I recently made a post on this subject, but I have additional information that some of you might find interesting. After looking back on some of my notes, those congeners (or impurities as some call them) are absolutely necessary for the good taste of bourbon, but the question is, how do they get rid of the bad ones? After a discussion with Bill Friel (and I hope I get this right as I am taking it from my notes), he informed me that the condensor on a still which is usually located at the highest point that you can see steam coming out of (approx. 4 to 5 inches in diameter) and you won't see a steady steam but it "puffs". When you see a "puff-puff-puff" that is what the condensor is doing, getting rid of the bad congeners. There are certain things like fusel oils that can't be puffed off by the main condensor by controling the temperature-like acetates and aldehydes can. The only way to control fusel oil levels in the product is to control how much is created in the fermenter. Every strand of yeast has its tendency to produce fusel oils, some more than others. Some yeast are stimulated to produce fusel oils if the fermenter is set too cold. Some produce fusel oils is the peak fermentation temp. is acheived too early in the cycle and held for the rest of the fermentation cycle. Distillers know their yeast and control accordingly, but the point is --once created--the fusel oils are there and cannot be extracted. It can be reduced in the final product by mixing a low level prod. with a high fusel oil level prod. The solution to this problem is dilution (an old environmentalist by-word)....the solution to pollution is dilution. The creation of acid is similar but most acids in bourbon are created by the good bacteria that come in with the malt. Malting is done below 148 degrees, so these good bacteria are not killed. At the end of the active fermentation-if beer is not distilled right away acids will continue to form unless the beer is chilled below 70 degrees and kept cold. Those good bacteria are called "Lactic bacteria" and acids cannot be "puffed off" either. Each distiller has his ideal level of acids for the BITE - remember how many times you have heard how Barton has that certain BITE -he wants in the product.