Purpose: The interactive effects of heating temperature (55 – 65°C), sodium chloride (NaCl, 0-2%), and green tea (GT, 0-3%) on heat resistance of a five-strain mixture of Listeria monocytogenes in ground turkey were determined.
Methods: Thermal death times were quantified in bags submerged in a circulating water bath set at temperatures 55, 57, 60, 63, and 65°C. The recovery medium was tryptic soy agar with added 0.6% yeast extract and 1% sodium pyruvate. The D-values (time for 10-fold reduction in viable cells) were determined by fitting the data to a survival equation with a curve fitting program. Thereafter, the D-values were transformed to the natural logarithm form and analyzed by second order response surface regression to develop a regression model for temperature, NaCl and GT.
Results: The multiple regression equation for the loge D-values yielded an R2 value of 0.97. The data indicated that all three factors interacted to affect the inactivation of the pathogen. Sodium chloride exhibited a protective effect and increased the heat resistance of L. monocytogenes. Green tea interacted with sodium chloride, and reduced the protective effect of NaCl on heat resistance of the pathogen. For example, increasing salt levels from 0.4% to 1.6% in turkey supplemented with 0.6% GT increased the D-value at 57.5°C from 15.42 min to 17.62 min. Conversely, increasing GT levels from 0.6% to 2.4% in turkey supplemented with 0.4% NaCl decreased the D-value at 57.5°C from 15.42 min to 10.52 min.
Significance: Food processors can use the predictive model to design an appropriate heat treatment for inactivating L. monocytogenes in cooked turkey products.