Cold plasma chemistry opens up new pathways for the synthesis of fundamentally new macromolecular surface structures through surface functionalization mechanisms. This might offer many advantages with respect to conventional chemical synthesis but can harbour as well unforeseen disadvantages. Our experiments have shown that under the action of active plasma species dehydrogenation and macromolecular backbone scission reactions of 1,4-benzopyrones are induced. This leads to oxidation and cleavage of the flavonoid skeleton. The decomposition clearly depends on the chemical structure of the investigated molecules. The origin of the observed reactions has to be clarified in more detail. We propose that the decomposition reaction probably starts with the generation of active substrate species (free radical sites, unsaturated bonds) and is followed by various inter-and intramolecular reaction processes of the substrates. As a result, mono- and polyphenolics are oxidized into volatile compounds which diffuse from the bulk to the surface and desorb. In addition, we suggest that during plasma treatment CO, CO2, H2, H2O, and other volatile low molecular weight compounds are formed from slow combustion.
These compounds and the non-volatile reaction products present on the remaining substrate surfaces unfortunately have not yet been identified. In case of the latter, chromatographical methods as used in conventional food chemical analysis have shown to be too insensitive to detect changes on the nanometer scale. The isolation and characterization of the volatile products so far failed due to rapid and extensive mixing with the surrounding air. The successful application of this novel reaction chemistry as a standard procedure not only in food industry though strongly depends on the basic understanding of plasma-surface reaction mechanisms and on the control of plasma species formation. To elucidate underlying reaction pathways, a identification and quantification of the different gaseous plasma species and the isolation and characterization of the volatile and non-volatile reaction products emanating from plasma-induced surface processes is mandatory.
While low-pressure plasmas are already well characterized with the current interest aiming towards a higher spatial and temporal spectroscopic resolution, the development of in-situ spatiotemporal diagnostic techniques for atmospheric pressure plasmas is still in its infancy.
This is particularly true for atmospheric pressure plasma jets, considered to be the ideal candidates for food industrial purposes. To this end,major focus should be put on the design
and development of spectroscopic and spectrometric tools (OES, LANGMUIR probes, etc.) for systems operating in an open environment. One very promising method to separate and identify trace chemicals in gaseous media, such as air, is ion mobility spectrometry, especially if combined with GC, LC, and/or MS. Even more powerful for the detection of volatile organic compounds (VOC) at ultra low concentrations is PTR-MS (Proton Transfer Reaction-Mass Spectrometry) reaching detection limits in the single-digit pptv-range. If turbulences are aggravating the spectroscopic detection, a modification of the experimental set-up is thinkable. By the use of a closed reaction chamber coupled with a supply and extract volume flow system, a constant and well-defined reaction atmosphere can be generated. In this case reaction products emanating from plasma-induced desorption processes could possibly as well be isolated and determined by less sensitive Headspace- or Thermal Desorption-GC/MS. Changes in the set-up are furthermore useful if ions, reactive oxygen species and photons can be separated to study their effect on the sample independently. The use of particle beams simulating the influence of electron, ion and neutrals bombardment will give deeper insight into the individual contributions of plasma-inherent species on the ablation or modification of surfaces. Under conditions where an experimental approach is limited, the use of first principles calculations may be helpful in addition to or as an alternative to experiment Molecular simulations of plasma elementary reactions may be gained from a combined density functional theory calculation coupled with transition state theory and ab initio kinetic Monte Carlo. The fact that most effects of the energetic impacting species are completed within picoseconds makes molecular dynamics particularly useful for studying plasma-surface interactions and rationalizing energy and angular distributions of sputtered particles. From a food technological point of view, effects of plasma induced changes on the plant postharvest physiology need to be studied. As demonstrated in this study, the impact of a cold plasma on plant based compounds and food ingredients is strongly influenced by matrix effects. An improved extractibility due to the disintegration of cellular membranes from rare gas ion sputtering or ROS induced oxidation is just as plausible to explain increased levels of specific molecules as the activation of specific biosynthetic enzymes as a plant response to wounding or harmful UV radiation.
Beyond sterilization, plasma treatment could thus enhance the bioavailability of a given flavonoid provided that the newly formed molecules are not lost along the food processing chain. Yet, most of the problems that probably arise when plant based food is treated with
plasma do not occur because the surface is oxidized in and polar and reactive functional groups are generated. The real danger seems to be an overtreatment: In a prolonged plasma exposure, epicuticular waxes are degraded and as already observed for several polymers form low molecular weight substances which can deteriorate the surface properties or can be washed off. This is particularly critical as the plant’s surface barrier usually hinders most microorganisms to penetrate and spoil the inner tissues. In addition, tissue respiration rates usually increase with the extent of tissue disruption (KADER, 1992). This is of great importance in harvested and stored plant products, as respiration reduces product quality and weight (catabolizm of e.g. carbohydrates, lipids, proteins to CO2) and increases storage costs (raising temperatures require ventilation or cooling systems) (KLOTZ, FINGER, AND ANDERSON, 2008). It is however as well conceivable that the ablation of the waxy layer offers distinctive advantages for further preservation steps. Attachment to the hydrophobic plant surface is usually believed to limit contact between chlorinated water and microbial contaminants (ADAMS,HARTLEY, AND COX, 1989). Changing the plants surface properties can thus impede microbial attachment and spoilage or improve conventional sterilisation procedures. Disintegration of cellular membranes from increased exposure to UV C radiation or rare gas ion sputtering should likewise be minimized, to avoid detrimental browning of tissues from PPO mediated polymerization reactions. In this context the investigation of enzyme activity and correlated genes by cell and molecular biology methods (Northern and/or Western Blotting, RT-PCR etc.) before and after plasma exposure of the plant matrix is another object of concern. Secondary metabolites produced and deposited by plant cells undergoing hypersensitive cell death can be visualized by UV stimulated autofluorescence.
Economic limitations include the use of rare gases as discharge working gases. These gases are very cost-intensive and future studies should aim at substituting rare gases by “cheaper”
molecular gases like O2 or N2 or gas mixtures like air. It is due to technological reasons that the APPJ we used in this study was almost exclusively limited to the use of rare gases and could not be applied to molecular gases. To this end, improvements in plasma source development have to be undertaken. We anticipate that these advances will help shed light on the complexities of low temperature plasmas altering bio-surfaces, aiming for a simple but controlled surface functionalization or food preservation. Whether the observed modifications are relevant to human health has to be elaborated by toxicological studies.