A model that fully describes the typical pH(t) profile representing the lactic acid production kinetics of Lactobacillus bulgaricus cultures is reported. The model, a four-parameter function [pH=(A-D)/(1+(t/C)(B))+D], is able to fit any change on the experimental pH-time curves, due to variations on the inoculum cell concentration of the culture. The four fitting parameters (A, B, C and D) of this model are closely related to the lactic acid fermentation and they have a physical meaning. Parameters A and D represent the initial and final pH of the culture, respectively. Parameter B is related to the slope of the linear decreasing region from the pH-time curve and C represents the time at which half of the total decrement of pH is achieved. The proposed model can be used not only for evaluating and comparing the acidifying capacity of homolactic cultures but also for predictions of final fermentation times.