Question: Salmonella enterica is an important prominent foodborne pathogen. Its ability to survive in acidic conditions raises safety concerns within the fruit juice industry. Additionally, the continuous exposure to stress conditions facilitates the development of tolerance rather than immediate bacterial death. The organism's adaptive response to environmental changes is vital to survive the challenges. Nonetheless, the precise timeline for such adaptations in the microorganism is still not completely understood. The objective of this study was to determine the thermal resistance of Escherichia coli ATCC 11229 as a surrogate for S. enterica. after pre-incubation under different environmental conditions.

Methods: The heat resistance of E. coli ATCC 11229 was determined, after 15 min and 3 h pre-incubation at 20°C. Three sugar content levels, namely 7° Brix, 12° Brix, 17° Brix, as well as three pH-ranges, from 3.1-3.3, 6.7-7.0, and 9.3-9.6, were investigated. To exclude other environmental factors, pre-incubation was performed in strawberry nectar (12° Brix, pH 3.5) and artificial nectar (12° Brix, pH 3.5). After the incubation time, thermal inactivation at 60°C in the strawberry nectar in biological triplicates and technical duplicates was performed.

Results: The heat sensitivity of the investigated strain remained consistent after 15 min pre-incubation in both strawberry nectar and artificial nectar with identical sugar content and pH levels. Therefore, the 3 h pre-incubation was performed only in strawberry nectar. Almost in all cases the determined D60-value was higher after the longer pre-incubation period. However, only in the 12°Brix strawberry nectar sample with a pH range of 9.3-9.6, a statistically significant higher D60-value was obtained.

Conclusions: This study showed clearly the impact of environmental stress, such as increased sugar content and extreme pH values, on thermal resistance of E. coli ATCC 11229. Nevertheless, more research is needed to verify the influence of incubation stress conditions and duration on bacterial thermal resistance.