Short communicationNovel insights on pink discoloration in cheese: The case of Pecorino Toscano
Introduction
The occurrence of defect in cheeses is of great concern for the dairy industry as it could lead to products recall and thus representing a significant economic loss for dairy companies (Carminati et al., 2019).
Defects may have different origins and may occur in various steps of the cheese making process. They can be related to variations in milk quality, milk treatment, hygiene practices and cheese processing parameters (manufacture technology and/or ripening). Defects can also be linked to differences in the starter cultures activity and acidity profiles, as well as to the nature and number of the non-starter microorganisms that come from raw milk or other environmental sources (O'Sullivan, Giblin, Mc Sweeney, Sheehan, & Cotter, 2013).
To date, several studies have focused on the understanding of the causes of cheese discoloration. In particular, the reason for the formation of pink defect in cheeses without colorant addition still remains unknown in some cases (Quigley et al., 2016). It has been mainly attributed to the metabolism of certain strains of thermophilic lactobacilli, propionic acid bacteria, Thermus spp., or Glutamicibacter arilaitensis as well as the activity of residual microbial enzymes or to Maillard reactions, particularly related to the presence of unfermented galactose (Cleary, Kolachina, Wolfe, & Sanchez, 2018; Daly, McSweeney, & Sheehan, 2012).
During summer 2018, a Tuscan dairy producing Pecorino Toscano, a PDO pasteurised sheep's milk medium ripened Italian cheese, observed an unusual pink discoloration on the rind of several cheese wheels (Personal communication; Fig. 1). With the aim of determining the cause of discoloration, microbiological analysis followed by molecular identification of the species recovered in the pink area, were carried out.
Section snippets
Sampling
To identify the bacteria species potentially responsible for the alteration, the crust and the closer inner part of six representative cheeses were sampled and homogenized 1:10 with sterile Ringer's solution, by means of a blender (Stomacher, 400 circulator; Oxoid, Basingstoke, UK) for 2.30 min at 230 rpm. Each cheese was analysed separately. The samples dilutions were then plated in triplicated on different growth media. Pseudomonas CFC agar (PS) (Oxoid) was used to count Pseudomonadaceae,
Microbial counts
In the present study, the total microbial counts was of 7.82 log cfu g−1 (mean value of the samples) (Fig. 2), which corresponds to the results found by Proroga et al. (2009) on Pecorino. Yeast and moulds count was 3 log cfu g−1, which is typical for ripened cheeses such as Pecorino Toscano PDO (Todaro et al., 2011). The presence of spore forming microorganisms was lower than 3 log cfu g−1 (Fig. 2), confirming what has been observed in the literature for similar cheeses (Palmas, Cosentino,
Conclusion
The microbial analysis of each sample from the six Pecorino Toscano cheese revealed the presence of S. liquefaciens, suggesting that an environmental contamination of the cheese rind could be the cause of the observed defect. In particular, it seems of central importance, when studying this kind of defect, the evaluation of brine and ripening chambers hygiene as well as the overall hygiene of the working areas. Another important aspect is the control of rind humidity, which may promote the
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