The blue discoloration of fresh cheeses: A worldwide defect associated to specific contamination by Pseudomonas fluorescens

Italian authorities reported in 2010 that mozzarella imported from Germany turned blue after opening the package. The “blue mozzarella” defect was associated with Pseudomonas fluorescens strains contaminating processing water in some dairy plants. Afterwards, similar episodes occurred in various European countries and the USA, all characterized by the development of an intense blue discoloration on the surface of fresh cheeses when exposed to air. Recent research on the presumptive causal microorganisms and the metabolites associated with the blue discoloration has contributed to elucidate the bacterial species and pigments responsible for this particular type of spoilage. Quorum sensing mechanisms may induce the production of blue pigment and free aromatic amino acids may enhance its biosynthesis. Physicochemical characteristics of fresh cheeses favorable for growth of psychrotrophic bacteria and long shelf life periods demanded by large supermarket chains contribute to explain the increasing incidence of blue discoloration. Appropriate cleaning and disinfection programs and addition of pepsin-digested lactoferrin are of great help in solving the problem.