EFFECT OF FOOD MICROSTRUCTURE ON THERMAL INACTIVATION DYNAMICS OF LISTERIA MONOCYTOGENES

摘要

The effectiveness of predictive microbiology is limited by the lack of knowledge considering the influence of food microstructure on microbial dynamics. In this study, fish-based model systems with various microstructures (i.e., liquid, xanthan - a more viscous liquid system, emulsion, aqueous gel, and gelled emulsion) were used to investigate the effect of certain isolated food microstructural aspects (i.e., growth morphology of the cells, nature of the food matrix, and the presence/absence of fat droplets) and initial cell conditions (i.e., grown inside the matrix from a low inoculation level or inoculated to a high inoculation level from a fresh preculture) on thermal inactivation dynamics of Listeria monocytogenes, as well as on the evolution of the percentage of sublethal injury of the cells over the course of the treatment. A peak in sublethal injury always occurred when the cells were in their log-linear inactivation phase, but the level of sublethal injury at the end of the treatment was rather limited. For experiments started from cells that were grown inside the matrix, a larger maximum specific inactivation rate (k_(max)) was observed than for experiments started from a fresh preculture, although only for the viscous systems (i.e., liquid, xanthan and emulsion). Due to the greater variation among biological replicates, the residual cell density (N_(res)) was also smaller for experiments started from cells that were first grown inside the matrix. For experiments started from a fresh preculture, k_(max) was considerably larger in the viscous systems than in the gelled systems, while N_(res) was larger in gelled systems than in viscous systems, regardless of the initial condition of the cells. The presence of a small amount of fat droplets had a considerable effect on k_(max) and N_(res), albeit of a complex nature.