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A set of 83 lytic dairy bacteriophages (phages) infecting flavor-producing mesophilic starter strains of the Leuconostoc genus was characterized, and the first in-depth taxonomic scheme was established for this phage group. Phages were obtained from different sources, i.e., from dairy samples originating from 11 German dairies (50 Leuconostoc pseudomesenteroides [Ln. pseudomesenteroides] phages, 4 Ln. mesenteroides phages) and from 3 external phage collections (17 Ln. pseudomesenteroides phages, 12 Ln. mesenteroides phages). All phages belonged to the Siphoviridae family of phages with isometric heads (diameter, 55 nm) and noncontractile tails (length, 140 nm). With the exception of one phage (i.e., phage ⌽LN25), all Ln. mesenteroides phages lysed the same host strains and revealed characteristic globular baseplate appendages. Phage ⌽LN25, with different Y-shaped appendages, had a unique host range. Apart from two phages (i.e., phages P792 and P793), all Ln. pseudomesenteroides phages shared the same host range and had plain baseplates without distinguishable appendages. They were further characterized by the presence or absence of a collar below the phage head or by unique tails with straight striations. Phages P792 and P793 with characteristic fluffy baseplate appendages could propagate only on other specific hosts. All Ln. mesenteroides and all Ln. pseudomesenteroides phages were members of two (host species-specific) distinct genotypes but shared a limited conserved DNA region specifying their structural genes. A PCR detection system was established and was shown to be reliable for the detection of all Leuconostoc phage types.

As the cheese market faces strong international competition, the optimization of production processes becomes more important for the economic success of dairy companies. In dairy productions, whey from former cheese batches is frequently re-used to increase the yield, to improve the texture and to increase the nutrient value of the final product. Recycling of whey cream and particulated whey proteins is also routinely performed. Most bacteriophages, however, survive pasteurization and may re-enter the cheese manufacturing process. There is a risk that phages multiply to high numbers during the production. Contamination of whey samples with bacteriophages may cause problems in cheese factories because whey separation often leads to aerosol-borne phages and thus contamination of the factory environment. Furthermore, whey cream or whey proteins used for recycling into cheese matrices may contain thermo-resistant phages. Drained cheese whey can be contaminated with phages as high as 109 phages mL-1. When whey batches are concentrated, phage titers can increase significantly by a factor of 10 hindering a complete elimination of phages. To eliminate the risk of fermentation failure during recycling of whey, whey treatments assuring an efficient reduction of phages are indispensable. This review focuses on inactivation of phages in whey by thermal treatment, ultraviolet (UV) light irradiation, and membrane filtration. Inactivation by heat is the most common procedure. However, application of heat for inactivation of thermo-resistant phages in whey is restricted due to negative effects on the functional properties of native whey proteins. Therefore an alternative strategy applying combined treatments should be favored – rather than heating the dairy product at extreme temperature/time combinations. By using membrane filtration or UV treatment in combination with thermal treatment, phage numbers in whey can be reduced sufficiently to prevent subsequent phage accumulations.

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