Saccharomyces cerevisiae is commonly used for the production of ethanol from glucose via fermentation. Typical kinetic parameters for this organism performing this process are:

Y = 0.1 gVSS/g glucose max = 0.46 h-1

Ks = 100 mg glucose/L b = 0.05 day-1

If ethanol is continuously produced in a chemostat (a reactor in which the hydraulic retention time is equal to the solids retention time) with a hydraulic retention time of 5 hours, and the influent glucose concentration is 25 g glucose/L, estimate:

1) Effluent glucose concentration (in mg glucose/L)

2) Biomass concentration in the reactor (in mg VSS/L)

3) Effluent ethanol concentration (in g ethanol/L)

Useful assumption: The S. cerevisiae biomass can be represented by the chemical formula C5H7O2N

Note: Answering 1 and 2 requires a simple kinetic calculation. A good approximate answer for 3 can be obtained by considering only the energy reaction to determine the moles of ethanol produced per mole of glucose. Answering 3 more accurately will require you to back calculate an fso value from the yield coefficient (i.e., as in Module 3) and then estimate the stoichiometry when some of the glucose is also used for cell synthesis (i.e., combining half-reaction equations as in Module 3). However, note that in a fermentation reaction there is no external electron acceptor, so you need to be careful on the construction of an appropriate energy reaction.