The hunt for lower high pressure
High pressure processing is a gentle and not least healthy way of increasing the shelf-life of food that has proven very well suited for meat products and juices. However, to be able to use this method on other foods, such as fish, we need to use high pressure at lower levels – without the bacteria surviving.
Food items preserved using high pressure processing look like fresh items, but have a much longer shelf life, as a result of the processing. For example, raspberries are still fresh and good 90 days after high pressure processing. This method can also be used to tenderise meat, de-shell shellfish (lobsters, for example) and thaw frozen goods. It is healthy because it allows reduced use of additives such as salt. Considerations such as taste, texture, quality and health all suggest that high pressure processing is a preferred processing technique to extend the shelf-life of foods.
“We would like to be able to use high pressure processing on fish products, such as smoked salmon, for example. However, in general fish will appear cooked and change colour when exposed to very high pressure,” explains Nofima scientist Trond Løvdal.
Experiments are therefore now being done through the SafeFood research project, led by the Norwegian University of Science and Technology (NTNU), to find out if it is possible to lower the pressure used during processing to below 3,000 bars and still achieve safe products with a long shelf-life.
“We know that smoked salmon only starts losing its colour and looking like it has been cooked when we exceed 3,000 bars, and even processing at low pressure will extend the product’s shelf-life. However, pressure this low is in itself not enough to ensure the food safety of the product. Listeria bacteria will only be temporarily inactivated and will be able to reactivate. For this reason, we need to customise the process so we are sure we kill all the bacteria completely,” he explains.
Listeria bacteria pose a major challenge because they cause a lot of waste and problems for the meat and fish industry. It is an extremely robust bacterium. It can withstand a lot of heat, it tolerates salt, and it manages to grow in both the presence and absence of oxygen.
“We therefore need a super-component in high pressure processing against listeria. In our pursuit of this super-component, we are using methods from molecular biology. Our partners in the project are looking at gene expression in the bacterium Listeria monocytogenes, and what effect high pressure has on this gene expression. Time, temperature, and other factors also play a role. Once we have mapped the bacterium’s gene expression, we will use fairly sophisticated models to calculate an appropriate design for high pressure processing. The goal is to find out which parts of the bacteria cell are affected by processing. Is it the cell membrane, cell wall or other biochemical or metabolic processes in the bacterium that cause it to either die or no longer manage to grow as a result of the processing? Once we have established what mechanism in the bacterial cell is affected by high pressure, we can start working on finding the right process to kill it,” Løvdal explains.
High pressure processing at lower levels will also be suitable to protect products such as cured meats against listeria.
Another reason that the food industry is keen to use lower high pressure is that pressure of 4,000–6,000 bars entails tremendous wear on the equipment. If slightly lower pressure can be used during processing, the machinery will also last longer.
So far the researchers have identified two strains of listeria that are very resistant to pressure. These super-resistant bacteria have been used in the study precisely because they can tolerate so much.
“If we can find a method that beats these strains, we will have come a long way. We will soon be at a stage where we can test out different components to get rid of listeria,” says the Nofima scientist.
What is unique in this project is that the scientists are using sophisticated molecular biological processes, in addition to this modelling, instead of a broad-spectrum empirical approach, based on trial and error until they hit upon a good method. In this project, the scientists are focusing on the underlying mechanisms that lead to inactivation and death of the bacteria. In this way, it will be possible to wage a targeted and highly efficient war on listeria.
The project is international and consists of eight research teams from six different countries in Europe: Norway, Finland, Romania, Germany, Spain and Portugal.
Once the best method for beating listeria with high pressure has been developed, it will be patented in the EU’s SafeFood programme, which is scheduled to last for a total of three years. SafeFood aims to provide safer, healthier food for consumers, better profitability and export value for the food industry, and last but not least, ensure less food is thrown away.
Read more about the project at safe-food.eu.