On-line estimation of lactic acid concentration during batch fermentations of Streptococcus thermophilus based on pH measurement

In this paper a model-based approach for on-line estimation of product formation in a fermentation broth is presented. The validation of the proposed method is demonstrated both in pH controlled and uncontrolled fermentation of the bacteria Streptococcus thermophilus. As the concentration of lactic acid produced by the bacteria can not be measured on-line, but is a requisite for predicting the harvesting time of the biomass, it has to be estimated from measurements that are available on-line. The only on-line available measurements are the pH value of the fermentation broth, and additionally, in pH controlled experiments, the amount of base added. Using a model-based approach based on chemical balance equations of the chemical components forming in the fermentation broth, changes in pH can be modelled even if the medium is complex and the ionic composition not exactly known. The model-based approach introduces a model acid-base system that exactly describes the acid-base characteristics of the medium. The model parameters such as concentrations and dissociation constants have to be identified off-line and are specific for the medium. Results from experiments show high accuracy in the estimation of lactic acid concentration.