Mucilage of yellow mustard (Sinapis alba L.) exhibits completely different rheological behaviour, depending on where it has been extracted from. Whereas mucilage isolated from intact mustard seeds or from separated brans acts like many hydrocolloids (e.g. increasing viscosity significantly at low concentrations) these and related properties (i. e. shear thinning behaviour) are lost, once mustard seeds are processed. Mucilage extracted from processed mustard only raises viscosity slightly. Although the supernatant of processed mustard consists of 3-5% mucilage (dry matter), it exhibits Newtonian flow. Changes of rheological properties of mustard mucilage were attributed to a change of molecular shape from an extended to a globular state due to processing. Chemical analysis has shown a significant raise of nitrogen content of mucilage after processing and during storage, indicating some chemical reaction taking place, possibly involving proteins.