Dynamic modelling of a continuous lactose fermentation system for control and optimisation of operation has been carried out. The bioreactor used was coupled with an ultrafiltration module and an electrodialysis unit. A unstructured model taking into account cell growth, substrate consumption, and metabolite (lactic acid) production, has been analysed. The metabolite production model was represented by the Luedeking-Piret equation, modified in order to improve the description of lactic acid production at low dilution rate.