Microstructure of cheese matrix modifies solute migration at the bacterial colony scale: examples with nisin and lactic acid
Résumé
Solute migration has rarely been studied in cheese matrix until today although it is crucial in the ripening process. Microstructure of the cheese matrix may modify the migration properties of solutes. The well-known methods to study migration are destructive and informative at a macroscopic scale. The aim of this work was to assess the impact of microstructure on solute migration at the microscopic level with in situ non destructive techniques. The model cheese matrix was renneted retentate from ultrafiltrated milk, and different percentages of gelatine were added to modify microstructure. Two technical strategies have been performed to assess solute diffusion: (1) measuring the loss of cultivability of a nisin-sensitive strain of Lactobacillus sakei in co-culture with a nisin-producing strain of Lactococcus lactis (2) characterising pH microgradients between bacterial colonies using pH sensitive fluorophores. As confirmed by confocal microscopy, the addition of gelatine modified the microstructure and proportionally increased the loss of cultivability of Lb. sakei. Many validations allowed us to conclude that the migration of solutes was speeded up by the addition of gelatine. The spatio-temporal kinetics of pH between colonies also depended on the initial composition and so the microstructure of the model cheese matrix, and on the distance between colonies. These results are a first step to understand the ripening mechanisms at a microscopic scale. They are also opening a new field of in situ investigations of the micro-environnement around bacterial colonies.