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The in¯uence of three well-known antifoaming agents (polypropylene glycol, silicone and soybean oil) on gas-liquid mass transfer in stirred tanks is studied, both in model and in fermentation media. The effect of antifoam concentration, ionic strength, viscosity, agitation speed and gas¯ow rate are investigated. It is found that antifoam addition at low concentrations markedly decreases the gasliquid volumetric mass transfer coef®cient, k L a, for the three antifoam agents tested. Although the major effect is on the ®lm coef®cient k L , some effect is also detected on the speci®c area, a. It is found that the in¯uence of viscosity and antifoam addition are not cumulative: each tends to attenuate the other's effect on mass transfer. Both for silicone and for soybean oil, but not for PPG in the concentration range studied, there is an antifoam concentration above which further antifoam addition starts to improve k L a.

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