Emulsion matrixes affect Salmonella survival during simulated gastric digestion

Salmonella enterica subsp. enterica serovar Typhimurium is an emerging prominent foodborne pathogen worldwide. While its acid resistance and pathogenicity have been investigated over the years, there is a necessity to systematically study how food matrices impact Salmonella's resistance to environmental stresses and survivability in the digestive tract. In this study, coarse water-in-oil (W-O) emulsion and oil-in-water (O-W) emulsion matrices were inoculated with Salmonella in the oil and water phase, respectively. The emulsion matrices were then challenged with simulated gastric acid (pH 2 HCl solution with 3 g L^-1 pepsin) facilitated with stomacher mixing at 37 °C, and samples were taken at designated time points for bacterial counts. The survival curves suggested that the W-O emulsion offered a significant protective effect against simulated gastric digestion, which achieved 1.55 ± 0.61 log(CFU ml^-1) reduction in 60 minutes. However, the same level of protection was not observed in the O-W emulsion, and it showed 4.54 ± 0.69 log(CFU ml^-1) reduction in 60 minutes. As for the acid resistance of Salmonella, there was no significant difference between water phase and oil phase inoculation. In addition, the protective effect could be mainly attributed to the W-O emulsion structure but not simply to the high viscosity of the W-O emulsion matrix. Furthermore, the results also indicated that more than 16.3% of bacterial cells were present in the oil phase of the W-O emulsion, which was critical to Salmonella survival. In conclusion, our results revealed an increased health risk of the W-O emulsion against gastric digestion when it is contaminated with foodborne pathogens.