Dough fermentation is an important step during the preparation of fermented baking goods. For the supervision of dough fermentation, a continuous-discrete extended Kalman filter was applied, which uses an image analysis system as the measurement. By estimation a fixed number of gas bubbles inside the dough, the radius of an average bubble was determined. A mathematical dough model was used by the extended Kalman filter to estimate the radius of the average bubble, the CO₂ concentration of the non-gas dough phase and the number of CO₂ molecules in the average bubble. During a fermentation of 50 min, the extended Kalman filter estimated that the average radius increased from 50 µm to 127 µm, the CO₂ concentration in the non-gas dough increased to 23 mol/m³, and the CO₂ amount in the bubble increased from 0.1 × 10⁻¹⁰ to 4 × 10⁻¹⁰ mol. Also, the specific CO₂ production rate was estimated to be in the range from 1.5 × 10⁻³ to more than 4 × 10⁻³ mol·m³/kg/s. The advantages of an extended Kalman filter for the supervision of the dough fermentation process are discussed.