A new method to enumerate low level of Listeria monocyogenes and Salmonella spp. in meat products

Micro-organisms counting techniques are a subject of major concern in the field of food hygiene. The European Community Regulation No 2073/2005 defined a quantitative limit for 100 cfu.g-1 which was is applicable for certain types of food during their shelf life. However an adequate counting method for low contaminating levels is still lacking. The EN ISO 11290-2 European and International Standard method for enumeration of Listeria monocytogenes is characterized by a limit of enumeration of 10-100 cfu.g-1, and a poor accuracy. Recently a new method has been developed to enumerate L. monocytogenes at low contamination levels in several food matrices using a membrane filtration method (Baudouin et al., 2009). This method uses a combined enzyme/surfactant treatment to solubilize food components which increases handling period. The aim of the present work was to develop a new method to enumerate L. monocytogenes and Salmonella spp. at low concentration on meat products. This method is based on membrane filtration method without using treatment to solubilize food components. White pudding has been used as “experimental matrix” and artificially contaminated on surface with 4-12 cfu.g-1 of L. monocytogenes or Salmonella spp. For enumeration, 25 g of samples were placed into a plastic bag and homogenised in water peptone buffer (dilution 1/10) for 30 sec using a Pulsifier. Food suspension was filtered on a single use membrane of 47 mm diameter and 0.45 µm pore size. The filters were laid on specific plated media. To compare with a control, 25 g of samples were placed in classic Stomacher bags with a mesh screen liner (280 µm pore size) and homogenised using a Mix 2 Stomacher apparatus with the same dilution as above. A volume of 1 ml of filtered food suspension was laid on specific plated media according to EN ISO 11290-2 standard. Enumeration results obtained with the two methods are statically similar but the new one is easier, cheaper to carry out, and more sensitive. Moreover this methodology is faster than methods previously described and non destructive: food components solubilisation step is no more necessary. Preliminary tests were carried out using minced meat and, due to meat texture, problems with clog membrane occurred, therefore this method should be improved. This new method is useful for simulating the contamination level commonly found in food industries.