Aspergillus flavus and Aspergillus parasiticus are the predominant species responsible for fungal contamination and subsequent production of aflatoxins in nuts in the field, at harvest and during postharvest operations and storage. Being a high contamination risk of these aflatoxigenic fungi, the decontamination methods for nuts are of great interest for economic and environmental reasons, as well as for public health.
A current potential alternative for inactivation of microorganisms is plasma treatment. It is well known that atmospheric pressure plasma is a source of reactive oxygen species (ROS) and reactive nitrogen species (RNS), which are considered to be the most important inactivation agents in plasma because they induce oxidative stress causing cell damage and cell death.
Turkish scientists have designed an innovative atmospheric pressure fluidized bed plasma (APFBP) system and have tested its inactivation effect on spores of A. flavus and A. parasiticus on the surface of hazelnuts. For the study, APFBP was applied at two different frequencies (25 kHz and 20 kHz) and varying reference voltages (100% V, 90% V, 80% V) for 1 to 5 min by using dry air as the precursor.
The hazelnuts were artificially contaminated with A. flavus and A. parasiticus and then were treated with dry air plasma for up to 5 min in the APFBP reactor at various plasma parameters. Scientists investigated also the effect of storage on the survival counts of aflatoxigenic fungi following plasma treatment after 30 days of storage at 25°C.
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Results showed significant reductions of 4.50 log (cfu/g) in A. flavus and 4.19 log (cfu/g) in A. parasiticus after 5 min of treatments at 100% V —25 kHz (655 W) by using dry air as the plasma forming gas. The decontamination effect of APFBP on Aspergillus spp. spores inoculated on hazelnuts increased with the applied reference voltage and the frequency.
The damage caused by the plasma process on spores was not temporary, since the viable spore count did not increase but decreased after 30 days following the APFBP decontamination process, while spores on control samples continued to grow at the same conditions (30 days at 25 °C).
Temperature change on hazelnut surfaces in the range between 35 and 90°C was monitored with a thermal camera, it was observed that the temperature increase taking place during plasma treatment did not have a lethal effect on spores, this proves that the reduction on spores achieved with plasma decontamination process was due to the detrimental effect of plasma active species, rather than the temperature change occurring during the treatment. The spores inactivation was a consequence of effects caused by the plasma decontamination process performed in the APFBP system. The damage caused by APFBP treatment on spores was also observed by scanning electron microscopy (SEM).
Scientists conclude: "APFBP reactor could be used for decontamination of aflatoxigenic fungi on hazelnuts with significant reductions. Thus, this process has the potential to be used as an alternative practical method for nut decontamination in the industry to enhance the microbiological safety during postharvest operations and storage. Parameter optimization of plasma processing of foodstuffs in atmospheric plasma conditions, performance tests against other types of spores and contaminants, and further scale up studies for pilot and industrial scales are still under investigation by our research group and many others."
The full study is available online (since September 2015): http://www.sciencedirect.com/science/article/pii/S0168160515301197
Source: Beyhan Gunaydin Dasan, Mehmet Mutlu, Ismail Hakki Boyaci, 'Decontamination of Aspergillus flavus and Aspergillus parasiticus spores on hazelnuts via atmospheric pressure fluidized bed plasma reactor', International Journal of Food Microbiology, January 2016, Vol. 216, pages 50–59.
Contacts:
Mehmet Mutlu
Department of Biomedical Engineering
Faculty of Engineering
TOBB University of Economics and Technology
Sogutozu, 06560 Ankara, Turkey
Email: m.mutlu@etu.edu.tr