Adding as little as 0.5 ml to a 23L batch of beer at mashout/before the boil is sufficient to reduce the risk of boil over in most circumstances.  FermCap AT antifoam agent does not adversely effect the foam stability in finished beer, what residues that are left after the boil settles out quickly at the end of fermentation. No flavour change occurs as a result of using FermCap AT.