What I experienced w/ my last (and first) AG batch might correlate your findings. Then again, it might just be coincidence, or my inexperience.
I used WLP002, which is supposed to be ridiculously flocculant, and pitched at 72 degrees in order to meet White Labs recommendations (which I'll likely change next time). I had planned on slightly under-pitching in an attempt to bring out more esters, so I was just going to pitch the vial w/o a starter. Problem was, I wasn't paying attention when I pitched (ok... I'll be honest. I was drunk by then. My first AG batch took much longer than anticipated!). So I ended up only pitching approx 1/2 the vial, as the remaining contents stuck to the inside. I also split the wort into 2 carboys (10 gal batch) and the other container got a full vial of irish ale yeast (wlp004). Both batches were very well aerated w/ a wine de-gasser, done after the pitch.
The irish ale took off fast, fermented vigorously, and floc'd out well. The 002, however, had a much longer lag time, fermented slowly, and did not floc out well. It is dropping... it's just taking a longer time. And the 'curds' aren't anything close to what I've seen from other brewers using this strain.
One possibility I've considered as the cause is yeast cell aging. My thoughts are this: If you underpitch, you're causing the yeast population to reproduce through many more generations than they would otherwise before reaching 'critical mass' when reproduction is suppressed. So you have a bunch of infants, adolescents, teens, 30-somethings, mid-lifers, seniors, geriatrics, and centarians all swimming in the same pool at once. All aging and dying (or dropping) over a much larger span of time. According to White Labs, one of the factors influencing flocculation is cell age... as the yeast cells grow older, their cell walls change (become 'rougher') which increases their chances of adhering to nearby cells. Another factor is cell promimity - they can't floc together if they don't touch. So I'm thinking it's possible that only a fraction of the cells are getting 'old' at any one time, and flocculation is therefore negatively impacted. The older cells most likely to adhere to others are few & far between, which means not only are there less of them dropping at any one time, there's less chance of them coming into contact w/ other cells they're likely to bind to.
Another factor might be the fermentation itself. If you underpitch, you can probably count on your fermentation taking longer to complete, which means the movement created by the emerging co2 helps keep the yeast in suspension longer than normal. Especially since they're not flocculating well.
If this theory holds true, then the reverse should hold true as well... if you pitch an 'adequate' cell count, or especially overpitch... you should see more aggressive flocculation as the majority of the cells all age and change at approximately the same time.
Thoughts ??
