For manufacturers of ready-to-eat (RTE) meat and poultry products, food safety is both a regulatory obligation and a brand-defining commitment. One of the core components of that commitment to food safety is thermal processing, where the controlled application of heat and humidity is used to reduce or eliminate the pathogenic bacteria that may be present in raw meats. While processors have long relied on established USDA Food Safety and Inspection Service (FSIS) guidance such as Appendix A time-temperature tables, evolving products, processes, and pathogens continue to raise an important question: How well do these generalized guidelines apply to what actually happens in commercial cooking systems?

Like many other fields, a persistent concern with any thermal processing research is whether or not the results obtained in a laboratory will be applicable to real-world situations. A recent, peer-reviewed study published in Meat and Muscle Biology may provide some clarity in this area. This study from University of Wisconsin-Madison took experimentally derived D-values and z-values for Salmonella, Listeria monocytogenes, and Shiga toxin–producing Escherichia coli (STEC) and validated them in pilot-plant systems designed to replicate commercial meat processing practices. The aim of this study was to help processors understand where traditional assumptions about thermal processing hold firm and where additional scrutiny may be warranted. The results from this study, when paired with modern computing tools such as the Hydrated Surface Lethality (HSL) calculator (https://meatsciences.cals.wisc.edu/hsl-calculator/) developed at the University of Wisconsin–Madison provide the industry with a powerful combination of strong science and practical application.

Basic thermal lethality calculations rely on two important concept: D-values and z-values. The D-value represents the time, at a specific temperature, required to achieve a 1.0-log (90 percent) reduction of a target pathogen population. The z-value describes the temperature change needed to shift the D-value by one log, effectively capturing how sensitive a pathogen’s heat resistance is to changes in temperature. Together, these parameters allow processors to estimate total lethality delivered by a given time–temperature profile. In the Meat and Muscle Biology study, researchers challenged three representative RTE products with high levels (approximately 8.0-log CFU per gram) of multi-strain pathogen cocktails. The products, uncured roast beef, uncured turkey breast, and cured ham, were selected to reflect products and processing styles commonly encountered in the industry and formulated to create a “Worst-case Scenario” for pathogen survival. Using pilot-plant cooking equipment that closely mirrored commercial operations, the team subjected the products to a range of thermal conditions and measured actual pathogen reductions.

The results were reassuring at conventional endpoint cooking temperatures. Across all products, heating to an internal temperature of 71.1°C (160°F) achieved greater than 6.5-log reductions for all three pathogens tested. These findings reinforce the robustness of widely used high-temperature lethality targets when processes are properly executed. The data also showed that moderate-temperature processes can be effective under the right conditions. For example, roast beef heated to 62.8°C (145°F) and held for five minutes delivered greater than 7.0-log reductions of Salmonella and STEC. This confirms that well-controlled time–temperature combinations below traditionally perceived “high” temperatures can still meet regulatory lethality expectations, provided temperature monitoring, temperature control, and holding times are appropriately managed. Where the study suggests caution is at lower-temperature processing conditions. When roast beef was heated to 54.4°C (130°F) and held for two hours, pathogen reductions fell short of 4.0 log for Salmonella and STEC. Additional experiments demonstrated that extending the hold at target temperature to four hours or more was required to achieve a target 5.0-log reduction for Salmonella. This result shows that processing with lower target temperatures may be done safely by following processes with time added to achieve target reductions. Another key finding with practical implications was the influence of thermal history on pathogen resistance to heat stress. Extended exposure to sub-lethal heat (<54.4°C) resulted in increased D-values for Salmonella, indicating higher heat tolerance. This underscores the importance of evaluating processes as an integrated whole rather than relying exclusively on static values generated under idealized laboratory conditions.

For processors, the message should be clear. Product and process-specific validation matters. Seemingly minor changes in formulation, geometry, humidity, or heating rate, to name a few, can meaningfully affect delivered lethality. While FSIS Appendix A guidance remains crucial as a source of safe harbor thermal processes, defensible food safety systems increasingly depend on data that reflect real products under real processing conditions.

Translating that data into actionable decisions is where the Hydrated Surface Lethality (HSL) calculator comes into play. Developed by Dr. Russ McMinn, a member of Dr. Jeff Sindelar’s research group at the University of Wisconsin–Madison, the web-based HSL tool allows users to estimate thermal lethality of a target pathogen at both the core and surface of a product. Available at hsl.meatsciences.cals.wisc.edu, the HSL calculator enables processors, food safety professionals, and regulators to input time–temperature data from a thermal process and apply validated D- and z-values to estimate process lethality at multiple locations on/in a product. The interface is designed to be intuitive, making sophisticated lethality modeling accessible without advanced statistical software.

From a practical standpoint, the HSL tool supports several critical industry needs. It provides a transparent, science-based method for evaluating whether a given process delivers sufficient lethality. It helps identify gaps in marginal processes before they become regulatory problems. And it generates documentation used in hazard analyses, validation studies, and inspection discussions. Perhaps most importantly, the combination of validated thermal inactivation metrics and user-friendly modeling tools encourages proactive decision-making. Processors can explore “what-if” scenarios, compare alternative schedules, and optimize processes with safety and efficiency in mind.

As RTE products continue to evolve and regulatory expectations remain high, success will depend on integrating solid microbiological science with practical processing knowledge. The latest thermal processing research, coupled with tools like the HSL calculator, provides a clear path forward that enhances food safety, strengthens compliance, and reinforces consumer confidence in today’s meat and poultry products.