Despite the fact that CO2 production by Propionic Acid Bacteria (PAB) fermentation is the driving force for eye growth in most hard and semi-hard cheese types, few coefficients of CO2 production have been assessed in literature. In this study, the diffusion free CO2 production rate coefficient was experimentally assessed in semi-hard cheese as a function of salt content (0-1.3% w/w), ripening temperature (13-25 degrees C) and ripening time (2 weeks). The CO2 production rate decreased linearly with increasing salt content due to the inhibition effects of salt on the PAB, while it increased with temperature, due to the vicinity with the PAB optimum growth temperature (25-30 degrees C). A lower rate of production was observed in the first days of ripening, followed by an increase in CO2 production rate and consequent decrease due to the lower availability of the main substrate (lactate). We proposed a polynomial equation for predicting changes in the CO2 production rate during ripening. This equation may be used to better describe the phenomenon of eye growth occurring in cheese with propionic acid fermentation. (C) 2016 Elsevier Ltd. All rights reserved.