In this work, the concentration of two commercials starters (YO-MIX™ (L) 204 and YOFLEX® (Y)) and sugar percentage (5, 7.5 and 10%) were evaluated by optimization response surface methodology (ORSM), in order to identify the higher acidification rate. The starters growth was followed during the acidification process. The results showed that optimal acidification conditions for L were at the lowest value of sugar and the highest value of the starter concentration. In the case of Y the optimal acidification conditions were with the highest starter concentration but at medium sugar levels. Total acidification times were 4.4 and 4.2 hours, respectively. The growth rate for each starter culture was determined by nonlinear regression resulting lower growth rates for streptococcus (0.705 h^-1 for L and 0.367 h^-1 for Y) than for lactobacillus (0.733 h^-1 for L and 0.389 h^-1 for Y) in each starter. Finally, significant differences were found in the time necessary to reach the maximum acidification rate for starter cultures evaluated at the same concentration of sugar. This kind of studies should continue because it offers the possibility of growth in the dairy sector by standardizing production processes.