Purpose: Pathogens that have been implicated in illness outbreaks due to post-processing contamination of cheeses include Listeria monocytogenes (LM), Salmonella spp. (Salm), Escherichia coli O157:H7 (ECO157) and Staphylococcus aureus (SA). This study identified hard and semi-hard cheeses that inhibit pathogen growth on storage at 25°C, while identifying intrinsic factors affecting pathogen survival.
Methods: Strains of Salm (6), ECO157 (5), LM (10) and SA (5) were grown separately in broth. Strains were combined into single-pathogen cocktails, and serially diluted to ~107 CFU/ml. 24 types of sliced-cheeses were inoculated (~104 CFU/g), vacuum-packaged, and stored at 25°C up to 15 days. At day 0, 3, 6, 9, 12 and 15, sample pH was measured, followed by enumeration of pathogens on specific agar: modified Eosin Methylene Blue (Salm and ECO157), Listeria Selective (LM), and Baird Parker (SA). Δlog CFU/g over 15 days was determined. Cheese % moisture and % salt were determined on day 0; titratable acidity (TA) and an enumeration of flora able to be grown on deMan-Rogosa-Sharpe (MRS) agar were determined on day 0, 6 and 15.
Results: pH ranged from 4.34 (Feta) to 5.98 (Gruyere); water-phase salt from 0.76% (Lacy Swiss) to 6.96% (Parmesan); and TA from 0.94% (Havarti) to 2.83% (Feta). For LM, Δlog CFU/g ranged from -4.04 (Feta) to +0.23 (Gruyere); Salm, -4.12 (Feta) to +1.57 (Gruyere); ECO157 -3.99 (Feta) to -0.23 (Colby); and SA, -3.49 (Feta) to +2.25 (Gruyere). Cheeses supporting growth were Gruyere, Provolone, and low-sodium Provolone.
Significance: These results provide scientific validation in support of extended room temperature storage of most hard and semi-hard cheeses tested. Further assessments are needed to determine the safety of room temperature storage of Gruyere and Provolone.