The Complex Effect of Food Matrix Fat Content on Thermal Inactivation of Listeria monocytogenes: Case Study in Emulsion and Gelled Emulsion Model Systems

Abstract

Previous studies on the influence of food matrix fat content on thermal inactivation kinetics of food pathogens have shown contradictory results due to the combined influence of fat content and other factors such as composition. Therefore, thermal inactivation of Listeria monocytogenes at 59, 64, and 69°C was systematically investigated in emulsion and gelled emulsion food model systems with various fat content (1, 5, 10, and 20%), such that the effect of fat content was isolated. Thermal conductivity and rheological properties of the model systems were quantified, as well as the effect of these properties on the thermal load of the model systems. Thermal conductivity was complexly related to fat content, the nature of the food matrix (i.e., viscous or gelled), and temperature. For the emulsions, the consistency index K increased with increasing fat content, while the flow behavior index n followed the opposite trend. For the gelled emulsions, the storage modulus G' was always larger than the loss modulus G″ (i.e., measure of elastic and viscous properties, respectively). The phase angle δ [i.e., arctan (G″/G')] was proportional with fat content, but this relation became more complex at higher temperatures. The thermal load of the model systems was not largely affected by food matrix fat content. Thermal inactivation of L. monocytogenes was investigated by means of the maximum specific inactivation rate k _max, log reductions, and sublethal injury (SI). Both for emulsions and gelled emulsions, k _max decreased with increasing fat content below approximately 60°C, while a more complex behavior was observed at higher temperatures. In the emulsions, log reductions were considerably lower (i.e., 2-3 log) at 1% fat than in systems with higher fat content. In the gelled emulsions, log reductions generally decreased with increasing fat content. SI decreased with increasing fat content, both in emulsions and gelled emulsions. In conclusion, the inactivation rate (i.e., k _max) of L. monocytogenes was affected by a complex relation between food matrix fat content, thermal conductivity, rheological properties, and inactivation temperature. Due to the small scale of the model systems, differences in k _max did not directly affect the final log reductions in a similar fashion.

Keywords: Listeria monocytogenes; fat content; food microstructure; predictive microbiology; thermal inactivation kinetics.

FIGURE 1

Storage modulus G′ (A, full lines), loss modulus G″ (A, dashed lines), and phase angle δ (B) of the gelled emulsions with different fat content (i.e., 1, 5, 10, and 20%), measured over a temperature range from 20 to 70°C. Three independent replicates were conducted.

FIGURE 2

Inactivation kinetics of Listeria monocytogenes in the emulsion (A–C) and gelled emulsion (D–F) model systems with different fat content at temperatures of 59 (A,D), 64 (B,E), and 69°C (C,F). Symbols (x, o, and □, for 5, 10, and 20% fat, respectively) correspond to the experimental data and lines correspond to the model fit of the Geeraerd et al. (2000) model. The detection limit (DL) is also indicated.

FIGURE 3

Estimated maximum specific inactivation rate k_max (1/min) in function of the inactivation temperature, according to the inactivation model of Geeraerd et al. (2000) for the thermal inactivation of Listeria monocytogenes in the emulsion (A) and gelled emulsion (B) model systems with different fat content. Data obtained from Verheyen et al. (2019a) was used to calculate k_max for the systems containing 1% fat.