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Byline: Roberto Carcoba (1), Carmen Pin (2), Ana Rodriguez (1) Keywords: Lactococcus lactis; Chemostat cultures; Lactose metabolism; Dilution rate; pH Abstract: Lactococcus lactis subsp. lactis IPLA 947 is a starter strain isolated from a Spanish farmhouse acid-coagulated cheese. To use this strain as starter culture in Afuega'l Pitu cheese manufacture, anaerobic lactose-limited chemostat cultures were used to optimise the active biomass production in a simple medium (BRFS). Growth, lactose consumption and metabolite production were measured at different pH values (5.5, 6.0 and 6.5) and dilution rates (0.06 to 0.96 h.sup.-1). The influence of both variables on the quoted parameters was described by means of mathematical models. These provided a satisfactory representation of the experimental data. The bacterial response surface showed a significant increase in the biomass when the pH increased and the dilution rate decreased. As dilution rate increased, biomass yield increased but only at the lowest pH. The lactose consumption and the ratio of metabolic end products were also influenced by both variables. As dilution rate decreased, the residual lactose decreased and increased amounts of metabolites other than lactate (acetate, formate, ethanol) were produced. The minimum lactose consumption was obtained at pH 5.5 and 0.96 h.sup.-1 dilution rate. A significant rise of acetate and formate levels occurred as pH increased. A maximum lactate level, along with an acetate, formate and ethanol production close to the minimum, was predicted by mathematical modelling at 0.5173 h.sup.-1 dilution rate and pH 6.054. Author Affiliation: (1) Instituto de Productos Lacteos de Asturias (IPLA-CSIC), Apdo. 85, 33300, Villaviciosa, Asturias, Spain (2) Institute of Food Research, Norwich Research Park, Norwich, NR4 7UA, UK Article History: Registration Date: 01/06/2004 Received Date: 28/01/2004 Online Date: 02/07/2004