Effect of food microstructure on growth dynamics of Listeria monocytogenes in fish-based model systems

Abstract

Traditionally, predictive growth models for food pathogens are developed based on experiments in broth media, resulting in models which do not incorporate the influence of food microstructure. The use of model systems with various microstructures is a promising concept to get more insight into the influence of food microstructure on microbial dynamics. By means of minimal variation of compositional and physicochemical factors, these model systems can be used to study the isolated effect of certain microstructural aspects on microbial growth, survival and inactivation. In this study, the isolated effect on microbial growth dynamics of Listeria monocytogenes of two food microstructural aspects and one aspect influenced by food microstructure were investigated, i.e., the nature of the food matrix, the presence of fat droplets, and microorganism growth morphology, respectively. To this extent, fish-based model systems with various microstructures were used, i.e., a liquid, a second more viscous liquid system containing xanthan gum, an emulsion, an aqueous gel, and a gelled emulsion. Growth experiments were conducted at 4 and 10 °C, both using homogeneous and surface inoculation (only for the gelled systems). Results regarding the influence of the growth morphology indicated that the lag phase of planktonic cells in the liquid system was similar to the lag phase of submerged colonies in the xanthan system. The lag phase of submerged colonies in each gelled system was considerably longer than the lag phase of surface colonies on these respective systems. The maximum specific growth rate of planktonic cells in the liquid system was significantly lower than for submerged colonies in the xanthan system at 10 °C, while no significant differences were observed at 4 °C. The maximum cell density was higher for submerged colonies than for surface colonies. The nature of the food matrix only exerted an influence on the maximum specific growth rate, which was significantly higher in the viscous systems than in the gelled systems. The presence of a small amount of fat droplets improved the growth of L. monocytogenes at 4 °C, resulting in a shorter lag phase and a higher maximum specific growth rate. The obtained results could be useful in the determination of a set of suitable microstructural parameters for future predictive models that incorporate the influence of food microstructure on microbial dynamics.

Food Microstructure and Fat Content Affect Growth Morphology, Growth Kinetics, and Preferred Phase for Cell Growth of Listeria monocytogenes in Fish-Based Model Systems

“…Food microstructure encompasses the spatial arrangement of the various structural elements (e.g., water and oil droplets, gas cells, particles, granules, strands, crystals, micelles, and interfaces) of a food product and their interactions (17). Microbial dynamics are affected by a plethora of food microstructural aspects, e.g., physical constraints on microbial mobility (18)(19)(20), the presence of fat in the food matrix (21,22), the nature of the food matrix (i.e., viscous or gelled) (22), and diffusion of oxygen, water, nutrients, preservatives, and metabolites (23)(24)(25)(26)(27).…”

“…The set of model systems consisted of three viscous systems and two gelled systems, i.e., (i) a liquid system; (ii) xanthan, a more viscous liquid system containing a small concentration of xanthan gum; (iii) an emulsion; (iv) an aqueous gel; and (v) a gelled emulsion, respectively. These model systems were used to investigate the effect of food microstructure on growth dynamics of L. monocytogenes at suboptimal temperatures, i.e., 4 and 10°C (22). The growth morphology of the cells (i.e., planktonic cells, submerged colonies, or surface colonies), the nature of the food matrix (i.e., viscous or gelled), and the presence of fat droplets were reported to exert a significant influence on the parameters of the growth model of Baranyi and Roberts (32).…”

“…The growth morphology of the cells (i.e., planktonic cells, submerged colonies, or surface colonies), the nature of the food matrix (i.e., viscous or gelled), and the presence of fat droplets were reported to exert a significant influence on the parameters of the growth model of Baranyi and Roberts (32). Since the study of Verheyen et al (22) relied solely on macroscopic growth experiments, the underlying mechanisms have mostly been left unraveled. More specifically, apart from a visual inspection during the macroscopic growth experiments, a detailed investigation of the growth morphology in which L. monocytogenes appeared in the different model systems was not conducted.…”

“…The aim of this study was to investigate the effect of food microstructure on L. monocytogenes growth dynamics at the microscopic level and relate the obtained results to findings at the macroscopic level. In order to compare microscopic and macroscopic observations, the bacteria were grown inside fish-based food model systems at 10°C, analogous to the macroscopic growth experiments conducted by Verheyen et al (22). Model system composition was based on processed fish products (e.g., fish soup, surimi, and fish paté), and the microstructure was simulated by including the major food microstructural aspects of those products (e.g., a visco-elastic matrix or fat droplets).…”

“…As a possible explanation for the affinity of the cells for a certain phase, hydrophobicity of the cells was quantified using the MATS (microbial adhesion to solvents) assay (40). The MATS assay was conducted for both the GFP strain and the L. monocytogenes strain cocktail used for the macroscopic growth experiments of Verheyen et al (22), enabling an improved comparison of microscopic and macroscopic results.…”