Modelling microbial growth in meat broth with added lactic acid under refrigerated storage

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Abstract
Summary: Lactic acid is a classical preservative in meat industry and it is used with high efficiency on sanitization of meat surfaces. This work analyses the effect of temperature and added lactic acid on the growth of different beef muscle‐isolated bacteria: Pseudomonas sp. and two enterobacteria identified as Klebsiella sp. and Escherichia coli, in a liquid model system. Inoculation levels between 104 and 106 CFU mL−1 in a culture medium of meat extract broth at 2% were studied, with addition of lactic acid in concentrations of 0.29, 0.39 and 0.58 m, which allowed to reach pH values of 6.1, 5.8 and 5.6 respectively. Culture media inoculated with each micro‐organism were incubated at three temperatures: 0, 4 and 10 °C in aerobiosis; bacterial counts as a function of time were mathematically modelled by applying Gompertz equation. Derived parameters: lag phase (LPD), specific growth rate (μ) and maximum population density were calculated. When the observed effect was bacteriostactic a linear regression was used. Results were compared with a meat extract broth control at pH = 7. At 10 °C added lactic acid produced a higher variation in μ values for Klebsiella sp. and E. coli, whereas LPD values were modified between six and three times for each micro‐organism. The highest LPD values were observed for E. coli, followed by Pseudomonas and finally by Klebsiella. LPD values for E. coli at 0 °C ranged between 40 and 50 days, whereas at 10 °C they varied between 3 and 6.5 days. Escherichia coli did not grow at 0 °C, except when the control system was at pH 7. The effect of temperature on μ values was modelled through an Arrhenius type equation and the corresponding activation energies were determined; μ and LPD values were correlated with the undissociated acid concentration for the three tested temperatures.