Purpose: To investigate the acid resistance of different serovars and strains of Salmonella spp. in mayonnaise after adaptation to sublethal concentrations of acetic acid and pH values.
Methods: Two strains of Salmonella Enteritidis and three strains of Salmonella Typhimurium were tested. Following growth in tryptic soy broth without glucose (TSB Glu(-)), cells were transferred to TSB Glu(-) supplemented with 15, 35, and 45 mM total acetic acid (AA). The pH of the medium was adjusted to values 5.0, 5.5, and 6.0, respectively, resulting in different concentrations of undissociated AA. After incubation for 90 minutes at 37°C, the cultures were inoculated to commercial packages of mayonnaise and the samples were stored at 5°C. Nonadapted (NA) inocula were grown in nonacidified TSB-Glu(-).
Results: The time needed for the reduction below the enumeration limit (<two log) was affected by the serovar tested. Both Salmonella Enteritidis strains were more acid sensitive compared to Salmonella Typhimurium strains, since they remained countable for 24 to 46 hours depending on the strain and the treatment tested, while Salmonella Typhimurium strains were enumerated for up to five to seven days on a general purpose medium supplemented with sodium pyruvate, depending again on the strain and the treatment tested. For some of the strains used, differences in the acid resistance were observed between NA inocula and some adaptation treatments. For example, adaptation at 45mM/pH6.0 resulted in higher population levels (P<0.05) compared to NA cells for one strain of Salmonella Enteritidis.
Significance: This study may provide new insights for the role of undissociated acetic acid and its impact on Salmonella stress response and survival in acetic acid based products.