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A new procedure for surface contamination of food products using absorbent paper

Tucci, Patrizia, Sperandii, Anna Franca, Salini, Romolo, Centorotola, Gabriella, Neri, Diana, Iannetti, Luigi, Migliorati, Giacomo, Pomilio, Francesco
Journal of food safety 2018 v.38 no.4 pp. e12461
Listeria monocytogenes, absorbents, bacteria, cross contamination, cutting, food contamination, gorgonzola cheese, inoculum, meat, paper, pasting properties, predictive microbiology, ready-to-eat foods, statistical analysis, wheels
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 concentration of about 5 log₁₀cfu/cm² or MPN/cm² was obtained starting from a contaminated suspension of 8 log₁₀cfu/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 applications 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 technique, 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 Gorgonzola 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.