Two recent research projects focusing on Listeria have helped boost understanding of the pathogen’s specific chemical and structural properties and improve food safety from farm to fork.

The first, led by the University of Surrey’s BioProChem group, revealed evidence about the way Listeria grows in foods.

The other saw a team from the University of Natural Resources and Life Sciences, Vienna (BOKU) identify conditions for inactivating Listeria while maintaining product quality.

Both projects used Listeria innocua – a strain that is harmless to humans.

Research led by the BioProChem group at the University of Surrey was published in the International Journal of Food Microbiology. The work was in collaboration with Surrey’s School of Biosciences and Medicine and KU Leuven in Belgium.

Scientists remodeled food products to examine how bacteria develop antimicrobial resistance in a highly controlled chemical and structural environment – and how Listeria responds to nisin, a natural antimicrobial produced by lactic acid bacteria in dairy products, and to heat in such a controlled environment.

They found that in food models containing both proteins and polysaccharides, a type of carbohydrate, the bacteria grow exclusively on the protein, suggesting Listeria has a mechanism that decides on wher growth will take place.

When cells are exposed to environments that are mildly stressful – such as a gradual increase in temperature – they develop resistance to that factor and others, via a mechanism called “cross protection.”

Conventional sterilization techniques are being abandoned because consumers demand foods that undergo minimal processing and have high nutritional value and sensory characteristics, which are lost during sterilization. Food manufacturers are looking at alternatives such as processing with natural antimicrobial compounds, ultrasonic treatment, hydrostatic pressure or milder heat treatment.

Dr. Eirini Velliou, who has led the project at Surrey, said: “Our research could enable manufacturers to tailor products and processing techniques more effectively. The long term aim is to ensure safety from ‘farm to fork’ and to find out how bacteria can develop resistance throughout the food chain as a result of a food’s specific chemical and structural properties.”

BOKU study
Meanwhile, the team from BOKU found low temperatures and an acidic environment created ideal conditions for applying an effective method of inactivating Listeria and other germs in the processing of whey proteins – without destroying valuable nutrients.

Harmful bacteria are inactivated by electroporation, a process that does not require drastic temperature increases, in contrast to conventional thermal preservation methods.

Pulsed electric fields (PEF) destroy bacterial cell membranes and has been used with fruit juices and other fluid foodstuffs.

BOKU researchers worked with an unnamed global food company on equipment supplied by partner organization EQ BOKU to identify conditions under which Listeria in more viscous whey-protein solutions can be eliminated by PEF, while preserving valuable nutrients.

Professor Henry Jäger, deputy head of the BOKU’s Institute of Food Technology, said temperature plays a decisive part in the process.

“Temperatures of 20 degrees Celsius (68 F) or so and an acidic environment with a pH value of about 4 are ideal conditions for electroporating bacteria in protein solutions like the whey protein we tested,” he said.

“The energy produced by the PEF causes the medium to warm quickly, which damages many valuable proteins. This effect increases in line with the strength and duration of the electric field, which in turn kills off the bacteria more efficiently. So it’s all about finding the right balance.”

The study, published in the Journal of Food Engineering, compared two different concentrations of whey proteins — 2 percent and 10 percent. A temperature of between 20 degrees Celsius (68 F) and 40 degrees Celsius (104 F), and a pH value of 4 or 7, was selected.