Nisin migration is dependant on the microstructure of the cheese matrix: an in situ investigation
Résumé
Bacterial growth in solid or semi-solid matrices is of importance in many food
systems. In cheese matrixes, bacteria grow as immobilized colonies, and the diffusion is the
main mechanism for the transfer of nutrients and metabolites. Migration limitation can then
act as a constraint to the bacterial growth. Very few data is available in the literature about
small solute diffusion in cheeses (Floury et al., 2010).
The aim of this work was to explore if nisin diffusion in cheese model is dependant on
model cheese composition or not. Our strategy was to study nisin migration using a nisin
producing strain (Lactococcus lactis diacetylactis UL719) and nisin sensitive strain
(Lactobacillus sake ATCC 15521) in co-culture. Death rate of Lb sake was considered as a
marker for nisin diffusion. The composition of the model cheese matrix was modified by
adding different percentages of gelatin (0, 4 and 10%). Lb sake death kinetics was followed in
the early stages of culture in model cheese.
In parallel, effective diffusion coefficients of nisin were estimated thanks to the
classical concentration profile method and Fick’s law second. Nisin concentration was
measured by ELISA (Enzyme Linked ImmunoSorbent Assay). In addition, the structural
properties of the products were observed by confocal microscopy and the textural properties
of the matrices were analyzed by rheological measurements.
Increasing gelatin percentage in model cheeses caused faster death for Lb sake. Gelatin
had changed the model cheese microstructure by forming pathways in the protein network
which most likely accelerate nisin migration. Formation of these pathways was verified by
using FITC-labelling of the gelatin before incorporation into the retentate. Lc lactis growth
and its in situ nisin production were not affected by the addition of gelatin. Furthermore, Lbsake
death was not affected by (1) the acidification by Lc lactis, (2) the nutritional
competition with Lc lactis, and (3) the addition of gelatin. Nisin concentrations obtained by
ELISA confirmed that nisin migration was faster in matrix with 10% added gelatin.
Such a dynamic investigation of nisin migration was achieved for the first time in
cheese matrix. Nisin diffusion in cheese matrix could be modified by changing the cheese
composition, which also affects the cheese structure. As a peptide, nisin could be a model of
peptides obtained from the proteolysis during cheese ripening.
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