What is psyllium husk?

The outer coating of the psyllium plant seed (Plantago ovata), sold as whole-flake husks or fine ground powder. Mostly soluble fiber. In gluten-free baking, psyllium hydrates into a gel that mimics gluten's elasticity — without it, GF doughs are essentially batters with no way to trap fermentation gas. Typical usage: 4-8% by flour weight in GF breads. The powder form hydrates fast and disperses evenly; the whole flakes are more visible in the crumb and don't fully dissolve. For breads, always use powder. Common brands: NOW Foods (psyllium husk powder), Yerba Prima, Anthony's. Whole husks can be ground in a spice grinder if that's all you can find.