A simple and parsimonious model originated from the Weibull frequency distribution was proposed to describe non linear survival curves of spores. This model was suitable for downward concavity curves (Bacillus cereus and Bacillus pumilus) as well as for upward concavity curves (Clostridium botulinum). It was shown that traditional F-values calculated from this new model were no more additive, to such an extend that a heat treatment should be better characterized by the obtained decade reduction of spores. A modified Bigelow method was then proposed to assess this decade reduction or to optimize the heat treatment for a target reduction ratio.