Understand the limits and basis of growth prevention of Listeria monocytogenes in processed meats by weak acid salts

Summary

Listeria monocytogenes is a foodborne pathogen that is not unusually resistant to heat but, unlike most foodborne pathogens, can grow at refrigeration temperature in the presence of >10 per cent salt and in the absence of oxygen.
These characteristics mean that L. monocytogenes poses a health threat to consumers of foods that have long, refrigerated shelf lives and that are eaten without further cooking prior to consumption. Such foods include vacuum-packed or modified atmosphere packed ready-to-eat processed - RTE - meats.
Methods to reduce the risk of L. monocytogenes in smallgoods include minimisation of contamination and minimisation of its growth in products.
In practice, minimising the growth of L. monocytogenes in RTE processed meats could most readily be achieved by including growth inhibitors in the product.
Permitted food additives such as weak acid salts, e.g. sodium diacetate or sodium lactate or potassium lactate, can prevent the growth of Listeria monocytogenes under some conditions of pH, water activity and temperature.
Organic acids and their salts, applied singly and in combination, have been shown by many studies to prevent, delay or greatly retard the growth of L. monocytogenes in processed meats under both recommended temperatures of storage and mild temperature abuse - e.g. up to 10°C.
Most studies have focused on the activity of sodium lactate but several also consider potassium lactate or potassium lactate in combination with sodium diacetate.
Most studies have considered the activity of organic acid salts in sausage products such as bratwurst, frankfurters or saveloys but others have also studied their effectiveness in cooked ham. Their effectiveness in sterile uncooked comminuted beef emulsion has also been considered in the literature.
A previous Australian study (AFSCoE, 2004) also indicated the efficacy of salts of organic acids as listeriostatic agents in sliced, MAP, ham. Preparations of salts of lactic and acetic acids, either singly or in combination intended for food preservation and their listeriostatic potential are commercially available.
In this study experiments were designed to examine the influence of two variables on the development of populations of Listeria monocytogenes in two commercial smallgoods - Modified Atmosphere Packaged sliced ham and shaved chicken. The two variable were:presence of preservatives, namely weak acid salts - either a potassium lactate-diacetate blend, or sodium lactatestorage temperature - either 4 or 8°C
The influence of total viable bacteria on the potential for growth of L.monocytogenes was also considered.