My current brew-improvement-mission is to reduce the amount of unwanted/unpredictable esters which keep occurring in by brews. I was recently able to effectively eliminate too high of a fermentation temperature as a cause, by buying a refrigerator. My last brew was kept at a constant 18°C for the first week of fermentation and only raised to 20°C for the last few days. Unfortunately, I still ended up with quite a bit more esters than I was expecting.... Remaining possible causes are under-pitching and stressed yeast. Considering I only use fresh yeast every 3rd brew (the other 2 are pitched using yeast harvested from the previous brew), and the esters have been occurring the strongest in the brews fermented using harvested yeast, I am fairly certain that I am at least under-pitching. The Starter Calc. here tends to back up my suspicion, though it is hard to be 100% sure, because I don't really know how much "good" yeast I am harvesting. Either way, my next step is to eliminate the possibility of under-pitching by building a stir-plate and boosting my starter by a lot. What I am not so sure about is.....stressed yeast. I always pitch horizontal (same OG) or up, never down. So that shouldn't be an issue, but I do dry hop in my primary (thanks to Larry for the time and effort saving tip! ). I recently read something *somewhere* that seemed to suggest that yeast shouldn't be harvested from anything containing non-cooked hops. Is that a real risk? ...or could it be, that I simply misunderstood and shouldn't worry? EDIT: PS, some general background info... I always brew IPAs using WLP007 and primarily German Pale Ale and CaraFoam malts. Most of my brews are ~14-15°P OG and I normally pitch yeast into 1.5l. 9°P wort for 24hrs to make a starter. My last brew was a 19°P OG IIPA (slightly more % crystal malt than normal) with a 2.5l. starter, using 2nd generation harvested yeast.