The usual method for counting Listeria monocytogenes in products has a reporting limit of 10 nucleating units per gram (kve/g). Nutrilab also has a method with a lower counting limit in its package, namely <1 kve/g. Here, we will explain when it is useful to use this method.
For products that are a feeding ground for Listeria, the shelf lives must be established through additional research (see Info sheet 85 NVWA). It must be demonstrated that the products concerned contain less than 100 kve/gram of Listeria monocytogenes at the end of the shelf life period. An essential part of this is establishing the maximum contamination level at the start. If historical data show that, in 25 grams of the product, Listeria is always absent at the start of the shelf life period, this is also the limit that can be applied. This can also be read as <1 kve/25 grams or <0.04 kve/gram. This can be determined with the help of a test of the presence or absence in 25 grams of the product. Regulation 2073/2005 prescribes that this must be done fivefold (n=5).
However, some raw materials incidentally contain Listeria. If these are not killed off during the processing phase, an intermediary limit higher than 1 kve/25 grams, such as <1 kve/g, must be applied. The usual reporting limit of <10 kve/g is then insufficient in order to prove this contamination level. By using ten times as many plates as usual, this will work. Unfortunately, the costs of this are higher. Because the method remains unchanged for the other parts, this determination can be conducted under accreditation. If the establishing of a sound intermediary norm is accepted by the NVWA, this norm must be monitored from that point forward.