The last couple fermentations of our IPA (15°P) using our first gen yeast (WLP090) have not attenuated as well as usual and I was wondering if anyone has any advice. Our original process was to brew with our 15BBL system into our 15BBL FV's. We would prop up the yeast the night before with ~35L of 10°P wort. Doing this we consistently had an AE of ~2.5°P in around 7 days. We recently brought in 30BBL tanks and have been brewing twice on concurrent days to fill them using a 15BBL yeast pitch (the first brew effectively becomes a starter for the 2nd). When we are using slurry from another batch this has produced similar results as before. The last two batches have been 1st gen's propped up as I mentioned before and the fermentations have been sub-standard. The first of these stopped at 3°P and the most recent one seems to have stopped at 3.5°P. The cell counts on the first batches have been a bit low, but not alarmingly so (~500,000 cells/ml/°P) and 24 hours later the yeast has roughly tripled its population which should be sufficient for the 2nd brew. I am fairly certain it is not a nutrient or oxygen issue as we provide ample amounts of both and the lower gravity brews we brew in this way have fermented very consistently. There are a couple things I am thinking. One is that perhaps we should be aerating the 2nd batch, but this seems unwise and unnecessary to me as there seems to be ample yeast and the yeast has moved into the beginnings of its anaerobic state already by the time we run in the 2nd batch. Another is that possibly we should be leaving another day between brews (we do this already to good effect with our lagers) as perhaps the yeast hasn't moved far enough into it's anaerobic phase and would benefit from more time before the 2nd brew gets run in. I am thinking the 2nd of these is more likely. The paranoid part of me thinks it might be something to do with White Labs new (and totally annoying) new "milk bag" format, as this problem occurred exactly as these came on the market. Anyone else experienced questionable yeast vitality with these packs? Anyways just wanted to throw this out and see what the consensus was on this issue. Cheers.
PS Fermentation temp 19°C
PS Fermentation temp 19°C
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