Evaluation of Staphylococcus aureus Growth Potential in Ham during a Slow-Cooking Process: Use of Predictions Derived from the U.S. Department of Agriculture Pathogen Modeling Program 6.1 Predictive Model and an Inoculation Study

Abstract

The U.S. Department of Agriculture has cautioned against slow cooking meat such that the interior temperature increases from 10°C (50°F) to 54.4°C (130°F) in ≥6 h. During a commercial ham-smoking process, the ham cold point is typically between 10 and 54.4°C for 13 h, but the ham is subsequently exposed to heating sufficient to eliminate vegetative pathogenic bacteria. Thus, production of heat-stable staphylococcal enterotoxin is the primary biological hazard. For this study, uncooked surface and uncooked ground interior ham were inoculated with a three-strain Staphylococcus aureus mixture, exposed to simulated surface and interior slow-cook conditions, respectively, and analyzed periodically using the Baird-Parker agar and 3M Petrifilm Staph Express count plate methods. For the surface and interior conditions, S. aureus numbers increased by no more than 0.1 and 0.7 log units, respectively. Predictions derived from actual time and temperature data and S. aureus growth values from a computer-generated model (Pathogen Modeling Program 6.1, U.S. Department of Agriculture) were for 2.7 (ham surface) and 9.9 to 10.5 (ham interior) generations of S. aureus growth, indicating that use of model-derived growth values would not falsely indicate safe slow cooking of ham. The Baird-Parker method recovered significantly (P < 0.05) greater numbers of S. aureus than the Petrifilm Staph Express method. For hams pumped with brine to attain (i) 18% (wt/wt) weight gain, (ii) ≥2.3% sodium lactate, (iii) ≥0.8% sodium chloride, and (iv) 200 ppm ingoing sodium nitrite, slow-cooking critical limits of ≤4 h between 10 and 34°C, ≤5 h between 34 and 46°C, and ≤5 h between 46 and 54.4°C could be considered adequate to ensure safety.