A model was established to predict growth rate as a function of temperature, pH and water activity. The model is based on two, earlier developed models, one for growth rate as a function of temperature and water activity and the other for growth rate as a function of temperature and pH. Based on the assumption that combinatory effects between pH and water activity do not exist, the two models were multiplied to produce one overall model. The overall model was then fitted to data sets measured earlier, and the parameters of the model were determined. A new data set with values for controlling variables outside the range of the earlier developed model was then used to validate the overall model statistically. The model was well able to extrapolate outside the measured data range. Finally, the model was updated with all measured data. No significant changes in the parameters were found. The approach followed underpins the gamma concept, since in the gamma concept it is assumed that the effects of controlling variables can be multiplied, and cardinal parameters are not a function of other variables (temperature, pH, and water activity).