Response Surface Models for Effects of Previous Sodium Chloride and Temperature on Growth Kinetics of Salmonella Typhimurium on Cooked Chicken Breast Thomas P. Oscar USDA, ARS Microbial Food Safety Research Unit Princess Anne, MD

Previous Growth Conditions b Broth Culture Previous pH (5.5 to 8.5)Previous pH (5.5 to 8.5) b Cooked Chicken Breast Burgers Previous temperature (16 to 34°C)Previous temperature (16 to 34°C)

Model Development b Full 3 x 9 Factorial 0.5, 2.5, 4.5% NaCl0.5, 2.5, 4.5% NaCl 10, 12, 14, 16, 20, 24, 28, 34, 40 o C10, 12, 14, 16, 20, 24, 28, 34, 40 o C b 55 Growth Curves

Stage 1: Pre-incubation b Brain Heart Infusion Broth, pH to 4.5% NaCl0.5 to 4.5% NaCl b Salmonella Typhimurium ATCC o C for 23 h34 o C for 23 h

Stage 2: Incubation b Cooked Chicken Breast Burgers 10 to 40 o C10 to 40 o C b Salmonella Typhimurium 100,000 CFU per burger100,000 CFU per burger

Growth Curve Fit

Data Set

Lag Time Model

Lag Time Data

Growth Rate Model

Growth Rate Data

Model Validation b Full 2 x 8 Factorial 1.5, 3.5% NaCl1.5, 3.5% NaCl 11, 13, 15, 18, 22, 26, 31, 37°C11, 13, 15, 18, 22, 26, 31, 37°C b 16 Growth Curves

Validation Terms b Relative Error Predicted - Observed/ObservedPredicted - Observed/Observed b Mean Absolute Relative Error (MARE) Prediction AccuracyPrediction Accuracy b Median Relative Error (MRE) Prediction BiasPrediction Bias

Validation Results

RE of Lag Time

RE of Growth Rate

In-House Comparison

Poultry Food Assess Risk Model (Poultry FARM)

Conclusions b The models provide reliable predictions within the matrix of factors modeled. b Previous NaCl (0.5 to 4.5%) is not a major factor affecting growth of S. typhimurium.