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

Authors: Papageorgiou, Demetrios K.; Bori, Mina; Mantis, Antonios

Source: Journal of Food Protection®, Number 9, September 1997, pp. 1019-1151 , pp. 1041-1045(5)

Publisher: International Association for Food Protection

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Abstract:

Pasteurized whole ewe's milk was inoculated to contain ca. 1.0 × 106 to 2.0 × 106 Listeria 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.

Keywords: FETA CHEESE; FROZEN CURD; FROZEN EWE'S MILK; LISTERIA MONOCYTOGENES

Document Type: Research Article

Affiliations: Laboratory of Milk Hygiene and Technology, Department of Food Hygiene and Technology, School of Veterinary Medicine, Aristotle University of Thessaloniki, 540 06 Thessaloniki, Greece

Publication date: September 1, 1997

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    First published in 1937, the Journal of Food Protection®, is a refereed monthly publication. Each issue contains scientific research and authoritative review articles reporting on a variety of topics in food science pertaining to food safety and quality. The Journal is internationally recognized as the leading publication in the field of food microbiology with a readership exceeding 11,000 scientists from 70 countries. The Journal of Food Protection® is indexed in Index Medicus, Current Contents, BIOSIS, PubMed, Medline, and many others.

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