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 × 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 times 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 CPU/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.

Traditional Balkan fermented milk products

“…The whey cheeses, traditional for the Balkan Peninsula, are: Myzithra, Xinomyzithra, Manouri, Urda, Curd, Skuta, and Lor [55][56][57][58]. The whey cheeses, traditional for the Balkan Peninsula, are: Myzithra, Xinomyzithra, Manouri, Urda, Curd, Skuta, and Lor [55][56][57][58].…”

Listeria monocytogenes growth potential in Ricotta salata cheese

“…Documented evidence on whether the product supports the growth of L. monocytogenes must be provided to determine an appropriate shelf-life. Several studies have been conducted on whey protein cheeses to extend shelf-life (Davies, Bevis, & Delves-Broughton, 1997;Martins, Cerqueira, Souza, Do Carmo Avides, & Vicente, 2010;Samelis, Kakouri, Rogga, Savvaidis, & Kontominas, 2003), but little information is available on the extent of growth of L. monocytogenes in whey cheeses without the use of preservatives or particular packing conditions (Lioliou, LitopoulouTzanetaki, Tzanetakis, & Robinson, 2001;Papageorgiou, Bori, & Mantis, 1996).…”

A survey of soft cheeses in Greek retail outlets highlights a low prevalence of Listeria spp.

“…The pH of the sample was 5.92 and its a w was equal to 0.952. Anthotyros can support the growth of L. monocytogenes when the pathogen is introduced as a post-processing contaminant, even when the cheese is stored at 4°C (Papageorgiou et al 1996), and interventions such as irradiation or addition of nisin have been proposed for the inactivation or the control of L. monocytogenes proliferation in Anthotyros (Samelis et al 2003;Tsiotsias et al 2002). The second positive sample contained Listeria grayi at levels of 1.5×10 2 CFU g −1 (Blue cheese, "shelf" sample).…”

“…Controlling L. monocytogenes growth in contaminated foods is problematic because the bacterium is resistant to low pH, low water activity, and low temperatures (Hado and Yousef 2007), conditions that constitute primary approaches for food preservation. Previous studies have documented the ability of L. monocytogenes to proliferate in soft cheeses under refrigeration (Back et al 1993;Genigeorgis et al 1991;Papageorgiou et al 1996), a phenomenon aided largely through the pathogen's ability for intracellular accumulation of cryoprotective compounds from its environment (Angelidis et al 2002).…”