Factors affecting the thermal resistance of Salmonella Typhimurium in tahini

Salmonella enterica is a leading human pathogen responsible for foodborne outbreaks worldwide. In the last decade, foods with low water activity (a_w) and high-fat content have been involved in an increased occurrence of foodborne outbreaks. This research focuses on the foodstuff tahini, which is often linked to Salmonella infection outbreaks and recalls. Thermal treatments are suggested to reduce microbial populations in tahini, but little is known about its effectiveness against Salmonella. Our major objectives were to study the survival of Salmonella Typhimurium in tahini treated at temperatures ≥ 70 °C, and to identify food related factors that could influence its survival. Based on our experimental results the thermal treatments at 70 °C, 80 °C and 90 °C are suitable to inactivate only a partial population of Salmonella. The death of Salmonella in tahini matches a biphasic logarithmic inactivation model, with a maximal 3-log reduction after 1 h at 90 °C. Moreover, we observed that a second thermal treatment the day after the first treatment, is significantly less effective compared with the first thermal treatment. The inactivation rates of Salmonella in 100% tahini are almost 4-log lower than in water/tahini emulsions at 70 °C, with negative linear correlation between D-value and a_w, and the Salmonella susceptibility to heat in sesame oil/tahini emulsions is affected by the matrix of pre-acclimation. Bacteria that had been acclimated in tahini kept their heat resistance, while acclimation in sesame oil before mixing in the preheated oil/tahini emulsions resulted in a sharp decline within 2 min at 70 °C. According to these findings, tahini producers' current pasteurization processes are not sufficient to achieve the required 5-log reduction. Furthermore, we suggest that due to the tahini heterogenicity, the a_w in the micro-environment of each bacterium, which is shaped by the tahini substances, plays an essential role in Salmonella's survival in tahini at temperatures ≥ 70 °C.