Occurrence and growth of Listeria monocytogenes in packaged raw milk

Abstract

The increased availability of packaged raw drinking milk necessitates the investigation of the occurrence and growth of Listeria monocytogenes in raw milk during distribution and storage. The occurrence of L. monocytogenes in 105 retailed raw milk bottles, 115 bulk tank milk samples, 23 in-line milk filter socks and in 50 environmental samples collected from an on-farm dairy establishment were investigated. Growth of inoculated low-level L. monocytogenes contamination was also investigated in two types of raw milk packaging, namely in 1-litre plastic bottles and 3-litre bag-in-boxes, both stored at three different storage temperatures of 6, 8 and 10°C. The occurrence of L. monocytogenes was higher (4.8%) in bottled raw milk stored until the use-by-date of the package compared to fresh bulk tank milk (1.7%). L. monocytogenes counts were ≤13CFU/ml in bottled raw milk and ≤1CFU/ml in bulk tank milk. L. monocytogenes was not detected in the packaging facility, but occurred very frequently (39%) in the milk filter socks. Subtyping of L. monocytogenes isolates using pulsed-field gel-electrophoresis revealed seven pulsotypes, of which two occurred in multiple samples. Targeted inoculum levels of 1-2CFU/ml yielded L. monocytogenes counts≥100CFU/ml within seven days of storage in 22% of the raw milk packages stored at 6°C, and in all of the raw milk packages stored at 8°C. The frequent occurrence of L. monocytogenes in raw milk and the ability of a low-level L. monocytogenes contamination to grow at refrigeration temperatures highlight the importance of consumer education regarding the appropriate raw milk storage and handling.

An assessment of the microbiological quality and safety of raw drinking milk on retail sale in England

“…2073/2005 (European Commission 2005; as amended). However, it has previously been demonstrated that fresh milk will readily support the growth of L. monocytogenes at refrigeration temperatures (Rosenow and Marth 1987), and that even within 5 days, L. monocytogenes can grow to levels of 100 CFU per gram or more at a temperature of 6 or 8°C, depending on the initial inoculum level (Castro et al 2017). Moreover, RDM is frequently sold for home-freezing and subsequent defrosting (which this bacterium will readily survive; el-Kest and Marth 1991), which leads to a further lack of clarity about how to apply the criteria in EC 2073/2005, as it seems probable that the overall shelf life before freezing and after defrosting may exceed 5 days on some occasions.…”

Listeria monocytogenes in Milk: Occurrence and Recent Advances in Methods for Inactivation

“…The consumer demand for raw milk occurs from perceptions of better sensory and nutritional qualities of raw milk over those of pasteurized milk, besides the desire of many consumers to support local and small-scale agriculture [11,12]. It is important to observe that epidemiological data have demonstrated microbiological health risks associated with raw milk consumption [13]. This enforces the necessity of raw milk consumption being accompanied by a risk of ingesting pathogenic bacteria, which pose an elevated health hazard [14].…”

Occurrence, Persistence, and Contamination Routes of Listeria monocytogenes Genotypes on Three Finnish Dairy Cattle Farms: a Longitudinal Study

“…Although a wide variety of ready-to-eat foods can mediate listeriosis, approximately half of the outbreaks are linked to contaminated dairy products (4,5). L. monocytogenes is a common contaminant of raw milk, as the observed prevalence of L. monocytogenes in bulk tank milk (BTM) of the dairy cow is frequently in the range of 2 to 7% (6)(7)(8)(9)(10)(11)(12). The consumption of unpasteurized raw milk contaminated with L. monocytogenes poses a listeriosis risk (13,14).…”