Online foam separation was proposed to recover nisin during fermentation of Lactococcus lactis subsp. lactis ATCC 11454. Firstly, the optimal pH profile of nisin fermentation was investigated including different realkalization set values and pH drop gradients. Then the selected pH profiles of 5.75 +/- 0.05 and 6.25-5.75 (+/- 0.02) were used to perform nisin fermentation coupling with foam separation. The results showed that pH profile of 5.75 +/- 0.05 was better than that of 6.25-5.75 (+/- 0.02) for online foam separation. With the optimal pH profile, an aeration of 20 ml min(-1) that started at 8 h of incubation and lasted for 2 h resulted in 6.6 times higher specific productivity than that of the fermentation without aeration. Nisin synthesis was therefore prolonged with low sucrose concentration in the culture broth, which indicated that the feedback inhibition of nisin is more influential than the substrate limitation of sucrose in the late phase of nisin fermentation. Total nisin production (4,870 +/- 180 IU ml(-1)) was increased by 30.3% with online foam separation. This effective online recovery method for nisin production could be easily scaled up due to the facile operation of foaming process.