Antimicrobial efficiency of non-thermal atmospheric pressure plasma processed water (PPW) against agricultural relevant bacteria suspensions

The consumption of about 400 g up to 800 g per day of fresh fruits and vegetables is recommended by many organizations like the World Health organization (WHO) [1], the World Cancer Research Fund (WCRF) and the American Institute for Cancer Research (AICR) [2] as well as the German Nutrition Society e. V. (DGE). 

Fruits and vegetables are the supplier of vitamins and minerals, of dietary fiber and phytochemicals with a low energy density [3]. 

Investigations lead to the conclusion that the more fruits and vegetables eaten, the lower the risk is not only for certain types of cancer, but also for obesity, hypertension and coronary heart disease [4-10].

However, fresh and fresh-cut produce have a limited shelf life of several days, which allows only a regional distribution of that produce. The limited shelf life and the associated losses of fresh produce have various causes, but especially depend on microbial contamination at all stages in the value chain. The microbial contamination may also cause foodborne illnesses, which occur annually and worldwide. The U.S. Food and Drug Administration (FDA) listed under the ten riskiest foods in their Center for Science in the Public Interest (CSPI) Report 2009 5 times fruits and vegetables. Whereby leafy greens are on the top [11]. European institutions and customer organizations like the German Institute for Risk Assessment (BfR) are aware of the risk of food borne illnesses caused by fresh fruits and vegetables, too [12, 13]. 

The European Food Safety Authority (EFSA) described in their zoonoses report of 2011 [14] 5648 reported food-borne outbreaks for 2011 with more than 200,000 confirmed human cases. The outbreaks were caused by Bacillus toxins, Campylobacter, Clostridium, E. coli (mainly Verotoxin-producing Escherichia coli (VTEC)), Listeria, Yersinia and some others. 

Initial microbiological load of fresh vegetables ranges between 102 and 107 cfu  g-1, whereas most germs are harmless Gramnegative bacteria like Pseudomonas spp. and Pectobacterium spp. For food safety relevant foodborne pathogens are in particular bacteria like Enterobacteriaceae (Escherichia spp., Samonella spp.) and Listeria spp. [15, 16]. Due to the low infection dose of E. coli, the guidance level for it can be reduced to 100 cfu  g-1 [17, 18]. 

In case of fresh fruits and vegetables, preservation methods such as heat treatment and freezing are not applicable because of the resulting loss of freshness properties. Conventional methods of decontamination and cleaning of fresh food are based on rinsing with water which may contain high amounts of chemicals, e.g. chlorine (50-200 ppm), chlorine dioxide or ozone. Although the poor stability of chlorine and the association of chlorine with a possible formation of carcinogenic chlorinated compounds in water have called the use of chlorine in food processing applications into question [19, 20]. Water containing disinfectant eliminates 3 to 4 log of microorganisms in solution and prevents them from attaching to the product surface. However, once bacteria are attached or internalized, no effective method exists to remove or destroy the contamination [21, 22]. 

Therefore the development of environmentally friendly alternative disinfection and cleaning methods is important, but also the product compatibility, costs, environmental impact, impact on product quality and regulatory provisions have to be taken into account [23].

 Alternative methods for both effective and safe disinfection of fresh food, especially fruits and vegetables, are needed to guarantee safe consumption of high-quality products. 

One possible alternative method could be the application of non-thermal atmospheric pressure plasma.

 Plasma is generated by supply of energy to a gas leading to an excitation as well as ionization of gas atoms or molecules, giving the opportunity to a direct absorption of electrical power. A wide range of different plasma types are known. One type is the non-thermal atmospheric pressure plasma [24, 25]. Plasma is always a cocktail of a variety of species including excited and reactive atoms, molecules, ions and radicals, but also radiation (VUV, UV) [26-28]. Plasma is currently used in various industrial fields such as electrical engineering, textile and packaging industry, optics, automotive industry, printing as well as environmental technology, and much more [25, 29, 30]. 

The application of non-thermal atmospheric pressure plasma is a discipline with increasing attention in the field of food processing and an emerging non-thermal technology for reducing microbial load on the surface of fresh and processed foods [31]. Thus the potential applications of non-thermal atmospheric pressure plasma for the food industry are manifold and it has specific potential for the treatment of foods. 

Recent reports include special applications like modification of seed germination or active packaging of fruits [32, 33], but also plasma applications for decontamination of different food products, in most cases with the objective of further shelf-life or storage-time extension [34-39]. 

However, independent of the application a humid or wet environment is given by microbial suspensions, biofilms, and cell tissue, fresh or liquid food. Therefore, the presence of a gas-liquid environment and a gas-liquid interaction is always given.

 The aim of the presented work was to investigate the antibacterial efficacy of plasma processed water (PPW) against foodrelated microorganisms in suspension. Investigations of buffering effects by phosphate buffer and nutrient broth should give an insight for the capability to use PPW in food washing plants. To exclude the pH value as single responsible antimicrobial component, HNO solution was examined separately.