A modelling approach is proposed to represent the decreasing evolutions observed on cell concentration, biosynthetic activity and substrate consumption at late fermentation times during processing when using a cell recycling (membrane) apparatus and continuous feeding. Indeed, during such processes, which are over rather long time periods, the cell populations may have to withstand various stresses (physical, environmental and chemical) involving a decrease of the global activity of the cell populations. This study was carried out with L-lactic acid fermentation using Lactobacillus casei as a model system and the modelling approach was based on the definitions of a critical cell concentration and a stress zone. The results obtained corresponded to what was expected. (c) 2004 Elsevier Ltd. All rights reserved.