Kill Step Calculator
What is a Kill Step Calculator?
The Kill Step Calculator is a tool that makes use of basic microbiology and thermal inactivation concepts to help bakers validate the effectiveness of the baking process against pathogens during production operations.
From a food safety standpoint, a kill step is necessary to eliminate harmful pathogens such as Salmonella spp. in ingredients and finished products. In baking operations, proper monitoring of oven temperature using a Kill Step Calculator allows bakeries to comply with Food Safety and Modernization Act (FSMA) requirements and offer safe products to customers.
The FSMA Final Rule for Preventive Controls for Human Food (21 CFR §117.155 and 21 CFR §117.160) mandates food processors to verify and validate kill-steps applied to guarantee food safety. Verification and validation must be carried out to ensure the consistency and effectiveness of applied preventive controls in controlling identified hazards.1,2,3
The Kill Step Calculator was developed by AIB International, in cooperation with Kansas State University, the American Bakers Association (ABA), and the University of Georgia. The goal is to deliver a practical tool that could help bakers comply with the new FDA food safety regulations.
How does it work?
The Kill Step Calculator has the following components:
- Calculator (Excel spreadsheet) that contains model, input and output data
- Procedure (instructions) for recording temperature profile in oven
Both can be downloaded, free of charge, from AIB’s website. Bakers need to have access to Microsoft Excel and a data logger equipped with temperature sensors (6-point wire probes) and software connectivity.
Kill Step Calculators currently exist for the following bakery products:
- Hamburger buns
- Basic round top cake muffins
- Nut Muffin
- 100% Whole Wheat
- Multigrain Bread
- Crispy Cookies
- Soft Cookies
- Yeast-Raised Doughnut
- Flour Tortillas
Oven temperatures are sensed and monitored using the data logger probes inserted into the core of the product (e.g. dough piece). Temperatures are recorded 5 times at 15 second intervals by the data logger, and then downloaded into an Excel spreadsheet and analyzed.
The probe data points that take the longest time to reach 170°F (77°C) should be selected for analysis in the Process Lethality Fahrenheit/Celsius Tab of the Kill Step Calculator. The coldest points are, thus, the worst case scenario for thermal destruction of Salmonella. 20 data points (time/temperature) are needed for proper analysis.
Data is retrieved at the end of the baking cycle (batch ovens) or after going through the coolest zone (continuous ovens). This process is repeated five times.
Once the data is imported into the Kill Step Calculator, Process Lethality will automatically be calculated.
The following video tutorial is available for a quick overview of the process:
For specific microbial inactivation kinetics (Salmonella for e.g.) in a specific bakery system or product, the Kill Step Calculator provides the following information:
- Product internal temperature
- Thermal inactivation parameters (heat tolerance coefficients) D-, z- and F-value
- Cumulative log reductions
- Total process lethality (e.g., 5 log) for Salmonella
The decimal reduction time (D) is the time necessary to obtain a 90% reduction (1 log-cycle) in the microbial population of concern (Salmonella in this case).4
The thermal resistance constant (z) is defined as the increase in temperature necessary to cause a 90% reduction in the decimal reduction time D. The z-value is constant for a given microorganism strain in a given product.4
The thermal death time (F) is the total time required to accomplish a stated reduction in a population of vegetative cells or spores. This time can be expressed as a multiple of D-values, as long as the survivor curve follows a first-order model. For example, a 99.999% reduction in microbial population would be equivalent to 5 log-cycle reductions or F = 5D.4
Salmonella has been identified as the major biological concern for bakers. Ingredients such as eggs, whole grains, peanut, chocolate, milk and inclusions create additional food processing challenges in controlling this hazard.
It is important to note that any changes made in the baking profile regarding time/temperature (i.e. belt speed in continuous ovens, use of steam, layout and number of burners), require re-validation of the kill step.
Bacteria respond and behave differently at varying moisture levels (i.e. water activities). Factors such as pH, total acidity, fat content, osmotic pressure due to presence of dissolved salts and sugars, also create different conditions within the food system and must be considered when studying heat tolerance coefficients.
- U.S. Food and Drug Administration. “21 CFR 117 – Current Good Manufacturing Practice, Hazard Analysis, and Risk–based Preventive Controls for Human Food.” 1 Apr. 2017, https://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfcfr/CFRSearch.cfm?fr=117.130. Accessed 21 October 2018.
- L.H. Channaiah et al., “Validation of the baking process as a kill-step for controlling Salmonella in muffins.” International Journal of Food Microbiology, Elsevier, 2017, 250. pp. 1–6.
- L.H. Channaiah et al., “Validation of Baking To Control Salmonella Serovars in Hamburger Bun Manufacturing, and Evaluation of Enterococcus faecium ATCC 8459 and Saccharomyces cerevisiae as Nonpathogenic Surrogate Indicators.” Journal of Food Protection, International Association for Food Protection, 2016, 79, pp. 544–552.
- Singh, R.P., and Heldman, D.R. “Preservation Processes” Introduction to Food Engineering, 5th edition, Academic Press, Elsevier Inc., 2014, pp. 421–470.