A nonthermal plasma is in general any plasma which is not in thermodynamic equilibrium, either because the ion temperature is different from the electron temperature, or because the velocity distribution of one of the species does not follow a Maxwell–Boltzmann distribution.
In the context of food processing, a nonthermal plasma (NTP) or cold plasma is specifically an antimicrobial treatment being investigated for application to fruits, vegetables and other foods with fragile surfaces. These foods are either not adequately sanitized or are otherwise unsuitable for treatment with chemicals, heat or other conventional food processing tools. While the applications of nonthermal plasma were initially focused on microbiological disinfection, newer applications such as enzyme inactivation, protein modification and pesticide dissipation are being actively researched. Nonthermal plasma also sees increasing use in the sterilization of teeth and hands, in hand dryers as well as in self-decontaminating filters.
The term cold plasma has been recently used as a convenient descriptor to distinguish the one-atmosphere, near room temperature plasma discharges from other plasmas, operating at hundreds or thousands of degrees above ambient (see Plasma (physics) § Temperatures). Within the context of food processing the term "cold" can potentially engender misleading images of refrigeration requirements as a part of the plasma treatment. However, in practice this confusion has not been an issue. Cold plasma may also refer to the barely ionized (<0.1%) plasmas in general.
The nomenclature for nonthermal plasma found in the scientific literature is varied. In some cases, the plasma is referred to by the specific technology used to generate it ("gliding arc", "plasma pencil", "plasma needle", "plasma jet", "dielectric barrier discharge", "Piezoelectric direct discharge plasma", etc.), while other names are more generally descriptive, based on the characteristics of the plasma generated ("one atmosphere uniform glow discharge plasma", "atmospheric plasma", "ambient pressure nonthermal discharges", "non-equilibrium atmospheric pressure plasmas", etc.). The two features which distinguish NTP from other mature, industrially applied plasma technologies, is that they are 1) nonthermal and 2) operate at or near atmospheric pressure. An upcoming technology will add the capabilities of nonthermal plasma to dentistry and to medicine, a field known as plasma medicine.
|NTP technology class|
|I. Remote treatment||II. Direct treatment||III. Electrode contact|
|Nature of NTP applied||Decaying plasma (afterglow) - longer lived chemical species||Active plasma - short and long-lived species||Active plasma - all chemical species, including shortest lived and ion bombardment|
|NTP density and energy||Moderate density - target remote from electrodes. However, a larger volume of NTP can be generated using multiple electrodes||Higher density - target in the direct path of a flow of active NTP||Highest density - target within NTP generation field|
|Spacing of target from NTP-generating electrode||Approx. 5 – 20 cm; arcing (filamentous discharge) unlikely to contact target at any power setting||Approx. 1 – 5 cm; arcing can occur at higher power settings, can contact target||Approx. ≤ 1 cm; arcing can occur between electrodes and target at higher power settings|
|Electrical conduction through target||No||Not under normal operation, but possible during arcing||Yes, if target is used as an electrode OR if target between mounted electrodes is electrically conductive|
|Suitability for irregular surfaces||High - remote nature of NTP generation means maximum flexibility of application of NTP afterglow stream||Moderately high - NTP is conveyed to target in a directional manner, requiring either rotation of target or multiple NTP emitters||Moderately low - close spacing is required to maintain NTP uniformity. However, electrodes can be shaped to fit a defined, consistent surface.|
|Examples of technologies||Remote exposure reactor, plasma pencil||Gliding arc; plasma needle; microwave-induced plasma tube||Parallel plate reactor; needle-plate reactor; resistive barrier discharge; dielectric barrier discharge|
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