Sanitation of medical cannabis flowers (Cannabis sativa L., flos): comparing current technologies and non-thermal plasma novelties from safety and quality perspectives-
Safety challenges associated with the use of cannabis
In medical, recreational, and nutritional applications, various factors determine the safety of consuming cannabis products. For example, these products should be purchased legally by consumers to ensure proper processing standards and legitimacy of the product. One pressing safety challenge regarding cannabis consumption in is the sanitation of cannabis flowers, particularly concerning the removal of harmful human and plant pathogens. Other contaminants, like pesticides, heavy metals, aflatoxins, foreign particles, and insects are also challenging. These need other preventive or reactive methods like sanitation to be reduced or avoided. However, preservation of flower quality under treatment and storage conditions needs to be considered for current and novel sanitation processes. Due to a lack of consistent laws and standards worldwide, current sanitation and safety monitoring of cannabis remains flawed, particularly for cannabis that is internationally traded. This is because the importer and exporter countries may have different acceptance thresholds for contamination, which could require double safety and quality checks. The flawed monitoring of cannabis may lead to the unnecessary waste of flowers as these thresholds are different and certain decontamination techniques may not be accepted in the receiving country. This underscores the need for more consistent and reliable sanitation techniques and safety monitoring of cannabis.
Microbiological risks of cannabis
During cultivation (outdoor and indoor) and at every stage of production, cannabis flowers are exposed to different sources of biological contamination. Microorganisms can be introduced by the water used for rinsing, by employees during production, or by the ambient air when the harvested cannabis flowers are dried (Holmes et al. 2015). Even producers who grow their cannabis flowers indoors and implement high standards of hygiene in their operations are not immune to contamination (Punja et al. 2019).
Under good manufacturing practice (GMP) of medical products, the contamination of the initial material can be reduced only through suitable hygiene measures and employee training. Production of cannabis flowers in the recreational sector provides comparable risks concerning microorganisms; therefore, GMP standards are also needed in this processing area.
Bacteria in/on cannabis
Bacteria are unicellular, prokaryotic microorganisms that do not have a cell nucleus (Slonczewski and Foster 2012). The vast majority of bacteria are nonpathogenic and fulfill essential ecological functions (Hahn et al. 2008). They play a central role in biogeochemical cycles and in symbiotic relationships with humans, animals, and plants, for example, in digestion, immune modulation, or nitrogen fixation.
Fungi in/on cannabis
To date, approximately 150,000 species of fungi have been scientifically described, classified, and named worldwide (Hawksworth and Lücking 2017). Fungi are capable of forming spores with a structural and functional design geared toward reproduction and efficient colonization of new substrates (Hahn et al. 2008). Depending on the group of fungi, spores can be formed exogenously on special structures or endogenously in fruiting bodies (Moore et al. 2020). While not the primary function of fungal spores, they are extremely resistant and can survive under extreme conditions.
Other microorganisms on cannabis
Fungi represent the greatest microbiological threat to plant and human health for cannabis, with bacteria also contributing to danger; however, other microorganisms, such as viruses, parasites, and archaea, may affect cannabis plants during cultivation. Since 1941, reports have increasingly revealed that viruses and viroids affect hemp and cannabis production worldwide (Chiginsky et al. 2021; Röder 1941). Recently, viruses such as cannabis cryptic virus (Righetti et al. 2018), hop latent viroid (Warren et al., 2019), lettuce chlorosis virus (Hadad et al. 2019) and beet curly top virus (Chiginsky, et al. 2021) have been shown to infect cannabis plants, leading to symptoms such as leaf wilting, malformation, brittleness, and yellowing.
Regulatory requirements
Like other medicinal products, cannabis flowers are subject to regulatory requirements. To produce medicinal cannabis in Germany and the EU, manufacturing authorization in accordance with GMP guidelines must be obtained. These manufacturing licenses are issued by the respective states or federal states. The cultivation of cannabis flowers must also be strictly monitored by growers. Some producers of cannabis flowers choose to grow their flowers under GMP conditions (DEMECAN 2022). However, other producers initially cultivate under the controlled but looser standards of the Good Agricultural and Collection Practice (GACP) and only refine the phytopharmaceutical under GMP conditions during further processing of the plant into a derived product such as an oil or extract (Cantourage 2024). If cultivation is not carried out directly by the producer of the medicinal product, the latter is obliged under GMP regulations to continuously monitor its growers with regard to their cultivation.
Quality requirements
The domestication of narcotic Cannabis sativa varieties involved the targeted selection of floral color characteristics, such as purple or white strains (Small 2015). These visual characteristics are considered indicators of high potency and are therefore preferred for further breeding. The popularity of such color characteristics clearly shows the influence of color trends and human aesthetics on cannabis breeding goals, which is also reflected in consumers’ purchasing decisions. The visual assessment of color and its association with the perceived quality of cannabis flowers is widespread in the recreational consumption market and largely agreed upon among consumers. Bright, shiny green tones are generally considered indicators of high-quality flowers, whereas dark green, brownish, or pale shades are often associated with inferior quality (Hartfield 2024). The texture of cannabis also plays a decisive role in the assessment of quality for many consumers. Ideally, cannabis flowers should be slightly sticky and spring back when pressed. These properties are mostly related to the residual moisture content of cannabis flowers.
Safety requirements
Pathogenic fungi and bacteria on cannabis can pose a particularly high risk to immunosuppressed patients. Additionally, the argument that inhaled smoke (50–60°C) is sterilized has been proven false, as some microorganisms present on cannabis are able to recolonize in cultivation media even after water filtration from smoking cannabis through a water pipe (Moody et al. 1982). Therefore, special care must be taken with medicinal cannabis flowers that have not undergone any germ-reducing treatment. This is because pathogenic germs such as Escherichia coli, Pseudomonas aeruginosa, Bacillus cereus, or Aspergillus spp. can colonize the pipes of growers’ irrigation systems and cause spot contamination on cannabis plants that may not be detected during random testing. To minimize this risk, cannabis flowers intended for medicinal purposes should always undergo germicidal treatment.
Current applicable irradiation methods for cannabis sanitation
Gamma irradiation
Gamma irradiation is an established physical process for the decontamination of cannabis flowers. In particular, in Canada and the Netherlands, it is often used to reduce the germination of cannabis flowers to comply with low microbiological requirements (Cannabis 2026). Using high-energy gamma photons, which are typically generated by a Cobalt-60 source (Haji-Saeid et al., 2007), the process of gamma irradiation uses ionizing radiation to damage the DNA of microorganisms, leading to the inactivation of bacteria, fungi, pests, and even spores (Virsik-Köpp n.d.).
Electron-beam (e-beam) sterilization
Electron beam (e-beam) sterilization is another physical process used for the decontamination and sterilization of materials via high-energy electron radiation. In this process, accelerated electrons are used to generate ionizing radiation. Like gamma radiation, direct DNA damage is caused in microorganisms, preventing them from replicating, including spores (Silindir Gunay and Ozer 2009). Compared with gamma irradiation, e-beam technology has a lower penetration depth because electrons interact extensively with a material as they pass through it, causing them to lose energy (Naikwadi et al., 2022). However, this makes it possible to achieve more precise dose control and shorter irradiation times. E-beam treatment can achieve comparable microbial reduction rates at very short exposure times (approximately 15 min compared with 24 h with gamma radiation) (Silindir and Özer 2009). The greatest advantage of this method is that no radioactive isotopes, such as Cobalt-60, are needed; therefore, no major harmful substance in the form of radioactive waste is caused by the method (AST 2026).
Ultraviolet C (UV-C) radiation
For this method, microbial inactivation is achieved by means of the electromagnetic spectrum, which has a particularly effective antimicrobial ability at wavelengths between 200–280 nm (nm). This range is referred to as UV-C (Rahmati et al. 2022). While UV-B radiation (280–320 nm) has a lower inactivation effect, UV-A radiation (320–400 nm) has a neutral to beneficial effect on microorganisms. UV-C is created using a source that can obtain a high temperature, which is typically a plasma source.
Current applicable gaseous sanitation agents for cannabis
Ozone
Ozone is a colorless and reactive gas that is denser than air in its gaseous form (Jadhav et al. 2021). In an ozone generator, oxygen molecules (O2) split into free oxygen atoms via energy induced bond breaking, react with each other, and form ozone molecules (O3) (Rahmati et al. 2022). Ozone generators usually employ plasma, such as a dielectric barrier discharge (DBD), by the action of UV radiation or electrical discharge for the high energy needed to form ozone (Siemens 1889). Due to the instability of ozone compared with atmospheric oxygen, it must be continuously generated on demand via an ozone generator (Epelle et al. 2023). The ozone is then injected into a chamber, either in a gaseous or aqueous phase, for decontamination of various surfaces, medical equipment, water, and food (Epelle et al. 2023).
Vacuum-Water Vapor-Vacuum (VWV) process
The VWV is a process for germ reduction based on the principles of pasteurization and the thermal destruction of microorganisms (Vegara et al. 2013), excluding spores and thermophilic bacteria. Pasteurization was initially used only for liquids and is widely used in the dairy industry (Holsinger et al., 1997). The process used for vegetables, herbs, spices, mushrooms, teas, and pharmaceutical raw materials such as cannabis flowers is a further development of this original process. VWV involves a cycle of evacuations and saturated steam exposure that are carried out in an autoclave chamber. An important aspect of this process is the absence of any chemicals, and only the combination of heat, water and vacuum contributes to germ reduction. Therefore, VWV is also approved for organically produced products. The aim of this process is to achieve a reduction in bacteria by one to four orders of magnitude, with the sensory properties of the products being affected as little as possible. This process may be effectively used to sanitize cannabis products without significantly changing the organic properties of the plant; however, public research is needed to understand its application in this field as current research is only available for companies with patents for this technology.
Innovative non-thermal plasma sanitation methods
As mentioned above, the current methods for decontaminating medicinal cannabis have several disadvantages, such as regulatory issues, cost barriers, and decreased customer approval. To counteract these disadvantages, new non-thermal plasma methods are being researched. Some of these plasma methods have already been used to decontaminate medicinal and aromatic plants which provides a scientific basis for their use in medicinal cannabis (Rahmati et al. 2022).
Plasma is generally regarded as the fourth state of matter. Plasma can be generated under a wide variety of operational conditions, as evidenced by naturally occurring plasmas such as lightning, the sun and stars, or the intergalactic nebula (Hu et al. 2024). In technical plasmas, plasma is defined as a partially ionized gas typically produced by electrical breakdown of a gas (Erich and Kunhardt 2013; Hale 1948). Through this ionization, free charge carriers, such as electrons and ions, are available to conduct an electrical current. This electrical conductivity enables the use of different types of electrical power supplies to sustain this plasma state.
Plasma-Processed Air (PPA)
Plasma-processed air is produced by energized interactions between neutral- and free-charged particles with air molecules in a closed space creating, primarily, reactive nitrogen species (RNS) (Schnabel et al., 2021). The composition of the plasma is influenced by factors such as the plasma source and working gas; however, the active species within the PPA from a microwave source mainly consist of NxOy RNS, such as nitrogen oxide (NO) and nitrogen dioxide (NO2) (Winter et al., 2023). In the case of PPA, plasma generated via microwaves or DBDs and compressed air has been shown to exhibit antimicrobial abilities across various applications, including the decontamination of food (Winter et al., 2023; García Casado et al., 2024), seeds (Wannicke et al., 2021), plants (Schnabel et al., 2018), and other surfaces (Kramer et al., 2020; Katsigiannis et al., 2022; Schnabel et al., 2014).
Conclusion
Cannabis is increasingly being used for medicinal and recreational purposes; nevertheless, it is subject to contamination as a natural product during its growth and processing. Microbiological contamination, particularly by pathogenic fungi and bacteria, poses risks to both the plant, affecting the yield and quality of the produce, and to people who consume cannabis, which may carry harmful pathogens. Some of these pathogens, such as Aspergillus, have been found to cause severe illnesses in consumers. Therefore, there are regulations on cannabis, particularly in medicine, for acceptable microbiological load limits, measured in terms of TAMC and TYMC. These limits are achieved by growing cannabis under proper GMP standards and by postharvest sanitation methods in an attempt to reduce the pathogens present on cannabis.
Cannabis is a sensitive product that is not suitable for decontamination methods that significantly increase its temperature, which affects its content of cannabinoids and/or terpenes and thereby its pharmacological and sensory properties, as well as methods that use chemical disinfectants due to the risk of lingering chemicals within the complex flower structure. Popular sanitation methods that are currently used on cannabis include gamma irradiation and electron beam irradiation. While these processes are proven to be effective at reducing the microbiological load, they present significant regulatory, cost, and customer approval challenges to the widespread acceptance of these methods. This is primarily due to skepticism around irradiation due to perceived consequences about the safety and quality of irradiated products. Some gaseous sanitation agent methods, including ozone and vacuum-water vapor-vacuum, are beginning to be applied in the cannabis industry, which may reduce this concern of using irradiation.
New and innovative technologies such as non-thermal plasma are therefore increasingly in research focus, as they offer the potential to achieve the microbial reduction required to ensure consumer safety while avoiding high temperatures and chemical disinfectants. These non-thermal plasma techniques are emerging as potential ways to sanitize cannabis, with the prospect of improved industrial feasibility and customer acceptance. One NTP method currently under research utilizes reactive nitrogen species within plasma-processed air to remove microbial contaminants. PPA has undergone laboratory testing for use in food applications that can help inform how it can be used on cannabis. Ozone and PPA treatment systems are currently being tested for the reduction of microbiological contaminants on cannabis specifically. These two methods appear to be the most promising methods, both generated with NTP techniques, because they take advantage of reactive species in a gaseous state that can possibly diffuse into the complex flower structure, destroying hard-to-reach microbiological contaminants.
The main challenge in developing a new germ reduction process is to achieve a microbial reduction that is comparable to existing germ reduction processes. Moreover, it must be ensured that the quality of the cannabis flowers meets the criteria of the release specifications. Not only must quantitative parameters such as cannabinoid and terpene contents be preserved, but organoleptic properties such as overall appearance, color, and odor must also remain unchanged. Therefore, future research must place a stronger emphasis on comprehensive stability studies to investigate potential long-term effects of novel decontamination methods. These studies should include microbial load, active ingredient content, and especially organoleptic properties, as the latter are closely linked to customer approval. Color changes require particular attention, as prior studies on dry food matrices treated with plasma-processed air have reported browning or pigment degradation, potentially linked to the destruction of carotenoids or chlorophyll. Additionally, terpene degradation and general plant degradation processes should be systematically analyzed. Based on such findings, treatment parameters, especially for ozone and PPA, will likely need to be optimized to balance effective microbial reduction with preservation of product quality, as adverse quality effects in plasma-treated foods have been associated with the most aggressive antimicrobial conditions.
For ozone treatment in particular, it remains inconclusive whether microbial efficacy is consistently associated with adverse quality changes in medicinal plants. Consequently, future data generated under optimized treatment parameters are required to clarify whether ozone can achieve sufficient microbial reduction on cannabis without compromising quality or consumer acceptance. In addition, NTP systems that do not rely on consumable chemical agents, such as hydrogen peroxide, may be advantageous for both industry and consumers. Processes such as PPA, which are generated from compressed air and do not require costly chemicals or noble gases, may therefore offer improved commercial viability and customer acceptance.
As is common for novel technologies applied to new industrial contexts, regulatory guidelines and standards for NTP-based decontamination are currently lacking. Therefore, the establishment and adoption of standardized metadata schemas for NTP applications are critical. Given the heterogeneity of non-thermal plasma sources and application conditions, including plasma generation, treatment medium, target material, diagnostics, gas composition, flow rate, temperature, and humidity, standardized reporting is essential to improve data quality and comparability. Recent metadata schemas, represent an important first step and may be adapted for cannabis flower applications in the future, provided they are widely adopted by the research community.
While this review primarily focuses on bacterial and fungal contamination as the most immediate risks, cannabis plants are also affected by other biological contaminants, including viruses, viroids, oomycete., and parasitic nematodes. The efficacy of ozone and NTP-based methods against these contaminant groups on cannabis flowers remains largely unexplored and should be investigated to establish a comprehensive safety profile.
In summary, non-thermal plasma methods that are known to efficiently inactivate microorganisms in food products may be used for cannabis sanitation. Ozone and PPA, in particular, present interesting alternatives to the irradiation methods currently used on cannabis as potentially effective, non-thermal, and residue-free sanitation processes. Another advantage of ozone and PPA is the possibility to treat cannabis locally, instead of sending it to a centralized facility, which means that even smaller growers can decontaminate their product. Current research indicates that up-scaling these processes is also likely possible if larger batches need to be treated. Ultimately, aspects such as the intensity of decontamination, the type of microorganism, the conditions of the treatment environment, safety, and economic factors must be taken into account to identify the best application of non-thermal plasma techniques. As further research progresses, the development of standardized regulatory and legislative frameworks will be a key step toward final industrial implementation. The goal of introducing novel decontamination techniques is to guarantee that medicinal cannabis, as well as recreational cannabis, maintains high safety and quality needs under feasible and effective industrial decontamination processes.
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