Impact of Sodium Reduction on Survival of Listeria monocytogenes

Introduction: Health risks associated with high sodium intact is prompting cheese manufacturers to develop low-sodium alternatives. However, these changes require a thorough evaluation of the potential food safety implications as salt is involved in controlling microbial growth. Listeria monocytogenes, frequently related to dairy foods, is the most likely candidate to assess the impact of salt reduction on its survival and growth.

Purpose: The purpose of this project was to assess the ability of Listeria monocytogenes to survive in low-sodium commercial sliced process cheeses.

Methods: The experiments were conducted with three different brands of commercially available sliced cheeses (brands A, B, C) packaged in bulk (slice on slice) and as individually wrapped slices. Brands A, B and C had sodium contents 44, 34 and 29% less, respectively, than their corresponding standard formulations. The effects of salt content and temperature were measured using surface inoculation of slices with mixtures of 5 strains of L. monocytogenes and determining bacterial counts during long term storage. The changes in Listeria counts were correlated with those factors as well as with pH, water activity and moisture content.

Results: At 4°C, the count of L. monocytogenes remained at approximately 4 log CFU/g  for the entire 60 days of storage in any of the three brands of regular and reduced salt processed cheese in both SOS or individually wrapped slices. As the storage temperature increased, the Listeria counts declined reaching undetectable levels at any condition after 25 and 15 days at 23 and 30°C, respectively.  At those temperatures, the survival rate in brand C appeared to be shorter than the other two brands.

Significance: The main finding of this research was that sodium reduction in processed cheese did not enhance survival nor promoted growth of L. monocytogenes at any temperature or brand tested.