Levels of pO2 are determined by the yeast oxygen uptake rate

It is important to clarify the relationship between high and low dissolved oxygen (pO₂) and the rate of oxygen consumption by yeast in a bioreactor. Briefly, the pO₂ is a balance between the rate of oxygen transfer from gas to liquid and the yeast oxygen uptake rate (OUR). Mathematically, the rate of change of pO₂ is given by

where pO₂* is the saturation level of dissolved oxygen (100%) that our chemostat would approach in the absence of any yeast respiration. The general solution to Eq. 5 is

The bioreactor oxygen mass-transfer coefficient (k) determines the timescale of how quickly an initial pO₂ reaches its new steady state after a change in yeast OUR. Our calibration measurements showed that steady state is reached quickly (k ≈ 1.2 min⁻¹). When the yeast OUR changes on a timescale that is slower than k, Eq. 6 is well described by the steady-state solution

If yeast OUR is high, then pO₂ is low (or 0%, the absolute minimum). Conversely, if yeast OUR is low, then pO₂ is high (or 100%, the absolute maximum). Our control experiments indicate that pO₂ accurately reflects the yeast oxygen uptake rate in our chemostat.