Running head : pathogen growth on cheese 1 2 Growth of Listeria monocytogenes , Salmonella spp . , Escherichia coli O 157 : H 7 , and 3 Staphylococcus aureus on Cheese during Extended Storage at 25 ° C 4 (original) (raw)

A potentially hazardous food (PHF) requires time/temperature control to maintain safety. The US Food and Drug Administration would classify most cheeses as PHF based on pH and a w, and a product assessment would be required to evaluate safety for >6 h storage at 21°C. We tested the ability of 67 market cheeses to support growth of Listeria monocytogenes (LM), Salmonella spp. (SALM), Escherichia coli O157:H7 (EC), and Staphylococcus aureus (SA) over 15-day storage at 25°C. Hard (Asiago and Cheddar), semi-hard (Colby and Havarti), and soft cheeses (Mozzarella and Mexican-style) were among types tested, and included some reducedsodium and reduced-fat types. Single-pathogen cocktails were prepared and individually inoculated onto cheese slices (~10 5 CFU/g). Cocktails were comprised of 10 strains of LM, six of SALM, or five of EC or SA. Inoculated slices were vacuum packaged and stored at 25˚C for < 15 days, with surviving inocula enumerated every three days. Salt-in-the-moisture phase (%SMP), calculated from measured moisture (%) and salt (%), titratable acidity (%), pH, and a w were measured. Pathogens did not grow on 53 cheeses, while 14 cheeses supported growth of SA, six of SALM, four of LM, and three of EC. Of the cheeses supporting pathogen growth, all supported growth of SA, ranging from 0.57 to 3.08 log CFU/g (avg. 1.70 log CFU/g). Growth of SALM, LM, and EC ranged from 1.01 to 2.05 log CFU/g (avg. 2.05 log CFU/g), 0.60 to 2.68 log CFU/g (avg. 1.60 CFU/g), and 0.41 to 2.90 log CFU/g (avg. 1.69 CFU/g), respectively. Cheese pH and %SMP most affected pathogen growth, with pH having a dominant effect. Pathogen growth/no-growth varied within some cheese types or lots. Except for Swiss-type cheeses, moldor bacterial-ripened cheeses, and cheeses made with non-bovine milk where insufficient data exists, the pathogen growth/no-growth interface could be modeled and boundary conditions established for safe, extended storage (<25°C) of cheeses based on pH and %SMP.