Cell viability of Bifidobacterium lactis strain in long-term storage butter assessed with the plate count and fluorescence techniques

Great interest in functional products containing bacterial strains displaying health-promoting properties is expressed worldwide and is as a result connected with a demand for developing new probiotic-based products, especially those containing bifidobacteria. The Bifidobacterium strains play a key role in gastrointestinal homeostasis, providing many health-related attributes, but as fastidious microorganisms require specific conditions (e.g. anaerobic environment, neutral pH) to survive in the long-term at the needed level above 106 cfu/g. In consequence, not every food product guarantees optimal maintenance of Bifidobacterium viability. From this point of view, the objective of the study was to examine the survival of Bifidobacterium lactis strain in butter during long-term refrigerated storage. Two enumeration techniques: microscopic LIVE/DEAD® and plating were compared by monitoring bifidobacterial counts for 4 weeks. The plate method was characterised by underestimation of the cell counts in relation to the results evaluated microscopically. However, the good survival exhibited by B. lactis was found with both techniques. Moreover, the microscopic LIVE/DEAD® method permitted to trace delicate changes in the viable/non-viable bifidobacterial population at the single-cell level.