Growth of Saccharomyces cerevisiae is controlled by its limited respiratory capacity: Formulation and verification of a hypothesis

Abstract

A novel mechanistic model for the growth of baker's yeast on glucoseis presented. It is based on the fact that glucose degradation proceeds via two pathways under conditions of aerobic ethanol formation. Part is metabolized oxidatively and part reductively, with ethanol being the end product of reductive energy metabolism. The corresponding metabolic state is designated oxidoreductive. Ethanol can be used oxidatively only. Maximum rates of oxidative glucose and ethanol degradation are governed by the respiratory capacity of the cells. The model is formulated by using the stoichiometric growth equations for pure oxidative and reductive (fermentative) glucose and ethanol metabolism. Together with the experimentally determinable yield coefficients (Y(X/S)) for the respective metabolic pathways, the resulting equation system is sufficiently determined. The superiority of the presented model over hitherto published ones is based on two essential novelities. (1) The model was developed on experimentally easily accessible parameters only. (2) For the modeling of aerobic ethanol formation, the substrate flow was split into two simultaneously operating (i.e., in parallel) metabolic pathways that exhibit different but constant energy-generating efficiencies (respiration and fermentation) and consequently different and constant biomass yields (Y(X/S)). The model allows the prediction of experimental data without parameter adaption in a biologically dubious manner.

A Model Based Simulation of Brewing Yeast Propagation

“…To describe the growth and the ethanol formation in the baker's yeast propagation process, many structured and unstructured modelling approaches are known from the literature. Unstructured modelling approaches, based on the Monod equation and its descendants with the assumption of balanced growth and product formation 27 , are often reproached with the fact that they inadequately describe the response of a microorganism population to environmental disturbances. According to Sweere et al 28 available structured models can be distinguished in different types from approaches based on a structured biochemical pathway 8,26,29 to more segregated modelling approaches regarding different cell ages 2,11 .…”

“…carlsbergensis (W34/ 70) required here, no comprehensive modelling approach is available, which takes into account the very limiting composition of the specific growth medium, i.e., brewer's wort. This medium is characterised by high concentrations of sugar (100 g/L), which leads to regulation effects such as the crabtree-effect expressed by reductive, oxidoreductive or oxidative growth of Saccharomyces sp 27 . A lack of nitrogen and trace elements, e.g.…”

“…Three areas were counted for each sample to avoid outliers. For the simulation software, the biomass concentration was calculated using the dry weight of the yeast, the yeast count, and the molecular weight of the biomass was set to 25.01 g/mol, according to the literature 27 .…”

“…For each substance a rate of metabolic turnover was defined. Here, biomass composition was adopted from Sonnleitner and Käppeli 27 .…”

“…For non-growth associated maintenance quantities of m ATP = 0.007-0.018 mol ATP/C-mol biomass and hour are given in the literature 23,29 . Under aerobic conditions, ethanol can be degraded (r 7 ), if the glucose concentration in the medium is very low 27 . In this case, ethanol is transformed to acetaldehyde, acetate and acetyl-CoA.…”