This paper will develop an observer based control using sliding mode techniques to obviate the need to measure biomass concentration. A new variable, the substrate consumption rate, consisting of a combination of substrate concentration, biomass concentration, specific growth rate and yield production coefficient is introduced to simplify the nonlinear differential equations of the fermentation process. A sliding mode observer is then developed to estimate this substrate consumption using readily available measurements of substrate concentration. It is shown that the sliding mode exhibited by the corresponding observer error dynamics is exponentially stable. This parameterisation and the resulting estimate of biomass concentration is then utilised within a feedback control strategy. Nonlinear simulation results in the presence of both parameter uncertainties and external disturbances illustrate the application of the approach.