Purpose: To assess the compositional factors that affect pathogen growth on cheese at non-refrigerated conditions.
Methods: We tested 67 natural market cheeses for their ability to support growth of Listeria monocytogenes (LM), Salmonella (SALM), Escherichia coli O157:H7 (EC), and Staphylococcus aureus (SA) during 15-days at 25°C. Hard (Asiago and Cheddar), semi-hard (Colby and Havarti), and soft cheeses (Mozzarella and Hispanic-style) were among types tested, and included regular, reduced-sodium, and reduced-fat types. Single-pathogen cocktails were prepared and individually inoculated onto cheese slices (~105 CFU/g). Cocktails were comprised of 10 strains of LM, six of SALM, or five of EC or SA. Inoculated slices were packaged and stored at 25°C for < 15 days, with surviving inocula enumerated every three days. Moisture (%), salt (%), titratable acidity (%), pH, aw, and lactic acid bacterial (LAB) count were measured.
Results: Pathogens did not grow on 44 cheeses over the 15 days, while 13 cheeses supported growth of SA, six of SALM, four of LM, and three of EC. Of the cheeses which supported pathogen growth, all supported S. aureus growth, ranging from 0.62 to 3.08 log CFU/g (avg. 1.70 log CFU/g). Growth of Listeria monocytogenes, Salmonella spp. and E. coli O157:H7 ranged from 0.60-2.68 (avg. 1.60 CFU/g), 1.01 to 3.02 log CFU/g (avg. 2.05 log CFU/g), and 0.41 to 2.90 log CFU/g (avg. 1.69 CFU/g), respectively. Cheeses which supported pathogen growth also supported LAB growth.
Significance: Cheese pH at time 0 and salt-in-the-moisture-phase (%SMP) were compositional factors most affecting pathogen growth, with pH having a dominant effect. Except for certain surface- and mold-ripened cheeses, or Swiss-style cheeses, pathogen growth on cheeses made with cows’ milk was correlated with initial pH and %SMP.