A stir plate does not aerate a culture. About all it does is keep the medium homogeneous and degas the culture.
Then why does an open stirred starter produce more biomass than an airlocked stirred starter?
The biomass increase had nothing to do with stirring. Both cultures were stirred. An open culture has more gas exchange and less top pressure than a closed culture. The same dynamic happens in open fermentation, which is why open fermentation is usually used with cultures with high O2
demands (although, some strains have such high O2
demands that post-inoculation aeration is required). A culture that is spun fast enough to create a vortex has more surface area than a gently stirred culture, but it does so by creating more turbulence; hence, the cells experience more shear stress.
A Fishtail Aerating a Fermentation Post-Pitching at Samuel Smith's Brewery
Here is the $10,000.00 question. If stirring was beneficial to brewing Saccharomyces strains, why are batch fermentation vessels not equipped with impellers? Continuous tower fermentation vessels are equipped with slow moving impellers, but they are there to continuously mix new substrate and O2
into the fermentation in order to keep the medium chemically static.