This work investigates the application of compact submerged hollow fibre membrane fermenters to boost the yield and performance of Lactococcus lactis ssp. cremoris fed with whey-based medium. The membrane allowed the removal of inhibitor-rich medium and simultaneous replacement with nutrient-rich medium by a novel media-backwash method. The simple submerged membrane system operated with sustained flux in the range of 10-65 L m(-2) h(-1) using only backwash to control fouling. It was found that replacement of fermented medium using submerged hollow fibre membranes more than doubled the lactic acid production and cell growth. Further testing with boosted lactose concentration of the backwash medium, improved cell yield (by 2.4 times) and yielded higher concentrations of lactic acid. The activity of the membrane treated cultures was higher than the control, even after 30 h of continuous fermentation. During the stationary phase of growth, membrane assisted fermentation continued to produce lactic acid, indicating that productivity became tied to the rate of nutrient addition and lactic acid removal. Simple compact submerged membrane bioreactors are a viable technology to enhance active cell and metabolic by-product yields, but careful control of nutrient replacement and metabolite removal are required to ensure the most optimal growth conditions. (C) 2012 Elsevier B.V. All rights reserved.