Koop's™ Mustard Dusseldorf Bratwurst Mustard. Since 1897. All natural ingredients. Gluten free. Our smooth German mustard perfectly complements any sausage. There really was a Peter Koops, a young Dutch adventurer who left his shipping business in the Netherlands for a new life in America. He began with a hand-crank mustard mill and a horse-drawn cart, selling mustard by the ladle to loyal customers. Our packaging and delivery systems have changed since then, but our carefully made stone-ground recipes are the same. Certified GF, gluten-free. www.koopsmustard.com.