In order to determine the effective radiation dosage for eliminating 50% (ED50) CFUs, a response curve with R 2 > 0.95 was produced for each treatment. CFU levels in the controls of each treatment were calculated as the 100%. This value divided by two was used as the Y value in the response curve formula of each treatment, and served as the radiation dosage required for reducing CFU levels by 50% (ED50). All other ED values were calculated using the same method.
While gamma and e-beam irradiation possess a similar mode of action, cold plasma treatment is a different method for sanitation and sterilization. The general definition of plasma is a state of ionized gas, with limited net charge. Natural examples of plasma are the sun and the aurora (Misra et al. 2019; Turner 2016). Cold plasma is usually achieved by deploying electrical discharges in gases at atmospheric or subatmospheric pressure. When a high enough voltage is reached a breakdown of the gas occurs, leading to the formation of a mix of antimicrobial elements. The mechanisms that take place during this phase of cold plasma reaction are numerous and include vibration and excitation of gas atoms, ion-ion neutralization, quenching and many more (Misra et al. 2019; Sahu et al. 2017). Addition of H2O2 to the plasma, augments the sterilization mechanism; e.g. it was shown that at a high concentration, ROS inhibit cell proliferation and cause apoptosis (Thannickal and Fanburg 2000). Many reports have reported the efficacy of cold plasma treatment in inactivating a wide spectrum of bacteria (gram positive and negative) and in many of these studies the method was shown to be even more effective in the reduction of fungal viability and spore CFU counts (Hertwig et al. 2015a, 2015b; Kim et al. 2014; Misra et al. 2019; Zahoranová et al. 2016). In recent years cold plasma sterilization has become more popular in various medical applications such as surface sterilization and sterilization of damaged tissues (Heinlin et al. 2010; Kolb et al. 2008; Xinpei et al. 2009). The direct mechanism of inactivation of fungi using cold plasma is still not entirely clear. Scanning electron microscopy examination of plasma post-treated Cordyceps bassiana spores revealed dried, cracked and flattened propagules, indicating that cold plasma treatment may cause cell wall leakage and destruction, resulting in reduced cell viability (Lee et al. 2015). Similar results were achieved with cold plasma treatments of Aspergillus spp. (Dasan et al. 2017), which is of paramount importance since Aspergillus spp. are very common in MC floral parts, and can cause serious health complications in immunocompromised patients when mycotoxin-contaminated products are inhaled in large quantities (Gargani et al. 2011; Hamadeh et al. 1988; Ruchlemer et al. 2015). Even more intriguing is the reported ability of cold plasma treatment to reduce the presence of these toxins as well as pesticide residues (Misra et al. 2015; Sarangapani et al. 2016). While gamma, e-beam irradiation and cold plasma treatments appear promising for MC sterilization, there is a lack of evidence and knowledge regarding the efficacy of each of these methods, specifically in the treatment of harvested MC inflorescences, and their effect on the desired active chemical compounds.
Cold plasma irradiation
Gamma irradiation is commonly based on the use of cobalt 60 isotope ( 60 Co) which is reported as safe for decontamination of both MC and various food products (Arvanitoyannis et al. 2009; Jeong et al. 2015; Sádecká 2007). Moreover, long term mammalian studies have shown that irradiated foods are both safe and nutritious for human consumption (Thayer et al. 1996). While gamma irradiation is more commonly used, e-beam is a newer method showing greater promise. This technique does not require a radioactive source as the radiation is created using an electron accelerator making it environmentally friendly. Moreover, it was reported that a similar efficacy of decontamination was observed when Botrytis cinerea (a major MC inflorescence fungal pathogen) was exposed to either gamma or beta irradiation (McPartland et al. 2017). Furthermore, another fungal pathogen Penicillium expansum, was more sensitive to e-beam than gamma irradiation (Jeong et al. 2015). While there was no direct mention of P. expansum as a specific phytopathogen of MC, Penicillium spp. spores are ubiquitous and common in dry MC products, suggesting that this fungus may be a potential pathogen of concern (McPartland et al. 2017; Punja et al. 2019).
The effective dosages calculated for reduction of percent population of CFUs for e-beam treatments in artificially inoculated Botrytis cinerea and uninoculated MC inflorescences are shown in Table 1.
CFU levels, [log10(CFU/g inflorescence)] of artificially inoculated Botrytis cinerea MC inflorecences, exposed to different e-beam irradiation dosages in two experiments. Bars represent SE of the mean of 9 replicates per sample. A value of 5.18 KGy was calculated, according to the polynominal formula (dotted line) to reduce CFUs by 50% (ED50), represented by the dashed line
This increased hydrophilicity can also increase the storage time of seeds, allowing for a greater amount of time between the treatment and the sowing of the seeds. Farmers and gardeners need only plant as much as they need, confident that the germination rates for the following season would still be acceptable.
The potential for this technology is amazing, as it could help preserve rare and disappearing plant species. A 2009 study looking at the effects of cold plasma treatment on the dormancy of seeds revealed that along with the increased germination of seeds, dormancy levels were reduced. Species with naturally long and extended periods of dormancy, which are disappearing due to plant competition by invasive species, could get a reprieve. Treated seeds could be re-introduced to allow these endangered plant species a better chance to survive.
Plasma is all around us. The sun is composed of plasma, and so is lightning. Fire is a plasma, as are the flickers of the fluorescent lights so many of us see on a daily basis. Any gas that is hot enough to be seen can be considered plasma. But plasma is not always hot bundles of energy. Plasma-treating seeds involve the use of cold plasma at an extremely low atmospheric pressure, as hot plasma would cook the seeds — popcorn, anyone? Temperature and pressure are directly proportional, so to lower the temperature to perform the cold plasma treatment, the pressure must be reduced.
Potential #1: Reduced Irrigation
We all want faster germination and better performance from our seeds. What if there was an emerging technology that could do that, plus improve the storage time of seeds and lead to faster-growing plants and increased yields?
Besides the obvious potential benefits of better germination, growth, and yield, the increased water absorption of plasma-treated seeds could mean that less irrigation water is needed. This has obvious benefits to all, but especially to farmers and growers living in arid parts of the world, or even those living in the suburbs facing water restrictions during the summer.
A study on plasma-treated tomato seeds found that treated tomatoes had a 28% higher germination rate, an 8% higher total survivability rate and an 11% higher number of plants surviving to the transplant stage than the non-treated control seeds. The treated plants continued to outperform the control plants as development continued—yields were 22-26% higher for the treated seeds and the weights per tomato were 9-16% higher in the treated versus non-treated plants (Meiqiang et al., 2005).
The air we breathe is primarily comprised of nitrogen and oxygen (about 78% and 21%, respectively). Another plus or minus 1% percent is argon, 1% water vapor and .04% carbon dioxide (this adds up to just over 100% because these percentages are approximate). Nitrogen and oxygen both have two atoms per molecule, so they exist diatomically.