Growth of Listeria monocytogenes in the whey cheeses Myzithra, Anthotyros, and Manouri during storage at 5, 12, and 22°C

Abstract

The fresh whey cheeses Myzithra, Anthotyros, and Manouri were inoculated with Listeria monocytogenes Scott A or California to contain ca. 5.0 X 102 CFU/g of cheese and incubated at 5, 12, and 22°C. The initial pH of the finished whey cheeses Myzithra, Anthotyros, and Manouri were 6.50, 6.41, and 6.30 respectively. Fat in dry matter was 16.3% in Myzithra, 65.9% in Anthotyros, and 71.7% in Manouri cheese; the moisture contents of the cheeses were 68.4, 66.9, and 52.2% respectively. The pH of the cheeses dropped gradually to between 5.30 to 4.97 during storage. Duplicate samples of whey cheeses were tested for numbers of L. monocytogenes and pH. Listeria counts were obtained by surface plating on lithium chlorite-phenylethanol-moxalactam agar (LPM). Selected Listeria colonies were confirmed biochemically. The growth of L. monocytogenes in the whey cheeses was very rapid. Generation times of L. monocytogenes at 5°C ranged between 16.16 and 20.16 h and were significantly longer than those observed at 12°C, which ranged between 5.07 and 5.81 h. Generation limes at 22°C ranged between 1.68 and 2.70 h. The generation time of L. monocytogenes Scott A at 5°C in Anthotyros cheese (20.16 h) was significantly longer than those of the same strain at 5°C in Myzithra cheese (16.16 h) and Manouri cheese (17.81 h). Also, the generation time of L. monocytogenes California at 22°C in Myzithra cheese (2.70 h) was significantly longer than the generation time of strain Scott A (1.93 h). Maximum populations of L. monocytogenes were reached after 24 to 30 days at 5°C. after 5 to 12 days at 12°C, and after only 56 to 72 h at 22°C and ranged from 6.92 to 8.81 log CFU/g. Maximum populations were significantly lower in Myzithra cheese at 5 and 12°C than maximum populations in Anthotyros and Manouri cheese at the same temperatures independent of the inoculated strain.