Cold plasma shows potential for Listeria reduction in packaged RTE foods

Cold atmospheric pressure plasma (CAPP) treatment shows potential as a way of reducing Listeria monocytogenes contamination on the surface of pre-packaged ready-to-eat (RTE) foods, according to new research.

Scientists from Denmark said that Listeria contamination was on the increase in many European countries particularly in meat and seafood products. They noted that surface contamination of RTE meats may occur during processing as Listeria ‘can persist in the processing environment and be transferred to products from equipment such as slicers’.

The Listeria contamination of RTE meats from a plant in Canada in 2008 which killed 23 people is the most high-profile example of this in recent years.

In their study, Susanne Knochel et al examined the ability of CAPP to combat this through surface decontamination of the product.

The research, published in the journal Food Microbiology, defined plasma as ‘an ionised quasi-neutral gas that can be generated by applying an electric field to an initially electrically neural gas’. It is a source of different antimicrobial substances including ultra violet (UV) photons, charged particles and reactive species such as superoxide and hydroxyl radicals. Previous research has established its capacity to inactivate a raft of organisms including gram-positive and gram-negative bacteria, endospores and viruses on different surfaces, said the Danish team.

Methodology and results

The team assessed the decontamination effect of indirectly applied CAPP – where the sample is palced at some distance from the plasma and only exposed to reactive species formed – on sliced bresola inoculated with Listeria innocua in sealed polyethylene bags. They also observed changes in oxidation and colour. Inoculated samples of the product were treated at 15.5, 31 and 62 W for between two and 60 seconds inside sealed linear low density polyethylene bags containing 30% oxygen and 70% argon.

The treatments resulted in a reduction of L.innocua of between 0.8-0.4 to 1.6-0.5 log cfu/g with no significant effects of time and intensity.

Multiple treatments at 15.5and 62 W for 20 seconds at 10 minute intervals increased reductions further. The higher the number of treatments the greater the reductions, said the study.

The scientists hailed the potential reductions as possibly ‘significant’ based on research done by the European Food Safety Authority (EFSA) on the prevalence of Listeria monocytogenes on the surface of RTE meats.

The research said that TBARS - Thiobarbituric Acid Reactive Substances – values of all plasma samples were low but ‘significantly higher’ than control samples. TBARS are formed as a byproduct of lipid peroxidation (i.e. as degradation products of fats). They exceeded control sample levels with power, treatments and storage time at 5C. But these levels were said to be below the sensory threshold levels. There were significant changes in surface colours of between 40% and 70%. But the reseachers said this due to the high oxygen content inside the packaging rather than the CAPP treatment.

Furter investigation is needed to explore potential for other pre-packaged products. This would include exploring the interplay between oxygen levels and products characteristics to identify the most promising areas of use.

Source: Food Microbiology