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 × 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.

Listeria monocytogenes in milk and dairy products

“…In sheep milk, frozen at -38 0 C, and subsequently stored at -18 0 C and -38 0 C during the period of 7,5 months, one strain of L.monocytogenes survived to great extent ( 95% ), whereas the other strain, under same conditions, survived in a lesser degree (40-50%). Destroying of L. monocytogenes in curdle of feta cheese obtained from said sheep milk stored under same conditions, was greater than in sheep milk (Papageorgiou et al, 1997). To which extent the L. monocytogenes develops or survives in milk and dairy products, stored at storage temperature or frozen, depends on the type of dairy product and strain of L. monocytogenes (El-Gazar et al, 1992;Theodoridis et al, 2006).…”

Effect of cold adaptation on the survival of Listeria monocytogenes in ice-cream formulations during long-term frozen storage

“…Marth (1990, 1991) observed an initial decrease of L. monocytogenes population in frozen milk and in frozen milk components over 7 weeks. Papageorgiou et al (1997) also showed that L. monocytogenes survived for 7.5 months in frozen ewe's milk and feta cheese curd, and the data indicate a decrease of 98% in L. monocytogenes population during frozen storage of curd at −38°C or −18°C for up to 7.5 months. Furthermore, their data indicated that strain Scott A can survive frozen storage at a higher percentage (60-80%) than strain California.…”

Μελέτη φυσιολογικών χαρακτηριστικών στελεχών του μικροοργανισμού Listeria monocytogenes απομονωθέντων από γαλακτοκομικές επιχειρήσεις

“…Επιπλζον ζχει αναωερκεί, ότι θ διάρκεια τθσ λανκάνουςασ ωάςθσ για κφτταρα τθσ L. monocytogenes, που ζχουν προθγουμζνωσ καλλιεργθκεί ςε χαμθλζσ κερμοκραςίεσ, είναι μειωμζνθ ςε ςχζςθ με τα αντίςτοιχα που ζχουν αυξθκεί ςε κερμοκραςίεσ περιβάλλοντοσ (Whiting και Bagi, 2002). Κατά τθν ςυντιρθςθ των τροωίμων ςε κερμοκραςίεσ κατάψυξθσ δφναται να παρατθρθκεί επιβίωςθ του πλθκυςμοφ των κυττάρων του πακογόνου ι μείωςι του, ανάλογα με ςτζλεχοσ και το υπόςτρωμα (Papageorgiou et al, 1997, Gougouli et al, 2008. Θ ανκεκτικότθτα του μικροοργανιςμοφ ςτισ χαμθλζσ κερμοκραςίεσ επιτυγχάνεται μζςω τθσ ενδοκυτταρικισ ςυςςϊρευςθσ των ςυμβατϊν διαλυτϊν ενϊςεων γλυκίνθβεταΐνθ και καρνιτίνθ (Angelidis et al, 2002).…”