Study of the effect of lethal and sublethal pH and aw stresses on the inactivation or growth of Listeria monocytogenes and Salmonella Typhimurium

During food processing, microorganisms are commonly exposed to multiple sublethal or lethal stresses (commonly aw, pH) sequentially or simultaneously. The objectives of the present study were: (i) to comparatively evaluate the survival of Listeria monocytogenes and Salmonella Typhimurium in lethal acid (pH 4.0 and 4.5 with lactic acid) or osmotic conditions (15 and 20% NaCl), applied singly, sequentially (pH then NaCl or NaCl then pH), or simultaneously at 5 and 10 °C; and, (ii) to quantify the effect of osmotic shifts at pH 7.0, 6.0 or 5.0 (adjusted with lactic acid) on the lag phase and growth rate of L. monocytogenes at 10 °C. In sequential lethal stress applications, the second stress was applied 2 or 3 days after the first for Salmonella and L. monocytogenes, respectively. Acid tolerance of L. monocytogenes was higher than osmotic tolerance and the opposite was observed for Salmonella. Higher inactivation was observed after exposure to pH 4.0 compared to pH 4.5 as well as after exposure to 20% NaCl compared to 15% NaCl. Exposure to stresses sequentially resulted in faster (P < 0.05) reductions than the exposure to single or double stresses applied simultaneously. The pH then NaCl sequence was more detrimental for pathogens than the reverse sequence. Incubation temperature (5 and 10 °C) did not show any profound (P < 0.05) effect on microbial inactivation. When L. monocytogenes was incubated at aw 0.930 or 0.995 at 30 °C, then the lag phase increased both in subsequent osmotic downshift and upshift, respectively, at 10 °C. Shorter lag phase and higher ability to initiate growth at lower aw was observed after pre-adaptation at pH 6.0 or 5.0 compared to neutral pH. The results may contribute to the review of critical limits in low pH (with lactic acid) and water activity products, considering the risk of L. monocytogenes and Salmonella survival. In addition, the present indications may address the points in processing where stricter sanitation procedures should be applied in order to minimize the risk of survivors.