Growth Comparison of Listeria monocytogenes in Laboratory Media Using Equimolar Concentrations of NaCl and KCl

Introduction: In an effort to reduce sodium content in foods, manufacturers frequently replace sodium with potassium salts.  However, due to differences in molecular weights, substituting salts on a percentage basis may result in reduced microbial inhibition.

Purpose: To compare the inhibition of Listeria monocytogenes (Lm) by various molar concentrations of NaCl and KCl.

Methods: Trypticase soy broth (includes 0.6% or 102 mM NaCl in the base) adjusted to pH 5.2 or 6.2 with HCl, was supplemented with an additional 0, 250, 500, 750 and 1,000 mM NaCl or KCl or equimolar mixture of NaCl: KCl to provide a total of 500 or 1,000 mM, and sterilized by autoclaving.  Treatments were inoculated with 3-log CFU/ml Lm (3-strain mixture) and five tubes/ treatment assayed three times per week for changes in absorbance_{600 nm} during storage at 4 and 7 °C for 8 weeks. In addition, treatments were analyzed for %Cl-, pH, and a_w.

Results: When compared on a molar basis, there was no difference in a_w for NaCl vs. KCl even though the w/v concentrations of the KCl salts were greater than the NaCl treatments; a_w values were 0.99, 0.98, 0.975, 0.965, and 0.960 for the 0, 250, 500, 750 and 1,000 mM salts treatments, respectively.  Similarly, time until growth (A_{600 nm} increase by 0.1) was not substantially different between the two salts.  For example, at 4 °C the pH 6.2 Control grew at D-19, whereas growth was delayed until an average 22, 28, and 32 days for the 500, 750 and 1,000 mM salt treatments.  Interestingly, Lm growth was slightly enhanced in the 250 mM added NaCl treatments compared to the Control regardless of the pH or storage temperature.

Significance: This study suggested that KCl will provide equivalent antilisterial activity as NaCl when substituted on a molar basis.  Additional studies should confirm efficacy in foods and with other pathogens.