The antimicrobial activity of sodium lactate

In this thesis, the action spectrum and mechanism of microbial growth inhibition by sodium lactate were examined, with special emphasis on its use in meat products.The concentrations (mM) of lactate needed to prevent growth of various spoilage organisms and pathogens in a broth were determined and compared to those of NaCl. The sensitivity towards lactate differed between microorganisms, as did the effect of pH on the minimum inhibitory concentration (MIC) of lactate. Especially, bacterial strains that were able to grow at low water activity (≤0.95) in the presence of NaCl were inhibited by sodium lactate (Staphylococcus aureus, Listeria monocytogenes, Brochothrix thermosphacta), indicating that the microbial quality of food products could be increased when part of the NaCl content would be exchanged by sodium lactate. This is also true when the influence of lactate on toxin production, spore germination and heat resistance of spores from Clostridium botulinum is considered.The effects on growth characteristics of pH, temperature and sodium lactate or NaCl concentrations below the MIC, as examined with Listeria innocua in a broth, were successfully translated into a relatively simple mathematical model that was subsequently validated in a Bologna-type sausage. Microbial growth in the sausage was much slower than in broth, but this might be related to the growth conditions being less favourable. The parameter value for the maximum specific growth rate in the absence of lactate was re-estimated using the data obtained in sausage and as a result, the model predictions were statistically acceptable.Listeria innocua and Lactococcus lactis which had been grown in the presence of sodium lactate were better capable of regulating their intracellular pH (pH in ) than control cells (grown without lactate). This is probably related to an increase in the amount and/or activity of the proton ATPase (F 1 F 0 -ATPase) in response to a decrease in the pH in . The mechanism of microbial growth inhibition by sodium lactate remains to be further elucidated, since the decrease in pH in was not enough to explain why microbial growth in the presence of lactate is prevented at low pH.