Food‑borne infections caused by Listeria monocytogenes are rare, but even low‑level contamination of meat or sausage products can lead to severe disease. This is because the bacterium continues to grow at cool storage temperatures and can survive milder heating steps. In addition to the established hygiene measures applied during meat‑product manufacturing, new technologies offer further opportunities to lower the occurrence of Listeria in the final products. These include non‑thermal high‑pressure treatment, which, however, may entail sensory losses, and bioconservation with bacteriophages. In a research project coordinated by the Research Association of the German Food Industry (FEI), a concept was developed at the MRI that combines both methods to reduce Listeria in raw and cooked sausages – while preserving product quality.
 

Opportunities and limitations of high pressure and bacteriophages
High‑pressure processing is a preservation technique, which inactivates microbial cells by applying very high pressure. At pressures of up to 600 megapascal, however, the sensory attributes of meat products, such as colour and texture, can be adversely affected.  

The use of bacteriophages (short: phages) does not have a negative impact on the food itself. Phages are viruses that infect only bacteria and kill them selectively. When applied to processed foods, the size of the product’s surface area becomes a limiting factor: not every pathogenic bacterial cell is reached during phage treatment. However, by combining Listeria monocytogenes‑specific phages with a mild high‑pressure processing, an effective protection against Listeria could be achieved.
 

Combined procedure increases microbiological safety
Experiments carried out within the project showed that a moderate pressure level (circa 300 megapascal) followed by a phage treatment yields a synergistic effect. The pressure markedly reduces the Listeria population, but does not eliminate it completely. The subsequently applied phages eradicate the remaining bacteria – even those that have developed increased pressure tolerance.  

Because of this synergy, the required pressure can be reduced compared with using high‑pressure processing alone. Consequently, the sensory quality of cooked sausages (e.g., Lyoner sausage) is largely retained. Slight quality losses were observed, however, in more strongly fermented products such as salami.  

Long‑term challenge tests under realistic production conditions demonstrated that the combined treatment controls the growth of Listeria monocytogenes throughout the entire low-temperature storage period. The combined moderate‑pressure and phage approach therefore not only enhances the microbiological safety of foods but also has economic relevance for meat‑processing plants, for example by lowering the risk of product recalls.
 

Funding
The IGF project 01IF22678N of the Research Association of the German Food Industry (FEI) was supported within the programme for promoting the Industrial Collective Research (IGF) of the German Federal Ministry for Economic Affairs and Energy on the basis of a decision by the German Bundestag.