In Nofima’s laboratory in Stavanger, fish cakes, smoked mackerel and cod roe caviar are placed side by side awaiting the verdict. Listeria bacteria have been added to all the products, which have then been stored at various common refrigeration temperatures at which Listeria can grow. The researchers are closely monitoring to see how much the bacteria multiply.
“We simulate a normal storage situation, with all the food remaining in its normal packaging as we find it in the store. Generally, the seafood counter and refrigerator maintain a temperature of approx. 4 °C, but we will take into account that deviation can happen. In the experiments, therefore, we store the seafood at temperatures at which the product might be exposed to and which can provide better growth for Listeria,” says Researcher Thomas Rosnes.
“If the Listeria doesn’t grow quickly and sufficiently in the higher temperature in the lab, it won’t do so in a cold seafood counter.”
The researchers want to enumerate the bacteria. In the project “Listeria 100”, the aim is to document that a food product will not support growth to a level higher than 100 Listeria bacteria per gram. In the foods in which Listeria may grow, such as smoked salmon, the limit should not be reached until after the best-before date. In foods in which Listeria cannot normally grow, such as shrimp in brine or salt-cured fish, the limit should not be exceeded at any time.
It is important to have control of the level of Listeria bacteria. A person eating foods containing Listeria may develop the disease listeriosis. It is primarily risk groups such as pregnant women and others with immunodeficiency that are in the danger zone, but people with a well-functioning immune system can also become ill if they ingest too high levels of Listeria.
Strict safety demands
Fish cakes and similar products such as fish pudding or fish balls undergo heat treatment to kill any Listeria bacteria which may be in the food either because they were in the raw material or were transferred from the surroundings. However, the products may be recontaminated after heat treatment for instance by equipment or staff.
Consequently, strict guidelines for hygiene are in place in food production. Exporters of Norwegian seafood continually experience their products being carefully checked by the customers. Both the EU and the Norwegian Food Safety Authority demands that ready-to-eat foods, in other words foods that do not need to be heated before it is eaten, do not contain more than 100 Listeria bacteria per gram.
But with such low levels of bacteria, it is a challenge to determine the exact number. Sampling does not provide an accurate answer about how many bacteria are present in the total product lot, unless a huge number of analysis are carried out. Consequently, the manufacturer must be able to document that the product will not support growth to a level of more than 100 bacteria per gram during the shelf life. However, as the guidelines to document this growth are so comprehensive and expensive to implement, many manufacturers are struggling to obtain the necessary documentation.
There are several methods to obtain such documentation, but they are in part very demanding. Both mathematical models and risk assessments may be developed if there is sufficient knowledge about how the bacteria behave in a given food. However, this is not the case for many seafood products.
The Norconserv Foundation invited seafood companies to use a so-called challenge test on their products at the laboratory of Nofima. This method is particularly relevant when there are multiple factors in the product that inhibit Listeria, such as salt, low pH and low temperature. In this test researchers at Nofima add relevant bacteria to the surface of the fish product, and subsequently storing it under as normal conditions as possible.
Packaged products are, for example, challenged by the bacterium being injected via a needle inserted through the plastic packaging. The plastic is then resealed. Vacuum-packed products are opened, the Listeria is added and the product is then vacuum-packed again. As caviar in a tube has no natural surface, the bacterium is injected into the tube before the cap is put back on. Subsequently the samples are left in storage until the best-before date has expired.
Waiting in excitement
EU regulations require that the test is repeated three times for each product, as there may be variations in production. Consequently, the food producers are still waiting anxiously to see what result their product receives.
“If we find seafood products that don’t comply with the target level then the manufacturer must take action. This may be to reduce the shelf life, change the production process or add preservatives to the product. There are many options. We know for example that some sectors of the industry use lactic acid to reduce bacterial growth,” says Rosnes.
“The strict regulations are already implemented and the seafood industry must take this seriously. But hopefully we find that the products deliver what they promise,” he concludes.
Food manufacturers wanting more information about the testing of Listeria may contact Nofima.