Modelling of microbial activity and prediction of shelf life for packed fresh fish

Prediction of shelf life based on growth of specific spoilage organisms (SSO) in model substrates was studied. The effect of CO₂ on the growth kinetics for Photobacterium phosphoreum and Shewanella putrefaciens was quantified and modelled. Results showed that microbial spoilage of packed cod stored with various concentrations of CO₂ was accurately predicted from the effect of CO₂ on P. phosphoreum grown in model substrates. The short shelf life extensions previously reported for packed cod therefore can be explained by the high CO₂ resistance of this Gram negative organism. S. putrefaciens was very sensitive to CO₂ and growth rates could not be related to the shelf life of packed cod. Growth curves without lag phases were found for all concentrations of CO₂ and for both the microorganisms studied. For the fitting of these growth curves the log-transformed Logistic models were selected after comparision with the 'modified Gompertz' models and with the model of Baranyi et al. (1993). The effect of CO2 on μmax was well described by a 2 parameter square root model. Validation of kinetic models by comparison of shelf life predictions with shelf life determined by sensory evaluations in product experiments was preferred for comparision of microbial growth rates determined in product and model system experiments. Kinetic modelling was found to be valuable for both evaluation and prediction of microbial fish spoilage and an iterative approach for development of kinetic shelf life models was suggested.