Survival of Listeria monocytogenes in Frozen Ewe's Milk and Feta Cheese Curd

Abstract

Pasteurized whole ewe's milk was inoculated to contain ca. 1.0 × 106 to 2.0 × 106Listeria monocytogenes Scott A or California (CA). Inoculated milk samples of 200 ml in sterile stomacher bags were frozen at −38°C and stored at −18 or −38°C for 7.5 months. Inoculated milk was also made into Feta cheese curd, according to a standard procedure. After 5 h of drainage, curd samples of 200 g in sterile stomacher bags were frozen at −38°C, and stored at −18 or −38°C for 7.5 months. The pH values of the ewe's milk and Feta cheese curd before freezing were 6.70 and 5.43 respectively. At l5-day intervals samples were thawed at 35°C and tested for numbers of L. monocytogenes cells by surface plating on tryptose agar (TA) and tryptose salt agar (TSA) for ewe's milk samples, or on lithium chlorite phenylethanol moxalactam agar (LPMA) for curd samples. A high percentage (ca. 95%) of L. monocytogenes Scott A cells survived during storage of frozen ewe's milk at −18 or −38°C for 7.5 months. The population of L. monocytogenes CA decreased by ca. 50 and 40% during storage of frozen ewe's milk for 7.5 months at −18 and −38°C respectively. The death rate of L. monocytogenes increased after repeated freeze-thaw cycles of ewe's milk at −18 or −38°C. Populations of L. monocytogenes Scott A decreased by ca. 40% in the center of the cheese curd samples but the rate of death was less than ca. 17% on the surface of the frozen cheese curd samples during storage at −38°C for 7.5 months. Populations of strain Scott A decreased by ca. 57% in the center of the cheese curd samples and by ca. 22% on the surface of the frozen cheese curd samples during storage at −18°C for 7.5 months. Populations of L. monocytogenes CA decreased by ca. 98% for samples both at the center and the surface of the frozen curd during storage at −38 or −18°C for 7.5 months.