Among the contaminants often found in mozzarella cheese, Pseudomonas spp. represent the most frequent one, causing a reduction in shelf life, possible discolouration of the product, and consequent economic losses in dairy companies. A smart strategy to contain Pseudomonas spp. contamination during the mozzarella production is the addition of bacterial cultures tested to be able to contain the Pseudomonas spp. spreading: a commercial blend of Carnobacterium spp. was tested against a Pseudomonas spp. cocktail (constituted of strains mostly isolated from mozzarella) directly in mozzarella brine and reduced the final Pseudomonas spp. count without impairing the sensory panel.

Contamination risks in Mozzarella’s production

IThe microbial spoilage of fresh cheeses is of great concern for the dairy industry because the changes in sensory characteristics of products reduce the quality, impair the reputation of the cheese manufacturers and may cause serious economic losses for dairy companies (del Olmo, et al., 2018).

Mozzarella cheese is the most popular member of the pasta filata cheese family: this diverse group undergoes a unique stretching process, which takes place in hot water at 65-85 °C. Subsequently, during the production, cheese balls are formed and allowed to cool in a second water bath at 14-15 °C before packaging in brine containing 0.45% salt.

The high moisture content of 50-60%, the pH value around 5.5 (mild acidity) and the low salt content, render Mozzarella cheese susceptible to growth of both spoilage and pathogenic microorganisms (De Candia et al., 2007).

Common contaminants of mozzarella cheese include Pseudomonas spp. which represents the main obstacle to extending the shelf-life of this kind of product.

Pseudomonas spp. are ubiquitous and psychrotolerant Gram-negative bacteria, frequently isolated from refrigerated raw milk. Although they are inactivated by pasteurization, Pseudomonas spp. can enter the processed products through post-pasteurization contamination from the dairy processing plants via raw materials, soil, and water. Pseudomonas spp. can survive and grow in different environments, it may colonize the surfaces of utensils and equipment used in dairy production lines forming biofilms onto the surfaces of dairy processing facilities which help these microorganisms to persist long in the environment.

The defects noted on mozzarella cheese due to their growth are caused by the release of enzymes or pigments, with a negative impact on the products’ structure, texture and sensory properties.

In order to reduce the initial microbial count, is essential the strict application of good hygiene and good manufacturing practices, but often there is the need for additional strategies to control the growth of Pseudomonas spp.

The use of cultures with protective effects represents one potential intervention to enhance the safety of dairy products and to contain the spreading of undesired cultures. Most strains composing the cultures with protective effects belong to the group of lactic acid bacteria (LAB), which have a long history of safe use as starter cultures in food fermentations (Holzapfel et al., 1995). Therefore, the addition of cultures with protective effects to fermented products like cheese does not require additional labelling thus aligning with consumer demand for “clean-label” foods. Furthermore, commercial cultures with protective effects are often screened for desirable properties including limited impact on the flavour and acidity of foods (Spanu et al., 2018). Among LAB, the genus Carnobacterium spp. was deeply studied for its ability to produce bacteriocins (Bhugaloo-Vial et al.,1996, 1999).

Carnobacteria are Gram positive rod shaped lactic acid bacteria (LAB) isolated from different ecological niches. thanks to its ability of producing bacteriocins, the genus Carnobacterium, has been

used as a protective culture to provide durability and safety to various types of foods (cheese, fish and meat products). Among the species included in the genus Carnobacterium, two, C. divergens and C. maltaromaticum, are frequently isolated from the natural environment and dairy products.

The objective of this work was to investigate the ability of Lyofast CNBAP (Sacco, Cadorago Italy), a commercial freeze-dried blend of C. divergens and C. maltaromaticum to control the spread of Pseudomonas spp. in mozzarella cheese, artificially contaminated by this spoilage microorganism.

Lyofast CNBAP can survive and grow in conditions of pH, NaCl concentration, and temperature similar to the psychotropic spoilage microorganisms: thus, the addition of CNBAP to the Mozzarella brine during the shelf life can limit the growth of Pseudomonas spp., thus improving the durability of chilled foods.