Division 2 Sample preparation and initial suspension preparation

Cross contamination between samples and from the surrounding environment shall be avoided at all stages. Samples are discarded once laboratory analyses have been initiated. If the analysis is stopped, for example due to unacceptable deviations in the analysis process, new samples must be obtained.

Either the entire product, or a representative test portion of 100 to 150 g, shall be taken to the initial dilution. Food shall be sampled to include surfaces reflecting the proportion that would be consumed (such as 20 % rind/surface and 80 % inside). When a packaged product is sliced, the respective sample is taken from more than one slice of the product. The test portion shall be cut in small pieces and placed into a stomacher bag, using a sterile instrument and an aseptic technique. From that mixture, a test portion of 10 g shall be taken for enumeration and a test portion of 25 g shall be taken for detection.

To the volume of the test portion (10 g), 9 volumes (90 ml) of diluent are added and subsequently the mixture is homogenised using a stomacher or a pulsifier for 1 to 2 min.

Buffered peptone water, as described in EN ISO 11290-2 ‘Microbiology of food and animal feeding stuffs — Horizontal method for detection and enumeration of Listeria monocytogenes — Part 2: Colony-count technique’, may be applied as a diluent for general use.

For the dilution of cheese, a sodium citrate solution, as described in EN ISO 6887-5 ‘Microbiology of food and animal feeding stuffs — Preparation of test samples, initial suspension and decimal dilutions for microbiological examination — Part 5: Specific rules for the preparation of milk and milk products’ may be used instead of buffered peptone water.

Detection and enumeration analyses of Listeria monocytogenes shall be performed in accordance with the following:

1. (a)

   for smoked and gravad fish samples two sets of analyses must be carried out:

   1. (i)

      immediately after sample collection at retail level; and

   2. (ii)

      at the end of shelf-life;

2. (b)

   for soft and semi-soft cheese samples and heat-treated meat product samples the analyses must be carried out only at the end of shelf-life.