I found something interesting re agitation (though perhaps not conclusive): http://www.academia.edu/3890122/Factors_affecting_the_growth_of_Saccharomyces_cerevisiae_in_batch_culture_and_in_solid_state_fermentation
"As can be seen from Fig. 1, the increase in agitation speed from 50 to 250 rpm led to an increase in biomass production (Fig. 1).
This paralleled with the increase in the specific growth rate, sugar and nitrogen consumption and pH reduction (Fig. 2). However, the effect of increased mechanical agitation on the viability of Saccharomyces cerevisiae appears to be negative, with higher levels of dead cells present at the end of the cultures (Fig. 3).
Previous works  on the effect of mechanical agitation on the viability of yeast cells did not report cell damage at agitation speeds values higher than the maximum value used in the present study. Since in this work, the nutrients (carbon and nitrogen) consumption increased with increased agitation intensity (Fig1), this last variable is unlikely to have a physical effect on the growth of Saccharomyces cerevisiae.Thus, the loss of viability observed at the end of the cultures could be due to the depletion of both carbon and nitrogen sources (Fig. 2) rather than to fluid-mechanical stresses caused by agitation .
However, the combination of both effects with the production of toxic metabolite (such as ethanol) may cause higher loss of viability than each factor alone."