Existence of a scaling relation in continuous cultures of Scheffersomyces stipitis: the steady states are completely determined by the ratio of carbon and oxygen uptake rates

Abstract

BackgroundRecently, we showed that steady-state continuous cultures of S. stipitis follow the principles of growth on mixture of two complementary substrates. More precisely, when such cultures are fed with progressively higher concentrations of glucose sf at fixed dilution rate D = 0.1 h−1, oxygen mass-transfer coefficient kla = 50 h−1, and oxygen solubility c_{text{o}}^{*}, they transition from glucose- to oxygen-limited growth through an intermediate dual-limited regime in which both glucose and oxygen are limiting, and ethanol is produced without loss of glucose. It is, therefore, of considerable interest to characterize the dual-limited regime. We found that the dual-limited regime occurs precisely when the operating parameters D, sf, kla, and c_{text{o}}^{*} satisfy the relation Y_{text{os}} < Ds_{text{f}} /left( {k_{text{l}} a cdot c_{text{o}}^{*} } right) < Y_{text{os}}^{prime }, where Yos and Y_{text{os}}^{prime } denote g of glucose consumed per g of oxygen consumed in the carbon- and oxygen-limited regimes. In this work, our goal was to determine if the above characterization of the dual-limited regime holds over a wider range of D, kla, and to understand why the dual-limited regime is determined by the dimensionless ratio Ds_{text{f}} /left( {k_{text{l}} a cdot c_{text{o}}^{*} } right).ResultsTo this end, we performed the foregoing experiments at three additional dilution rates (D = 0.07, 0.15, and 0.20 h−1) and one additional mass-transfer coefficient (kla = 100 h−1). We find that the above characterization of the dual-limited regime is valid for these conditions as well. Furthermore, the boundaries of the dual-limited regime are determined by the dimensionless ratio Ds_{text{f}} /left( {k_{text{l}} a cdot c_{text{o}}^{*} } right), because the steady-state concentrations are completely determined by this ratio. More precisely, if the steady-state concentrations of biomass, glucose, oxygen, and ethanol are suitably scaled, they collapse into a single curve with Ds_{text{f}} /left( {k_{text{l}} a cdot c_{text{o}}^{*} } right) as the independent variable.ConclusionThe dual-limited regime is characterized by the relation Y_{text{os}} < Ds_{text{f}} /left( {k_{text{l}} a cdot c_{text{o}}^{*} } right) < Y_{text{os}}^{{prime }} over the entire range of operating condition 0.07 h−1 ≤ D ≤ 0.20 h−1 and 50 ;{text{h}}^{ - 1} le k_{text{l}} a le 100;{text{h}}^{ - 1}. Since the effect of all operating parameters is embedded in the single parameter Ds_{text{f}} /left( {k_{text{l}} a cdot c_{text{o}}^{*} } right), the dimensionless plot provides a powerful tool to compare, with only a handful of data, various ethanol-producing strains over a wide range of operating conditions.