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Piercing Salmonella shield could unlock full potential of cold atmospheric plasma

By Rory Harrington , 03-Jun-2011
Last updated on 07-Jun-2011 at 13:05 GMT

Use of cold atmospheric plasmas (CAP) to tackle Salmonella in fresh or minimally-processed foods shows great promise - but key questions remain to be answered before the technology can be integrated into the supply chain, said scientists.

In a recent review of CAP, researchers from the Institute of Food Research (IFR) hailed its potential in meeting the consumer demand for fresh food that is free from microbials but also minimally processed.

But scientists still don’t fully understand how the treatment works and found that the uneven distribution of Salmonella cells can form a shield-like structure that can be CAP resistant, said experts from the Institute.

Plasma power

Drs Arthur Thompson and Ana Fernandez explained that plasmas are created when gases are excited by externally applied energy sources. They consist of a variety of highly energetic particles, which when joined together are able to inactivate microorganisms.

Their overall conclusion was that CAP treatment could be a highly efficient way of combating Salmonella, eliminating cells very quickly

But, as the pair said in their review in the journal Food Research International, the reasons for this bacterial inactivation are not yet fully understood. Previously published research has yet to clearly identify this because of the diverse experimental conditions used to evaluate CAP inactivation.

A greater understanding of the antimicrobial compounds in plasma is needed to fully grasp how they combat Salmonella. The IFR scientists cautioned that this understanding is key to ensuring no harmful by-products are generated, and that the process doesn’t adversely affect quality and shelf-life of the product.

Bacterial shield

More knowledge is needed on how Salmonella resists CAP and why different strains of the bacteria demonstrate “highly variable rates of inactivation”.

The IFR said it has launched a research project to explore strategies employed by Salmonella cells to survive CAP treatment by gathering data on the effects of the treatment when Salmonella is exposed on packaging or plant surfaces.

Recent research has also highlighted how Salmonella concentrations can curb the efficiency of CAP treatment – with microbial inactivation dropping as the concentration increases.

The researchers found that the efficiency did decrease as they expected but that increased amounts of biomass were not the only factor in this. They said that adding previously killed cells to increase biomass decreased the inactivation efficiency of CAP, but not in a dose-dependent manner.

This indicates that live cells are important for resistance to CAP treatment. Microscopy revealed at higher concentrations the Salmonella cells weren’t evenly distributed, but ‘clumped’ and that the multiple layers formed from this could be providing a physical barrier protecting against the CAP treatment”, said the IFR.

Exploring the link between CAP efficiency and bacterial concentration will be key to enabling further practical application in the food industry, said the scientists.

Dr Fernandez is also examining genes that are activated when Salmonella cells survive CAP treatment – with a view to identifying the mechanism by which they achieve this and boosting efficiency of the process to development a treatment that ensure food free of microbial contamination.

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