Changes

A: It depends on the off-flavor. Typical yeast character such as moderate amounts of diacetyl, banana, clove, and other esters will often be changed by Brett. However, many other flavors from things like fusel alcohols will not. Brett can esterify a small amount of [[Isovaleric Acid]] and [[Butyric Acid]], but if the beer has high amounts of these then they are likely to stay. As the old saying goes, garbage in, garbage out. It's better to brew a clean beer and then add Brett, rather than try to recover a badly brewed beer by adding Brett.