends to show otherwise.