Mathematical modelling of microbial growth in ground beef from Argentina. Effect of lactic acid addition, temperature and packaging film.

The effects of: (i) storage temperature (0, 4 and 10°C), (ii) gaseous permeability of the packaging film (polyethylene and EVA SARAN EVA for vacuum packaging), and (iii) natural beef pH (5.6, 5.8 and 6.1) on the growth of different bacteria isolated from beef muscle were examined. The bacteria were Klebsiella, Pseudomonas sp. and Escherichia coli. Microbial growth was modelled using Gompertz and linear equations. The effects of temperature on microbial growth rate (μ) and on lag phase duration were modelled using an Arrhenius type equation. In polyethylene, E. coli was the microorganism, that showed the highest μ values and also the greatest effect of pH on μ, especially in samples stored at 4 and 10°C. In the case of Klebsiella sp., neither pH nor temperature had marked effects on μ and on LPD. In ESE film, μ of all the microorganisms were less affected by pH and temperature than in polyethylene. In ESE film E. coli showed the highest effect of pH on μ, at 4 and 10°C. LPD increased significantly with respect to the values in polyethylene, with Klebsiella sp., showing the highest values of LPD, followed by E. coli. Experiments in ground beef with added lactic acid producing a decrease of the original muscle pH from 6.1 to 5.6 showed that the kinetic parameters of the microbial flora did not differ significantly from those of beef samples in which the original pH was 5.6.

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