I recently brewed a cocoa stout. problem is attenuation is too low : 55%
any insights? here are the specs:
1st fermentation : 2 weeks at 70F, 2nd : 1 week at 70F. attenuation didn't change from racking.
yeast used : WLP036 dusseldorf (altbier) close to the expiring date, though well kept in fridge.
2 litres starter 18 hours prior to pitching for a 40L batch, which perhaps wasn't vigorous enough - though the actual fermentation seemed to be quick and sufficiently vigorous.
oxygenation of the wort was just as usual.
mash procedures: too hot at first (80 celcius) and brought back to 67 celcius with the addition of cold water after 15 minutes. perhaps denatured too many enzymes (to break down long chain sugars), thus not enough fermentables and too many unfermentables in the wort? though the conversion with the iodine test was OK after 90 minutes of mashing...
I'd say that 80C (167F) denatured all of the Beta enzymes and a lot of the Alphas, which would give you a lot of unfermentable wort and a high finishing gravity.
70F is pretty warm for fermenting, btw.
Did you measure the gravities with a hydrometer or a refractometer? I ask because alcohol will not allow an accurate reading with a refractometer after fermentation has begun.