Diauxic Behavior of Yeast in a Fed-Batch Reactor
Yeast exhibits diauxic behavior with respect to the glucose and ethanol in the medium as alternative substrates. When glucose levels are high, fermentation will occur instead of respirative oxidation, thereby reducing biomass yields. The constant designates the fraction of respiring biomass and , the fraction of fermenting biomass. The rates of the processes are controlled by three enzymes, designated , and .
The results give us the concentrations of glucose (Glu), ethanol (Eta) and biomass () during the fed-batch process. Enzyme activation for the transformation of ethanol to biomass has an initial concentration of starting enzyme . This is converted to enzyme , which catalyzes growth on ethanol through an intermediate enzyme . Thus, the production rate of enzyme is strongly inhibited by glucose,
and the production rate of enzyme controlling the conversion of biomass to ethanol depends on ,
Glucose inhibits the formation of enzyme , and controls the level of this enzyme. You can experiment with the influence of this constant by modifying the values. The initial value of is critical since the concentration of this enzyme decreases throughout the fermentation. You can change the initial value to see its influence. The Demonstration has a control to feed the reactor and thereby maintain the biomass in a respiring condition.
 I. J. Dunn, E. Heinzle, J. Ingham and J. E. Prvenosil, Biological Reaction Engineering: Dynamic Modelling Fundamentals with Simulation Examples, 2nd ed., Weinheim: Wiley-VCH, 2003.