Modelling the dynamics of microbial populations and Salmonella spp. in milk kefir

Abstract

Kefir is a fermented dairy product based on the fermentation of milk by bacteria and yeasts. It is produced by adding kefir grains, consisting of a consortium of microorganisms, to milk in order to start a natural fermentation (Garofalo et al., 2020). Kefir is well recognized for its potential health value as a source of probiotics, however, there have been concerns about the potential growth of pathogenic microorganisms in kefir and the potential health hazards associated (Leite et al., 2013) A mathematical model was developed to describe the evolution of microbes present during kefir fermentation and the potential growth of Salmonella spp. as one example of a potential food safety hazard. For this, equations previously described in the scientific literature were combined and adapted to the milk kefir matrix. To assess the safety of the product, the growth of Salmonella was predicted considering its interaction with the medium and the other species. The drop in pH; generation of yeasts metabolites such as ethanol; and buffer capacity was described and considered when modelling Salmonellas’ kinetics. Interaction between the pathogenic species and the background microflora was included in the model. Parameters of some well-described systems were taken from literature. Alongside, some other parameters describing specific assets from the system were estimated using experimental data of microbial population kinetics during kefir fermentation. The growth of yeasts, lactic acid bacteria, Salmonella together with the pH were experimentally collected at critical processing times and fitted to the mathematical model by minimizing the residual sum of squares (RSS). Confidence intervals of 95 % were calculated. For further validation, the output of the model was contrasted with an independent data set. It was concluded that Salmonella is able to increase its population during the first hours of the kefir fermentation process. Inactivation can be apportioned to the drop of pH as a consequence of the LAB metabolism, once the pH reaches values below 4. After this no growth of Salmonella seems to be found in milk kefir as confirmed by the model. No residual population of the pathogen is observed. This suggests that by controlling the growth and metabolism of the background microflora, safety can be assured in milk kefir. This process was successfully described by a novel mathematical model and the estimation of its´parameters.