The Monod Model Is Insufficient To Explain Biomass Growth in Nitrogen-Limited Yeast Fermentation

The yeast Saccharomyces cerevisiae is an essential microorganism in food biotechnology, particularly in wine- and beermaking. During wine fermentation, yeasts transform sugars present in grape juice into ethanol and carbon dioxide. The process occurs under batch conditions and is, for the most part, an anaerobic process. Previous studies linked nitrogen-limited conditions with problematic fermentations, with negative consequences for the performance of the process and the quality of the final product. It is therefore of the highest interest to anticipate such problems through mathematical models. Here, we propose a model to explain fermentations under nitrogen-limited anaerobic conditions. We separated biomass formation into two phases: growth and carbohydrate accumulation. Growth was modeled using the well-known Monod equation, while carbohydrate accumulation was modeled by an empirical function analogous to a proportional controller activated by the limitation of available nitrogen. We also proposed to formulate the fermentation rate as a function of the total protein content when relevant data are available. The final model was used to successfully explain experiments taken from the literature, performed under normal and nitrogen-limited conditions. Our results revealed that the Monod model is insufficient to explain biomass formation kinetics in nitrogen-limited fermentations of S. cerevisiae. The goodness of fit of the model proposed here is superior to that of previously published models, offering the means to predict and, thus, control fermentations. IMPORTANCE Problematic fermentations still occur in the industrial winemaking practice. Problems include low rates of fermentation, which have been linked to insufficient levels of assimilable nitrogen. Data and relevant models can help anticipate poor fermentation performance. In this work, we propose a model to predict biomass growth and fermentation rates under nitrogen-limited conditions and tested its performance with previously published experimental data. Our results show that the well-known Monod equation does not suffice to explain biomass formation.