Researchers from the IFR, which receives strategic funding from BBSRC, have recently reviewed the use of cold atmospheric plasmas to inactivate Salmonella to assess their use by fresh or minimally processed food producers.

A growing demand for fresh produce is presenting a challenge to the food industry to supply safe food with minimal processing. It is crucial that these foods are supplied without microbial contamination as many of them are eaten raw, and so there has been much interest in novel ways of preserving food and destroying microorganisms without affecting quality.

One such emerging technology that has shown promise is the use of cold atmospheric plasma (CAP) treatment, and a recent review by IFR microbiologists has summarised what is known about how Salmonella is inactivated by CAP and suggests what further research is needed before it can be integrated into the food supply chain. For example, IFR projects are now widening our understanding of how Salmonella can resist these treatments.

Plasmas are created when gases are excited by externally applied energy sources, and they consist of a variety of highly energetic particles, which in combination are able to inactivate microorganisms. Exactly how this is done is not fully understood. IFR scientists Dr Arthur Thompson, a research leader working on Salmonella and Dr Ana Fernández have published a literature review in the journal, Food Research International on the inactivation of Salmonella by CAP treatment.

Overall, they concluded that CAP treatment can be highly efficient at inactivating Salmonella, killing cells in a very short period of time. The published literature does not yet clearly identify what factors are behind this inactivation, in part due to the different experimental conditions and diverse experimental conditions used to study CAP inactivation. Inactivation rates also vary greatly between different experiments. A greater understanding of the active antimicrobial compounds in the plasmas is needed to understand how they inactivate Salmonella. It is important to be certain that no harmful by-products are generated, and that the process doesn't adversely affect quality and shelf-life of the product.

In addition, more needs to be known about how Salmonella may be able to resist CAP treatment. Different types of Salmonella show highly variable levels of inactivation, and work at IFR is researching the strategies that the bacteria deploy to survive CAP treatment by getting accurate data on the effects of the treatment when Salmonella is exposed on both plant tissue or surface and packaging.

Research recently published in the International Journal of Food Microbiology investigated how the efficiency of CAP treatment is affected by the concentration of Salmonella. Generally, microbial inactivation drops as the concentration increases, but this was the first study that looked at CAP treatment and Salmonella.

The group from IFR found that the efficiency of Salmonella inactivation by the CAP process did decrease as expected. This was not simply due to increased amounts of biomass however. Adding previously killed cells to increase biomass decreases 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 were not evenly distributed, but clumped and that the multiple layers formed from this could be providing a physical barrier protecting against the CAP treatment.

This characterisation of the link between inactivation efficiency and Salmonella concentration will be crucial to the practical application of the technique in the food industry, and will also help inform further studies needed to fully understand the process of inactivation and resistance.

Funded by the European Union, Dr Fernández is looking at the genes that are activated when Salmonella survives the treatment. By looking at the changes in gene expression, it will be possible to identify which cellular processes are employed by Salmonella. Knowledge of these processes can then be used to optimise the CAP treatment technique to prevent the resuscitation, so refining the technique and helping to ensure that food is free of microbial contamination.