Beware (from what I understand) that fermenting under pressure can result in underattenuation and increased diacetyl/acetaldehyde production. Harvesting yeast from this and repitching can also lead to issues over time.Do you have any references for this? I'd appreciate any information.
The chart Chris had that I'm trying to get a hold of. Temp, pressure, and yeast strain all played parts in the tests White Labs did. That's why I wasn't too surprised at your results at that pressure.
Did some more reading, don't know how I missed this before.
Briggs, Boulton et al. (p. 482) refer to trials under "moderate" top pressure leading to suppressed yeast growth as well as yeast damage, higher than normal pH, reduced head retention and other effects suggestive of autolysis and effects of increased dissolved CO2. It should be noted that what they call "moderate" pressurization is 1.2-2 Bar, and the trial conditions also included increased temperature. Nonetheless, they make an important distinction:
Brulosoohy (always taken with a grain of salt) suggested that 5-6 psig top pressure is merely equivalent to the hydrostatic pressure yeast experience in commercial fermentation. I wondered about this, whether it was apples and oranges. Briggs, et al. clearly state that the effects of hydrostatic pressure are quite distinct from those of top pressure, primarily, I gather, because a sealed (top pressurized) vessel does not allow the escape of deleterious substances.
Since I may ( I know I tried to dismiss it yesterday) have experienced three of the key effects mentioned in Briggs -- suppressed yeast growth indicated by or the slow start and low attenuation, a higher than normal pH, and possibly reduced foam capacity (can't confirm this yet, but samples may have suggested it) -- I'm now having serious second thoughts about continuing with this procedure even at 5-6 psig.