The present work seeks a set of reasoning rules that permit the automatic selection of the start point for inverted sucrose feeding in the Cephalosporin C batch production process, using fuzzy methodology. By monitoring the percentage of CO2 in the outflow gases, it is possible to observe a point of maximum evolution when the microorganism growth phase finishes. Therefore, the moment when the feeding should begin is characterized by a transition from increasing (positive variations) to decreasing (negative variations) CO2 evolution rates. A fuzzy controller was then conceptualized to operate on three reasoning levels: attention, action and a protection level to prevent errors produced by the exchange of the drying column used before the analyzer for CO2 measurement. The algorithm was implemented in C. Two experiments were accomplished to establish, adjust and validate the fuzzy system. The results obtained indicated that the algorithm is robust for the tested conditions, allowing a safe automatic operation.