A chemical that reduces the surface tension of foams that form on the surface of broths in fermentors because of aeration or agitation. Stearyl decanol, octal decanol, vegetable oils, silicones, sulfonates, and polypropylene glycol are typically used, though they can cause problems in refining the broth later on.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.