Mathematical modelling of microbial growth in packaged refrigerated beef stored at different temperatures

Gompertz and logistic models were fitted to experimental counts of microorganisms growing in beef stored at 0, 4, 7, 9 and 10°C. Samples were packaged in polyethylene (high gaseous permeability) and in EVA/SARAN/EVA (low gaseous permeability) films, being EVA ethyl vinyl acetate and SARAN polyvinyl and polyvinylidene chloride copolymer. Lag phase duration (LPD) and specific growth rate (μ) were obtained as derived parameters for lactic acid bacteria, Enterobacteriaceae, Pseudomonas sp. and psychrotrophic microorganisms. The reciprocal of LPD was fitted to an Arrhenius type equation; LPD of lactic acid bacteria showed a marked dependence on temperature, with activation energy values (ELPD) of 222.2 and 216.9 kJ/mol for polyethylene and ESE respectively. The effect of initial microbial population at different storage temperatures on adaptation period was analyzed. As the initial microbial population increased, adaptation period decreased for all studied microorganisms and for both packaging films. The effect of temperature on specific growth rate was better interpreted by the Arrhenius model than by the linear or the square root equations. Psychrotrophic microorganisms in beef showed the highest activation energy values for specific growth rate (Eμ) in both packaging films, being Eμ 85.50 and 103.10 kJ/mol for polyethylene and ESE film respectively. In both films, Enterobacteriaceae showed the lowest Eμ values, being 15.33 and 59.89 kJ/mol in ESE and polyethylene respectively. The final number of microorganisms (maximum population density) did not show significant changes with storage temperature.