On the need for another type of predictive model in structured foods

Most of the models discussed up till now in predictive microbiology do nor take into account the variability of microbial growth with respect to space . In structured (solid) foods, microbial growth can strongly depend on the position in the food and the assumption of humogeneity can thus not be acce pted: space must be considered as an independent variable. Indeed, experime ntal evidence exists of bacteria competition on agar not showing the same b ehavior as the competition in a well-mixed liquid culture system. It is con jectured that this is due to the spatially structured habitat. Therefore, i n the current paper, a prototype two species competition model proposed in previous work by the authors is extended to take space into account. The ex tended model describes two phenomena: (i) local evolution of biomass and (i i) transfer of biomass through the medium. The structure of the food produc t is taken into account by limiting the diffusion through the medium. The s maller mobility of the micro-organisms in solid foods allows spatial segreg ation which causes pattern formation. Evidence is given for the fact that t aking space into account indeed has an influence on the behavior (coexisten ce/extinction) of the populations. Although the reported simulations are by no means to be interpreted as accurate predictions, the proposed model str ucture allows one to highlight (i) important characteristics of microbial g rowth in structured foods and (ii) future research trends in predictive mic robiology. (C) 2001 Elsevier Science B.V. All rights reserved.