A microbial bioreactor based on calcium alginate immobilized Lactobacillus cells coupled to a pH electrode was developed for quantitative determination of carbohydrate fermentation activity. A high biomass (10(10) cfu mL(-1)) and particular pregrowth conditions were needed. Reduction of catabolite repression by monosaccharides was achieved by pregrowth in lactose. The evolution of acid production in a continuous flow-stopped flow bioreactor was monitored for different sugar solutions in contact with the immobilized bacteria. The resulting slopes (Delta mV/Delta t) were used to quantify the fermentation capability for a defined sugar related to that of glucose, which was taken as 100%. The procedure is simple, being based on pH variation that can give quantitative results compared to other reported techniques for carbohydrate fermentation pattern from which only qualitative results are obtained. In addition, it offers reduction in time and costs and is a suitable tool for the rapid analysis of isolated strains and in studies of modifications of sugar metabolism in mutants.