Fresh produce decontamination by an atmospheric pressure plasma-jet

Abstract

The consumption of fresh produce is commonly considered to be an important part of a wholesome and balanced nutrition. Despite the application of good hygienic practice and good manufacturing practice during the post-harvest period, occurrences of high initial loads of pathogenic bacteria increase the risk of outbreaks of foodborne diseases. The application of conventional thermal processes is well developed and reliable. However, it is limited by its detrimental effects on the quality of fresh fruit and vegetables. This, in turn, compels the introduction of new highly efficient sanitation methods such as non-thermal atmospheric pressure plasma (NTP). The objective of this study was to evaluate the effects of NTP on vegetative bacteria (Escherichia coli) as well as on the physiological properties of fresh produce (lamb’s lettuce). Applying argon as working gas the plasma was generated between the electrodes of a miniaturized rf-driven atmospheric pressure plasma-jet. The microbiological tests were conducted using a polysaccharide gel as model matrix and enumeration by a conventional plate count method. Treatment time, generator voltage, gas flow rate, and gas composition were varied to improve the antimicrobial efficacy. The potential impact of plasma on leaves of the heat sensitive lamb’s lettuce was assessed with the help of chlorophyll fluorescence image analysis (CFIA). With the non-destructive CFIA, the response of the samples to the plasma treatment was measured immediately after plasma treatment and after subsequent storage. Infrared temperature measurements were conducted to exclude potential adverse thermal effects and to control the effects of plasma on the surface of the samples. Escherichia coli on model system were inactivated by 3 to 5 log CFU/cm2 within 2 min. The CFIA showed that lamb ́s lettuce could be treated with the process parameters needed for successful inactivation of E. coli with minor effects on product quality. Relevant treatment parameters to inactivate microorganisms without affecting the fresh food properties were derived. Non-thermal plasma; fresh produce; antibacterial treatment; INTRODUCTION The antimicrobial efficacy of plasma has been known for almost 50 years. Research mainly focussed on application on abiotic surfaces like medical devices. Furthermore, the bulk has been directed in the new emerging plasma medicine (reviewed by Fridman et al. (2008)). Over the last two decades interest and intensity of research on potential applications of plasma have raised in the field of food engineering. Especially developments of new plasma sources which allow generation of non-thermal plasma at room temperature appear suitable to treat heat-sensitive food surfaces. Over the last decade first attempts to treat foods were made. The antibacterial efficiency of a needle-to-plate-system was tested on apple juice, where a pulsed plasma was ignited directly in the juice resulting in a 7 log cycles reduction of Escherichia coli O157:H7 [1]. The effects of a helium-oxygen plasma, generated between two plate-electrodes, were examined on biofilm-forming bacterium Pantoea agglomerans and on colour alterations of bell pepper. Corresponding to a 2 log cycles inactivation, bell pepper could be treated without significant effects of discoloration [2]. Escherichia coli on almonds was reduced by more than 4 log cycles applying a dielectric barrier discharge plasma (DBD). Using a so-called one atmosphere uniform glow discharge plasma, a special kind of DBD, samples were treated in a chamber, remote to the plasma generation zone. Strains of E. coli O157:H7, Salmonella and Listeria monocytogenes were inoculated on apples, cantaloupe, and lettuce, respectively. All strains were reduced by at least 2 log units [4]. Using a low pressure plasma [5], in an antifungal treatment of nut surfaces Aspergillus parasiticus was reduces by up to 5 log units and concentrations of fungal aflatoxins could be reduced by 50 %. Further experiments on different seeds inoculated with Aspergillus spp. and Penicillium spp. resulted in a 3 log units reduction [6]. A gliding arc plasma inactivated E. coli O157:H7 and Salmonella Stanley on the surface of apples by 3 log units [7]. Klockow and Keener [8] conducted an in-package treatment of spinach leaves inoculated with E. coli O157:H7. The efficacy of a plasma-jet on a pathogenic strain and different spoilage microorganisms was examined on the pericarps of mango and melon and resulted in 3 log cycles inactivation [9]. Further investigations on cut fruit surfaces showed a resistance of microorganisms due to migration into the fruit tissue [10]. As examples of complex food systems sliced cheese and ham were treated using a type of plasma-jet working in helium. Three strains of Listeria monocytogenes were inactivated by 5.8 log units in sliced cheese whereas on ham a 1.7 log cycle reduction could be realised. These experiments indicate a strong effect of the matrix on the obtained inactivation results [11]. As a special case of a biological inorganic sample surface the afterglow of a resistive barrier discharge was applied on shell eggs in an after glow chamber. 2.5 to 4.5 log cycles reduction of Salmonella enteritidis and Salmonella typhimurium were achieved after 90 min treatment time dependent on the gas moisture [12]. Aside from sufficient antimicrobial efficacy the quality of the plasma-treated foods should be assessed. In spite of categorization as non-thermal plasma, depending on generator power amounts of residual heat can cause harm. Given that artificial nonthermal plasmas are partially ionized gases, the amount of charged particles may influence the membranes ́ potential of the biological samples as well. Therefore, the aim of this study was an alignment of antibacterial efficiency and quality assurance. Different process parameters were varied to gain enhanced inactivation. Successful parameter combinations were applied on Lambs lettuce and their suitability was monitored using chlorophyll fluorescence image analysis.

5 Figures and Tables

Cite this paper

@inproceedings{Geyer2011FreshPD, title={Fresh produce decontamination by an atmospheric pressure plasma-jet}, author={Adi Geyer and b Ehlbeck and Christin Knorr}, year={2011} }