Membrane filtration for enumeration of Listeria monocytogenes at low levels on salad leaf

Abstract

The reference method BS EN ISO 11290-2:2017 is insufficiently sensitive to enumerate Listeria monocytogenes in most commercially prepared leaf salads. A more sensitive method for enumerating low levels of Listeria spp. is required to evaluate the impact of process changes and the effect of Listeria control mechanisms in chilled foods including salad leaf. Membrane filtration allows larger volumes of food to be tested but care must be taken to avoid excessive pigmentation of the membranes by damage to leaf during sample preparation and bag homogenisation. Blue-green colonies can be identified after 48 h of incubation of the membranes on Listeria Ottaviani and Agosti selective agar plates, these colonies are indicative of Listeria spp., but halo zones cannot be distinguished. Results obtained by this technique are reported showing enumeration of 5 CFU/g added L. monocytogenes.