Is there any evidence that whatever shear stress might be placed on the yeast has detrimental effects on fermentation performance or the flavor of the resultant beer?
Anything that affects yeast health affects fermentation performance. I did not start out as a doubting Thomas. It was the difference in performance that made me question the claims that stir plate starters were superior to shaken starters. I never had to decant supernatant before switching to a stirred starter. Lag time increased, and high krausen was not as active. Lag time was increased and fermentation vigor was further reduced when I slowed the stir speed down enough to get rid of the foul smell.
Part of the problem can be attributed to stirring 1L of wort in a 2L flask, which is what most home brewers do. Increasing the size of the flask to 4 or 5 liters should reduce the need to stir as aggressively due the large increase in head space and surface area. One thing that I encountered while studying the physics behind shaker tables is that the culture volume should be between 10% and 25% of the flask volume, which makes a lot of sense with a conical-shaped flask. An orbital shaker further increases surface area by turning the surface of the medium into an inclined ellipse.
With that said, I do not see how a shaker or a stirrer can get around the maximum cell density problem. While there may be more actual biomass at the end of the process, is there a major increase in viable biomass? By major, I mean at least a two-fold increase because anything less than two-fold is insignificant. A viable count difference between two cultures of less two-fold results in both cultures needing to undergo the same number of replication periods to reach high krausen due to fact that yeast biomass grows exponentially at a rate of 2
n. Without a two-fold increase in viable biomass, the variable that matters is yeast health. Yeast health is dependent on the amount the amount of ergosterol and unsaturated fatty acids (UFA) that the cells hold in reserve when they are pitched as well as any stressors that the cells encountered during propagation. Given equal amounts of dissolved O
2 at the start of fermentative reproduction (all growth in starters and wort is fermentative due to the Crabtree effect) while holding all other variables constant, the culture that is pitched at high krausen will have higher ergosterol and UFA reserves, resulting lower initial O
2 demand and a shorter lag time upon pitching.
Whether or not stir plates provide any appreciable level of aeration (either before or during active fermentation in the starter) is a topic I have seen debated almost endlessly. Are you aware of any studies on this issue? I'd love to get a definitive answer; all I've ever seen is conjecture.
I have seen no peer-reviewed publications on stir plates because shakers and rollers are the preferred cell culturing devices in laboratories. It's almost a given. Magnetic stirrers are primarily used for mixing. They are less complex and cheaper than hermetically-sealed stirrers.