Is Mustard Always Considered Gluten Free? - Tasting Table
Briefly

When it comes to mustard, while the seeds are naturally gluten free, store-bought varieties can pose risks due to added ingredients like wheat flour and certain vinegars.
Regulations in the U.S. mandate that any major allergens, including gluten, are listed on ingredient labels, but celiacs should still seek out mustards that are specifically marked gluten free.
Brands like French's Classic Yellow Mustard are gluten free, but others such as Heinz and Grey Poupon don’t contain gluten ingredients yet lack the gluten-free label, which can confuse consumers.
Consumers looking for gluten-free mustards should be cautious and examine labels, especially since some popular varieties may contain hidden sources of gluten.
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