Thermal Inactivation of Stationary-Phase and Acid-Adapted Escherichia coli O157:H7, Salmonella, and Listeria monocytogenes in Fruit Juices

This study investigates the thermal resistance of three major foodborne pathogens—Escherichia coli O157:H7, Salmonella, and Listeria monocytogenes—in common fruit juices (apple, orange, and white grape). Specifically, it examines how acid adaptation, a common survival mechanism for bacteria in acidic environments, affects their ability to withstand pasteurization temperatures.

The researcher evaluated the heat resistance of stationary-phase and acid-adapted cultures of these pathogens in single-strength juices adjusted to pH 3.9. Thermal death time experiments were conducted using an end-point procedure across various temperatures (56–62°C). The study also assessed the influence of pH on thermal resistance in tryptic soy broth to understand how acidity levels impact pathogen survival. Finally, the author calculated the necessary time-temperature combinations to achieve a 5-log reduction of these pathogens, providing a benchmark for juice safety.

The results demonstrate that acid adaptation significantly increases the heat resistance of all three pathogens. Among the tested organisms, acid-adapted E. coli O157:H7 showed the highest heat resistance at the temperatures evaluated. However, the study notes that L. monocytogenes possesses a higher z-value, meaning its heat resistance remains more stable as temperatures increase; extrapolation suggests it could be more resistant than E. coli O157:H7 at temperatures above 65°C. Salmonella was consistently the most heat-sensitive of the three pathogens. The study concludes that standard hot-fill pasteurization processes currently used in the industry provide a lethality far exceeding the 5-log reduction requirement for these pathogens.

As fruit juices have been linked to several foodborne illness outbreaks, regulatory bodies like the FDA have proposed HACCP-based performance criteria to ensure safety. This research provides critical data for juice processors to select appropriate target organisms and validate pasteurization parameters. By accounting for the increased resistance of acid-adapted cells, producers can ensure their safety protocols are robust enough to handle even the most resilient, stress-hardened pathogens.