Sampling and Laboratory Analysis

Abstract

Due to the ubiquitous nature of L. monocytogenes, contamination of food processing facilities is common. Although transient contamination may be somewhat unavoidable, L. monocytogenes strains can persist for long periods of time and cause more serious problems, as described earlier. They often occur in the processing environment with little competition for space/ nutrients etc. from other bacteria. Sampling for L. monocytogenes in a processing environment is an important tool for identifying not only if L. monocytogenes contamination is present, but also if more than one L. monocytogenes strain is present and if the contamination is persistent. The EURL Lm has produced a guidance document on sampling premises for L. monocytogenes (EU 2012). Sampling of a food processing environment is best performed using a pre-moistened sponge swab stick for testing surfaces (Fig. 4.1) (both food contact surfaces and non-food contact surfaces) and sterile dippers for liquids. Swab samples should be taken from a surface of approximately 1 m² if possible, in a zigzag pattern, and the swab then returned to its sterile bag for transport to the laboratory. Although it is important to focus on sampling areas which come into direct contact with food, other areas in which L. monocytogenes contamination would be expected, such as pooled water on the floor, drains, the under-side of shelves and trollies etc. should also be sampled as areas such as these often harbour strains which can spread contamination to food contact areas and subsequently to the food itself.