Evaluation of a post-lethality treatment against Listeria monocytogenes on Ricotta salata cheese C. Spanu, V. Spanu, C. Pala, S. Virdis, C. Scarano * , E.P.L. De Santis Department of Veterinary Medicine, University of Sassari, Via Vienna 2, 07100 Sassari, Italy article info Article history: Received 13 April 2012 Received in revised form 10 July 2012 Accepted 17 July 2012 Keywords: Listeria monocytogenes Whey cheese Post-lethality treatment abstract As a control measure for Listeria monocytogenes contamination a post-lethality heat treatment was applied in vacuum packaged Ricotta salata, a traditional whey cheese obtained from sheep’s milk. In Ricotta salata L. monocytogenes usually occurs as post-process contamination and its concentration can increase during storage at refrigeration temperature. Ricotta salata wheels were challenged spraying the rind with a L. monocytogenes suspension, obtaining a contamination level of ca. 10 6 cfu g 1 . In order to take into account strains differences in growth, the inoculum contained a mixture of the reference strain NCTC 10887 (serovar 1/2b) and two wild type (serovar 1/2a) strains. 87 artificially contaminated Ricotta wheels were randomly selected and subjected to water bath heat treatment at 85  C for 90 min, while 21 served as negative controls. Samples were stored at refrigeration temperatures until analysis. L. monocytogenes, background microflora and physico-chemical properties were assessed 6 h, 2, 4, 6 and 12 months after the artificial contamination. Pulsed filed gel electrophoresis was conducted to distin- guish between the inoculated and recovered strains. L. monocytogenes cells were below the limit for the enumeration method in samples submitted to the lethal treatment. Heat treatment was effective to achieve a 6 log 10 reduction of L. monocytogenes cells. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Ricotta salata is a whey protein cheese traditionally made in Sardinia (Italy) from sheep’s milk. Great part of this product is intended for export to several European countries and in North America. Whey protein cheeses include dairy products obtained through the heat coagulation of whey (CAC, 2010). The whey remaining after cheese production is heated in large open kettles to temperature above 85  C and the floating curd is collected using perforated scoops, transferred into plastic moulds and allowed to drain and to cool. The curd is then pressed to enhance drainage and dry salted (5% w/w) for about 10 days. The final product is indi- vidually packed in vacuum bags and stored at refrigeration temperature with a set shelf-life which differs from three weeks up to several months, depending on the food business operators. Although the presence of Listeria monocytogenes in cheeses is quite rarely detected at levels greater than 100 cfu g 1 (EFSA, 2010; NZFSA, 2005), outbreaks and sporadic cases of listeriosis associ- ated with dairy products have been reported in different countries (Carminati, Gatti, Bonvini, Neviani, & Mucchetti, 2004; De Buyser, Dufour, Maire, & Lafarge, 2001; Farber & Peterkin, 1991; Goulet et al., 1995; Kozak, Balmer, Byrne, & Fisher, 1996; Makino et al., 2005; Manfreda, De Cesare, Stella, Cozzi, & Cantoni, 2005; McLauchlin, 1996). Most of the cases were linked to the consumption of fresh soft, blue-veined and mould-ripened cheeses (WHO/FAO, 2004). Cheeses are ranked at number six in the “FDA Top Ten” riskiest foods report (CSPI, 2009). Large numbers of viable Listeria cells present in milk are killed by heat treatment applied during cheese-making and whey protein coagulation (Buazzi, Johnson, & Marth, 1992; Casadei, Esteves de Matos, Harrison, & Gaze, 1998; ICMSF, 1996, chap. 8; Villani et al., 1996). However, the production of Ricotta salata includes manual manipulation of the curd after floating and the exposition to environmental contamination (Lioliou, Litopoulou-Tzanetaki, Tzanetakis, & Rob- inson, 2001; Pintado & Malcata, 2000). Recall of Ricotta salata imported from Italy due to L. monocytogenes contamination have been reported by the North Carolina and Georgia Department of Agriculture, respectively in 1999 and 2002. European Commission documented in 2008 a case of L. monocytogenes infection associated with Ricotta salata cheese (RASFF, 2008). The main route of contamination of dairy products with L. monocytogenes is the contact with surfaces of the processing plant environment and equipment (Alessandria, Rantsiou, Dolci, & Cocolin, 2010; De Buyser et al., 2001; Ho, Lappi, & Wiedmann, 2007; Kabuki, Kuaye, * Corresponding author. Tel.: þ39 079 229454; fax: þ39 079 229458. E-mail address: scarano@uniss.it (C. Scarano). Contents lists available at SciVerse ScienceDirect Food Control journal homepage: www.elsevier.com/locate/foodcont 0956-7135/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.foodcont.2012.07.023 Food Control 30 (2013) 200e205