Preliminary experiments showed that Listeria survived for up to 60 days in 15 and 21 percent salt solution, which are equivalent to the salt concentrations in salt-cured fish and clipfish respectively.
The experiment was repeated with Listeria being added to cod. The cod was then salt-cured and stored at a temperature of 4 °C for three weeks before being rehydrated.
The conditions for salt concentration, temperature and time were in accordance with practices in the salt-cured fish industry.
"The results pointed to the fact that the bacterium survived the salt-curing process. In the rehydrated product, the bacterium started to grow during the cold storage. This is an important finding, as it can set new scopes for assessment of food safety for salt-cured products," says Nofima Senior Scientist Grete Lorentzen, who recently presented her PhD dissertation at the University of Tromsø.
There has been a prevailing opinion that the high salt concentration in salt-cured fish and clipfish was sufficient to prevent the survival of undesired bacteria.
Around half the cod caught annually in Norway is salted and cured. In 2009 Norway exported around 120,000 tonnes of salt-cured fish.
New food habits
Salt-cured fish and clipfish are highly appreciated products because of the characteristic flavour, texture and good storage qualities. These are important reasons why these products are traditional ingredients in Portuguese, Spanish and Brazilian cooking.
The raw materials are prepared by rehydration and the rehydrated product is an ingredient in many read-to-eat products, such as clipfish balls.
Traditionally the consumer has rehydrated the fish immediately prior to use. However, rehydration is a time-consuming process and in time buying rehydrated fish at the fish shop has become more common and consequently time can now pass between rehydration and consumption.
If Listeria is present in the rehydrated product, the storage period can lead to the growth of the bacterium.
"If the rehydrated fish is used in bacalao, which is generally cooked for two to three hours, the Listeria will be killed and the food will be safe to eat," says Lorentzen.
"Rehydrated salt-cured fish is also an ingredient in clipfish balls, Bolinhos de bacalhau. The balls are generally prepared a while before they are sold then eaten straight away. If the balls have not received adequate heat treatment, and the raw material contains Listeria, the bacterium can grow during the intermediate storage period and people who then eat the food can become ill. We should also be aware that Spanish and Portuguese people also use salt-cured fish as an ingredient in various salads and in such cases the fish is eaten raw."
Changes certain characteristics
The doctoral research also points to the fact that Listeria changes certain characteristics after being exposed to high salt concentrations.
Heat treatment kills Listeria. Today’s regulations for heat treatment of food are based on a linear survival curve for the bacterium. This implies that it is based on an equal number of Listeria being killed per unit of time throughout the entire heat treatment process.
However, the results of Lorentzen’s research point out that this is not always the case.
"When Listeria is exposed to high salt concentrations, this leads to some bacteria not surviving, some with unchanged characteristics and some emerge stronger from the salt stress as they can tolerate a new type of stress better than they could before, for instance heat treatment," says Lorentzen.
"Formation of various ‘groups’ is demonstrated by the survival curve for Listeria no longer being linear. In practice this implies that products which previously have been salt-cured require a longer period of heat treatment to safeguard that any Listeria is killed compared with other products. These results show that it is important to consider the production history when making guidelines for heat treatment."
Listeria exposed to a lot of salt – still dangerous?
The disease listeriosis, which is caused by Listeria, is rare. Most people occasionally consume foods containing the bacterium without becoming ill.
In 2008 there were 1381 confirmed cases in EU countries. In USA around 2500 people develop listeriosis annually with a mortality rate of around 20 percent.
If the bacterium manages to penetrate the cell wall in a human’s digestive system, it can be extremely dangerous. Groups that are particularly vulnerable are the elderly, pregnant women, infants and people with weakened immune systems.
If the Listeria remains in the digestive tract, it will produce symptoms that can be confused with those of a light influenza.
There are no reported cases to date of people falling ill after eating salt-cured fish, clipfish or rehydrated products.
Lorentzen’s research points to the fact that Listeria which has been exposed to large quantities of salt has a weakened ability to penetrate through human cells than Listeria which has not been exposed to salt.
"The results indicate that when you expose the bacteria to large quantities of salt, it reduces their ability to pass the cell walls," says Lorentzen. "This can play a part in explaining why listeriosis has not been associated with such products. However, more research remains until these results can be used to assess real food safety."
Parts of the doctoral research were part of the EU project SEAFOODplus.
Grete Lorentzen has an MSc in Food Science specialising in quality control from the Norwegian College of Agriculture Engineering in Ås (now the Norwegian University of Life Sciences). Lorentzen was employed as a scientist at the then Fiskeriforskning in Tromsø in 1991, which in 2008 became part of the research group Nofima.