Purpose: This study compares the behavior of multidrug-resistant and pan-susceptible Salmonella during the manufacture and aging of Gouda cheese and to compare current enrichment and detection protocols.
Methods: Cheeses were manufactured, in triplicate, in a lab scale cheese vat from raw milk inoculated with a 6-strain cocktail of either resistant or susceptible S. Newport and S. Typhimirium at an approximate concentration of 20 CFU/ml. Samples of milk, whey, curd and finished cheese throughout aging were analyzed using each of the following enrichment protocols: ISO 6759:2002; AOAC-RI for BAX PCR; modified versions of AFNOR (BAX) and Health Canada MFLP-29. All results were culture confirmed.
Results: Overall, Salmonella counts increased significantly during manufacture to a mean count of 734 CFU/g of cheese on day one followed by a significant decrease over 60 days aging to a mean of <1 CFU/g. Levels fell and stayed below the direct plating detection limit of ≥5 CFU/g after 54 days on average, yet remained detectable following enrichment for 210 ± 40 days. Changes in pathogen levels observed throughout manufacture and aging did not differ significantly between cheeses inoculated with resistant or susceptible Salmonella. The ISO method with and without PCR detection produced the least false negative results followed by AOAC-RI for BAX. The remaining methods, notably those with elevated primary incubation temperatures, produced results that disagreed significantly with the true result according to McNemars test.
Significance: Our results suggest that while multidrug-resistant strains of Salmonella may not display increased survivability in Gouda cheese, the 60-day aging requirement alone is insufficient to completely eliminate viable levels of this pathogen.