ORIGINAL ARTICLE A new procedure for surface contamination of food products using absorbent paper Patrizia Tucci | Anna Franca Sperandii | Romolo Salini | Gabriella Centorotola | Diana Neri | Luigi Iannetti | Giacomo Migliorati | Francesco Pomilio National Reference Laboratory for Listeria monocytogenes, Istituto Zooprofilattico Sperimentale dell’Abruzzo e del Molise “G. Caporale,” Teramo, Italy Correspondence Patrizia Tucci, National Reference Laboratory for Listeria monocytogenes, Istituto Zooprofilattico Sperimentale dell’Abruzzo e del Molise “G. Caporale,” via Campo Boario, 64100 Teramo, Italy. Email: p.tucci@izs.it Abstract A new method was developed for artificial contamination of food products during challenge test studies, in products where superficial contamination is needed. Rinds of Gorgonzola cheese wheels were divided into quarters and then squares with 12 cm of side and contaminated with absorbent paper soaked with a bacterial suspension of Listeria monocytogenes left in contact with the surface of the blue-veined cheese, in order to achieve homogenous contamination of the whole surface. To verify the performance of the method of contamination, after 24 hr L. monocytogenes was enumerated to evaluate surface contamination. With the method described, an average concentra- tion of about 5 log 10 cfu/cm 2 or MPN/cm 2 was obtained starting from a contaminated suspension of 8 log 10 cfu/ml. The homogeneity of the contamination was demonstrated by the application of statistical tests. Practical applications Challenge tests are carried out to simulate contaminations as they happen during the processing and aging of cheeses, cured meats or ready-to-eat foods, for cross contamination studies, for appli- cations of predictive microbiology, and to evaluate potential dragging of bacteria during the cutting of slices. In all cases, the inoculum should be applied on the product using the appropriate tech- nique, in order to mimic natural contamination. This study reports a new method of contamination that allows the achievement of homogeneous microbial distribution on the whole flat surface of foods, using absorbent paper soaked with a bacterial suspension of Listeria monocytogenes. It was tested in the framework of challenge tests aimed to quantify the dragging of bacteria from Gor- gonzola cheese rinds to paste during different processing steps. The procedure could be, however, profitably used also in other food products, when uniform surface contamination is required. 1 | INTRODUCTION The definition and validation of the product’s shelf-life is crucial to ensure the microbiological safety of foods and the consumers’ health. In the framework of the Regulation (EC) No 2073/2005 (European Commission, 2005) on microbiological criteria for food- stuffs, FBOs shall carry out shelf-life studies in order to assure that their foods does not exceed the food safety criteria throughout the defined shelf-life. Challenge test are included among the indicated shelf-life studies (Spanu et al., 2014). A challenge test study consists in an artificial contamination of the food with a target microorgan- ism under controlled experimental conditions. A number of docu- ments published by International Health Authorities and Research Institutions describe how to carry out challenge test studies (Euro- pean Reference Laboratory for L. monocytogenes [EURL Lm], 2014; Health Canada, 2012; NACMCF, 2010), and since the publication of the first European Union Reference Laboratory for Listeria monocy- togenes Technical Guidance document for conducting shelf-life stud- ies in RTE foods in 2008 (EURL Lm, 2008), many studies have been performed applying challenge tests to determine the ability of a wide range of foodstuffs to support L. monocytogenes growth. How- ever, strict adherence to the Guidelines was not observed in most cases, as often one or more of the criteria were modified (Alvarez- Ordonez, Leong, Hickey, Beaufort, & Jordan, 2015). In fact, concen- trations of the inocula used in the various studies usually depend from the objective of the study. J Food Saf. 2018;e12461. https://doi.org/10.1111/jfs.12461 wileyonlinelibrary.com/journal/jfs V C 2018 Wiley Periodicals, Inc. | 1 of 9 Received: 18 November 2017 | Revised: 20 February 2018 | Accepted: 1 March 2018 DOI: 10.1111/jfs.12461