Model for the combined effects of temperature, pH, and sodium lactate on growth rates of Listeria innocua in broth and Bologna-type sausages.

A modified Monod equation was successfully applied to describe the maximum specific growth rate of Listeria innocua in a broth model in the presence of various concentrations of sodium lactate or NaCl. The combined effects of temperature and pH were assessed by translating the parameters of the modified Monod equation mu(m), alpha, and p') as functions of pH and/or temperature. As a result, the area in which the growth rate could be predicted was extended to include as a variable not only the salt concentration but also pH and temperature. The number of parameters needed to describe the experimental data was thereby reduced from 48 to 4 (NaCl) and from 42 to 5 (sodium lactate). The decline in the goodness of fit that accompanied the reduction in the number of parameters was within statistically acceptable ranges. The resulting model was compared with a polynomial fit, and it was proposed that the former was more suitable for the purpose of this study. The broth model for sodium lactate was evaluated with Bologna-type sausages. Because of the "worst-case" design of the broth model, it was necessary to reestimate one or all parameters to obtain a good description of the growth rate of L. innocua in the meat product. However, the simplicity of the model and the practical usefulness of its parameters offer considerable prospects for its use in predictive microbiology.

Modelling growth rates of Listeria innocua as a function of lactate concentration.

The effect of sodium lactate (NaL) concentration on growth of Listeria innocua in a yeast-extract/peptone broth at pH 5.5, 6.0, 6.5 and 7.0 at 4, 10, 20 and 30 degrees C was modelled with the modified Gompertz model. NaCl was used as a reference to distinguish between the water activity effect and the specific inhibitory effect of NaL. Minimum inhibitory concentrations (MIC) of NaCl appeared to be significantly higher than MIC values of NaL, indicating that NaL had a specific inhibitory effect on growth of L. innocua. The MIC values of NaL and NaCl were not much influenced by the temperature. The pH of the growth medium was shown to have influence on the MIC values of NaL but not on the MIC values of NaCl. Total growth inhibition of L. innocua at low pH (5.5) took place at lower NaL concentrations (217 mM) than at neutral pH (1071-1339 mM), indicating that the undissociated lactic acid plays a role in the mechanism of inhibition. However, MIC values for undissociated acid increased with decreasing pH from 0.8 mM at pH 7 to 5 mM at pH 5.5. It is therefore likely that besides acidification of the cytoplasm due to diffusion of undissociated acid into the cell, other mechanisms are involved. Growth rates at NaL concentrations between 0 and the MIC value decreased progressively with increasing concentrations down to 0 at the MIC value, and were strongly influenced by both temperature and pH. Growth rates in the presence of NaCl were influenced by the temperature only. It was shown that a modified Monod equation with three parameters was effective for description of growth rates of L. innocua at NaL and NaCl concentrations over the whole experimental range.

Effect of Sodium Lactate on Toxin Production, Spore Germination and Heat Resistance of Proteolytic Clostridium botulinum Strains.

The effect of sodium lactate and sodium lactate combined with sodium chloride (NaCl) on toxin production by proteolytic strains of Clostridium botulinum was determined in peptone-yeast extract medium, pH 6.1. Both inhibitors were also tested for their effect on thermal destruction of spores. Additionally, the effect of sodium lactate on germination of spores was assessed. The inhibitory effect of sodium lactate was dependent on the applied incubation temperature. The best inhibition was obtained at low temperatures. Toxin production was delayed at 15 and 20°C by sodium lactate concentrations of 2 and 2.5%, respectively. Complete inhibition of toxin production at 15, 20 and 30°C occurred at concentrations of 3, 4 and >4%, respectively. Further, sodium lactate inhibited germination of the C. botulinum spores, which may partially explain the inhibitory effect of sodium lactate on growth and toxin formation. The inhibitory effect of NaCl at concentrations resulting in an identical water activity value as obtained by sodium lactate was negligible, indicating that the inhibitory effect of sodium lactate was not caused by decreasing water activity. No clear synergistic effect of sodium lactate (1.5 or 2.5%) and NaCl (2.1%) was observed.

Minimum inhibitory concentration (MIC) of sodium lactate for pathogens and spoilage organisms occurring in meat products.

Pathogens and spoilage organisms occurring in meat products were screened in laboratory media, according to a method of Eklund (1983), in order to determine the specific inhibitory effect of lactate on growth of these microorganisms under optimum growth conditions (pH 6.5, 20 degrees C). In general, Gram-positive bacteria were more sensitive towards lactate than Gram-negative bacteria. It was shown especially, that strains that were able to grow at water activities of 0.95 and below in the presence of NaCl (Staphylococcus aureus, Listeria monocytogenes, Brochothrix thermosphacta) were inhibited by sodium lactate (NaL). It appeared, that yeasts were resistant to large amounts of NaL (> 10% w/v). However, NaL had a specific inhibitory effect on growth of these organisms when compared to the effect of NaCl. These results indicate that lactate addition to food products with a pH near neutrality offers good prospects for shelf life prolongation.