Microbiological characterization of the biofilms of wooden shelves used for ripening of traditional raw milk cheeses produced in Sicily (South Italy).

The production of artisanal Sicilian cheeses is generally carried out with wooden equipment. For centuries, wood has been the main material used in cheese making due to its availability, low cost and resistance over time. Among dairy wooden equipment, the wooden vat used for milk curdling plays a role of paramount importance to inoculate the bulk milk with desired lactic acid bacteria (LAB) responsible for the acidification of the curd. In the last few years, the biofilms associated to these containers have been object of study of different research groups, mainly Italian and French, but so far, very little is known about the microbiological characteristics of the wooden shelves used for cheese ripening and their influence on the safety of the final products. With this in mind, the aim of the present study was to investigate and enumerate several microbial populations associated to the wooden shelves used for the ripening of traditional Sicilian cheeses. To this purpose, the biofilms of nine wooden shelves used for the ripening of Caciocavallo Palermitano and PDO Pecorino Siciliano made with raw cows’ and raw ewes’ milk, respectively, were investigated. The shelf biofilms (400 cm2) were sampled by the brushing recovery method using sterile plastic squares (Biogenetics s.r.l., Padua, Italy). The cell suspensions of wooden shelves were subjected to the decimal serial dilutions in Ringer’s solution (Sigma-Aldrich, Milan, Italy). The microbiological investigation carried out by plate count involved pro-technological and spoilage/pathogenic bacterial groups and unicellular/filamentous fungi. Listeria monocytogenes, Salmonella spp. and coagulase-positive staphylococci were never detected, total coliforms were at low numbers with E. coli found only onto two shelves. The spores of clostridia were not detected in any shelf. LAB dominated the surfaces of seven shelves, but they were below the detection limit onto two shelves. In general, the dominance was showed by mesophilic LAB cocci found in the range 105 - 108 CFU/cm2. Enterococci were found only onto three wooden shelves, but almost at 2 Log cycles lower than the levels of mesophilic LAB. Pseudomonas spp. were found only onto three shelves, also positive for the presence of enterobacteria. Yeasts were found in all samples and their levels ranged between 1.60 and 6.81 Log CFU/cm2. Moulds were below the detection limit only in one sample and their levels ranged between 1.78 and 5.45 Log CFU/cm2 on the other shelves. This study represents a first report on the microbiological investigation of the biofilms associated to the wooden shelves used for cheese ripening in Sicily. The results of the present work confirmed that from a hygienic point of view, the wooden shelves do not represents a risk for the safety of consumers. However, further studies are necessary to better investigate the microbial composition of these dairy equipment at species level in order to evaluate their influence on the centripetal ripening.