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Plasma potential for tackling strawberry spoilage

By Joe Whitworth+

22-Jan-2014
Last updated on 22-Jan-2014 at 12:53 GMT

Atmospheric pressure cold plasma (ACP) can reduce microorganisms on strawberries without major impacts on colour and firmness, according to research.

Scientists evaluated ACP for the microbial decontamination of the fruit inside a closed package.

In-package decontamination of fresh foods is desirable as this minimises the possibility of post-processing contamination.

Strawberries were treated with ACP, generated with a 60 kV dielectric barrier discharge (DBD) pulsed at 50 Hertz, across a 40mm electrode gap.

The process involves applying a high voltage and using the packaging as a dielectic barrier, something that prevents electrical current flow but allows movement of voltage.

ACP effect

Background microflora (aerobic mesophillic bacteria, yeast and mould) of strawberries treated for five minutes was reduced by 2 log10 within 24 hours of post-ACP treatment.

Respiration rate of ACP treated produce, measured by the closed system approach, showed no significant increase and it had little effect on strawberry colour and texture.

The study involves use of a dielectric barrier discharge (DBD) to generate cold plasma from humid atmospheric air inside a package.

The DBD system achieved the desired effects with a power input of 15–20 W, without much increase in the temperature of the samples.

Competing technologies

Chlorine-based washing for decontamination is used by fresh produce processors but is prohibited in Germany, The Netherlands, Switzerland and Belgium.

Nonthermal technologies such as high pressure processing (HPP) and pulsed electric field (PEF) provide good results but the equipment and set-up for HPP is capital intensive (Hugas et al., 2002), while PEF is only suitable for liquid foods.

Nonthermal approaches for decontaminating whole fruits and vegetables include pulsed light processing, ionising radiation, ultrasound or ozone assisted washing and other chemical or packaging techniques.

However, challenges vary from the shadowing effect in UV light processing, consumer acceptance and facility set-up for ionising radiation and the lack of industrial scale processing units for ultrasound processing.

Microorganism count

The mesophilic and yeasts/mould counts for untreated samples were 4.99 and 4.96 log10 cfu/g. Storage at 10 °C for 24 hours of untreated control samples had no effect on the reduction of viable population on strawberry surfaces.

In-package, indirect ACP treatment of strawberries for five minutes achieved reductions of 2.4 and 3.3 log cycles of total mesophiles and surface yeasts/moulds.

A change in the colour parameters of ACP treated strawberries was seen (lightness, redness or greenness) but it was insignificant (p > 0.05) compared to the untreated control stored under same conditions. The same conclusion was reached for firmness.

Future studies will focus on inactivation of bacteria inoculated on the surface of fresh produce and shelf life studies will be done to assess long term effects on food quality.

Source: Journal of Food Engineering Volume 125, March 2014, Pages 131–138

Online ahead of print, DOI: 10.1016/j.jfoodeng.2013.10.023

“In-package atmospheric pressure cold plasma treatment of strawberries”

Authors: N.N. Misra, Sonal Patil, Tamara Moiseev, Paula Bourke, J.P. Mosnier, K.M. Keener, P.J. Cullen

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