Decontamination technologies for medicinal and aromatic plants: A review

Abstract Microbial quality assurance has always been an important subject in the production, trade, and consumption of medicinal and aromatic plants (MAPs). Most MAPs have therapeutic and nutritional properties due to the presence of active substances such as essential oils, flavonoids, alkaloids, etc. However, MAPs can become infected with microorganisms due to poor hygienic conditions during cultivation and postharvest processes. This problem reduces the shelf life and effective ingredients of the product. To overcome these problems, several technologies such as using ethylene oxide gas, gamma irradiation, and steam heating have been used. However, these technologies have disadvantages such as the formation of toxic by‐products, low consumer acceptance, or may have a negative effect on the quality of MAPs. This requires a need for novel decontamination technology which can effectively reduce the biological contamination and minimize the food quality losses. In recent years, new technologies such as ozonation, cold plasma, ultraviolet, infrared, microwave, radiofrequency and combination of these technologies have been developed. In this review, biological contamination of MAPs and technologies used for their decontamination were studied. Also, the mechanism of inactivation of microorganisms and the efficacy of decontamination techniques on the qualitative and microbial characteristics of MAPs were investigated.

conditions basically determine the quality of the final MAPs. These problems are more prevalent in tropical and subtropical regions, because the high temperature and moisture contents are favorable to fungal growth and toxin production (Zhang et al., 2012). Microbial contamination of MAPs is an important subject with respect to consumer safety, negative impacts on active substances, and nutritional properties of these plants, their export, and the quality standards of importing countries. Additionally, it reduces the shelf life of the product and also the accumulation of mycotoxins (Kneifel et al., 2002;Kosalec et al., 2009;Stępień et al., 2011;Waśkiewicz et al., 2008).
Mycotoxins are secondary metabolites that are formed by a wide range of contaminating fungi in a variety of foods and agricultural products around the world and potentially endanger human health (Science, 2003). According to the Food and Agriculture Organization of the United Nations, about 25% of the world's crops have been contaminated by mycotoxins during growth or storage (Wu, 2007).
Therefore, the microbial load in the product must be minimized or completely eliminated. Although the use of preventive measures such as good manufacturing practice (GMP) and guidelines on good agricultural and collection practices (GACP) can control these contaminants, the necessary infrastructure has not yet been provided in many areas of the world (Kosalec et al., 2009). Considering that medicinal plants and spices are collected from different areas of the world, their quality control and microbial safety play an important role in their trade and consumption. A lot of equipment and techniques have been employed to decontamination of medicinal and aromatic plants and their derived products. The most common commercially available methods for decontaminating MAPs are ethylene oxide and methyl bromide, heat treatment, and gamma irradiation.
Ethylene oxide and methyl bromide fumigation is currently banned in the United States and the European Union due to the formation of toxic and carcinogenic by-products (Sánchez-Maldonado et al., 2018;Shirkole et al., 2020). In the gamma-irradiation method, the maximum dose used should not exceed 10 kGy, as it endangers the health of consumers and can also cause structural damage to the food products such as, odor, color, flavor, and the reduction of volatile compounds regarding these technologies. Also, high cost of the decontamination process, formation of radioactive materials in packaged products, and a poor consumer acceptance toward irritated foods have been reported (Cho et al., 2017;Verma et al., 2021).
Steam treatment also has adverse effects on physicochemical qualities, nutritional properties, and quality parameters of the product.
Furthermore, it requires a heat treatment step because of dampened surface of the product, which does necessitate high energy consumption (Cheon et al., 2015;Molnár et al., 2018;Schweiggert et al., 2007). This requires a need for a new decontamination technology that can effectively reduce the biological contamination and minimize the food quality losses.
In recent years, new methods include: physical (ultraviolet, cold plasma), chemical (ozone), and thermal methods (infrared, microwave, and radiofrequency) or a combination of them (combination of two or more technologies to achieve synergistic effects) which have been used by the researcher. Novel technologies for decontamination have attracted the attention of a lot of food manufacturers. Some new technologies are all under research in laboratories, while other novel technologies are still undergoing initial testing.
To the best of our knowledge, no comprehensive research has been done on the equipment and systems used to decontaminate MAPs.
Hence, the path taken in this article is to review the technologies used in MAPs' decontamination and their effects on the physicochemical and microbial properties of MAPs.

| Ethylene oxide and methyl bromide injection
Ethylene oxide and methyl bromide treatment is a decontamination method that has been commonly used to decrease microbial infection in MAPs due to its efficiency and relatively low cost. Despite the above advantages, this method was banned for the formation of toxic by-products, carcinogenicity, safety, and environmental issues in 1991 by the European Union and many other countries (Schweiggert et al., 2007;Asill et al., 2013).

| Gamma irradiation
Irradiation is the amount of energy required for ionization that is transferred from the radiation source ( 60 Co or 137 Cs) to the food. Its main mechanism of inactivation of microorganisms is damaging the DNA of the cell (Khawory et al., 2020). Gamma irradiation was approved in 1983 by the Codex Alimentarius Commission (CAC) for microbiological decontamination of MAPs and is currently used in at least 51 countries up to a maximum dosage of 10 kGy (Khawory et al., 2020). One of the main advantages of gamma irradiation is its effective penetration depth and also its power to decontaminate the internal parts of the product. Numerous studies have shown the positive effects of gamma irradiation on reducing the microbial load of MAPs (Al-Bachir, 2007;Al-Bachir et al., 2004;Kamat et al., 2003). Irradiation reduces the damage caused by microbial contamination and insects. In addition, this method is fast, convenient, and user-friendly. As well, the chance of recontamination of MAPs reduces because disinfestation takes place after packaging (Farkas, 1998;Khattak & Simpson, 2009).
Gamma irradiation can be used at controlled doses, and higher doses can only be used in certain cases. In other words, in gamma irradiation, the maximum amount of absorption for food should not exceed 10 kGy, as it endangers the health of consumers and affects the structural and sensory characteristics of food. However, irradiation has been reported to have disadvantages such as potential impacts on the quality of MAPs, the high cost of the process, the formation of radioactive materials in packaged products, and the general lack of acceptance of products by consumers (Akbas & Ozdemir, 2008;Ban et al., 2018;Chytiri et al., 2005;Gumus et al., 2011).

| Steam heating
The steam heating system has been successfully used in the MAPs' industries for decontamination. Depending on the operation temperature, steam heating systems are divided into two categories: saturated steam (SS) and superheated steam (SHS). Saturated steam is a common method used for decontaminating spices in the United States and Europe in two ways: continuous or batch (Abba et al., 2009;Schweiggert et al., 2007). The general schematic of the saturated steam/superheated steam decontamination machinery with its components is shown in Figure (1).
The amount of steam produced in this system varies according to the inlet power to the system (inlet power to the boiler and superheater) and the capacity of the boiler. The working mechanism of this method is executed on the basis of steam use at a temperature of 100-200°C. For this purpose, the steam flow conveys heat to the surface of the product (convective heat transfer) to increase the temperature of the product and also to decontaminate it. Microbial contamination in plants has been reduced greatly with respect to temperature and time treatments, and gradually settled within the standard range. However, treatment with steam heating system encounters some disadvantages such as high energy consumption, complexity of equipment, color and sensory alterations, and reduction of volatile compounds (Ban & Kang, 2016;Brodowska et al., 2014;Rico et al., 2010;Tateo & Bononi, 2006;Waje et al., 2008).
There are some companies which manufacture steam systems to decontaminate MAPs, such as Napasol AG (Rotosol ® and Statisol ® ), Ventilex ® (Ventilex continuous steam sterilizing system), Log5 ® (continuous HT-ST "In-Flow" steam TEMA Process BV decontamination process), ETIA (Safesteril ® ), Revtech (Revtech ® ), etc. In these technologies, saturated steam is used for decontamination, and additional equipment is taken to control condensation and the uniformity of the decontamination operation. Also, these systems are useful for decontamination of MAPs, but they are expensive.
Therefore, the development of new technologies is needed considering environmental issues, optimal energy consumption, and production of high-quality goods.

| Ozone injection
The use of ozone in 2001 in both gaseous and water-soluble forms has been approved by the US Food and Drug Administration as a strong oxidation for decontamination and food processing (Khadre F I G U R E 1 Schematic diagram of the custom-made saturated steam (SS)/ superheated steam (SHS) decontamination machinery. (a) Water reservoir, (b) steam boiler, (c) superheater, (d) outer reacting chamber unit, (e) inner reacting cell, (f) power control unit, (g) temperature monitoring system, (h) temperature data processing system, and (i) temperature controller for the band heater et al., 2001). Using ozone as a safe method, to inactivate microorganisms, has been increased with respect to the food industry, especially for fluid foods. The schematic diagram of the ozone treatment system (water-soluble) is shown in Figure (2). The system consists of four main parts: an oxygen chamber, an ozone detector, an ozone generator, and a treatment chamber. For this purpose, all the oxygen molecules enter the generator and decompose to single oxygen molecules during reactions by ultraviolet (UV) irradiation, then the oxygen atoms react with each other to form ozone molecules (Mohammadi et al., 2017). The produced ozone is then injected into the chamber and the decontamination process triggered subsequently. Ozone antimicrobial property is associated with the oxidation of double-bond cellular compounds such as phenolic rings and sulfhydryl groups, which ultimately lead to cell death (Aponte et al., 2018;Pandiselvam, Mayookha, et al., 2020).
Decontamination efficiency with ozone technology is affected by factors such as the type of microorganisms, the amount of microbial contaminations, temperature, pH medium, relative humidity, additives, and the amount of organic matter around the cell (Han et al., 2002;Kim et al., 1999;Manousaridis et al., 2005). However, the main disadvantage of this technique is the potential toxicity of ozone molecules to the operator. Therefore, the decontamination process should only be performed in an isolated and well-ventilated chamber. Ozone should also be allowed to be decomposed to oxygen molecules, which usually takes 20 to 50 min at room temperature (Skåra & Rosnes, 2016;Thanushree et al., 2019).

| Cold plasma
Since the mid-1990s, plasma has been used to inactivate microorganisms. However, this method has recently been studied in the food industry (Li & Farid, 2016). In general, plasma is of two types: cold plasma and thermal plasma. Cold plasma is a relatively ionized gas that is coupled from energy sources such as corona discharge, dielectric barrier discharge (DBD), microwave discharge, pulse discharge, high-frequency discharge with gaseous medium such as nitrogen, oxygen, air, hydrogen, halogen, argon, or combination of them (Sakudo et al., 2020;Scholtz et al., 2015). Common electrical discharge equipment for the generation of cold plasma is shown in Figure (3). Reactions of various plasma compounds such as free radicals, charged particles, ultraviolet photons, ions, and heat lead to the oxidation of microbial cell membrane, DNA alteration, and thus inactivating microorganisms (Gallagher et al., 2007;Laroussi & Leipold, 2004;Lee et al., 2015). Cold plasma is a relatively fast, F I G U R E 2 Schematic diagram of the ozone system F I G U R E 3 Schematic diagram of electrical discharges for generating the cold plasma environmentally safe, and low-temperature processing method that has been successfully recruited to inactivate microorganisms of MAPs (Kalkaslief-Souza et al., 2007;Kim et al., 2014;. Plasma decontamination efficiency depends mainly on the type of gas, voltage, the energy source, gas composition, treatment time, type of product, and the relative humidity (Li & Farid, 2016;Patil et al., 2014).
However, cold plasma has disadvantages such as poor penetration capacity into food (especially solid foods) and unavailability on a commercial scale. In addition, information about its impact on the quantity and quality of active substances of product is limited (Ebadi et al., 2019).

| Ultraviolet irradiation
Ultraviolet irradiation was first used in France in 1906 to decontaminate beverages (Li & Farid, 2016). Ultraviolet is a nonthermal technology that uses electromagnetic spectrum (100-400 nm) to inactivate microorganisms. Its antimicrobial effect has been effectively confirmed within the range of 200-280 nm (ultraviolet-C) (Pedrós-Garrido et al., 2018). Inactivation of microorganisms comes from the ability of ultraviolet to penetrate the cell membrane and to damage the DNA or RNA of microorganisms, thus preventing their proliferation (Escalona et al., 2010;Gabriel, David, et al., 2020). This method can potently inactivate microbes and has recently been successfully used to decrease the microbial load of solid food samples (Fonseca & Rushing, 2006;Gabriel et al., 2017;Pérez-Gregorio et al., 2011). Decontamination efficiency by ultraviolet depends on factors such as the type of microorganisms, ultraviolet dose, and temperature of the treatment and the surface characteristics of the food (Fan et al., 2017). Failure to expose the total and effective surface of the product to ultraviolet and nonuniform decontamination operations are among the disadvantages of ultraviolet irradiation.
To solve this problem, it is proposed to use a combined fluidized bed system with ultraviolet ( Figure 4). However, this method has a low penetration depth and its application is limited to inactivating surface microorganisms (Guerrero-Beltr·and Barbosa-C·novas, 2004).

| Infrared radiation
In recent years, infrared radiation (IR) has been studied as a new technology for MAPs' decontamination. IR is a region of electromagnetic spectrum with a wavelength in the range of 0.76 μm-1 mm between ultraviolet and microwaves. IR is generally divided into 3 spectral regions: near-infrared sections (0.76-2 μm), mid-infrared (2-4 μm), and far-infrared (4-1000 μm) (Eliasson et al., 2015). In general, far-infrared is used for food processing because most food compounds absorb radiant energy within this range. Infrared radiation is absorbed by organic compounds in low-water activity foods such as protein, lipids, and sugars. The mechanism of infrared inactivation takes place through the absorption of energy by organic food compounds and damage to DNA, RNA, and proteins in microbial cells (Gurtler et al., 2014;Rifna et al., 2019;Sandu, 1986). The schematic of the IR heating system is shown in Figure (5). Several studies had examined the antimicrobial effects of infrared radiation (Bingol et al., 2011;Brandl et al., 2008;Eliasson et al., 2015;Erdoğdu & Ekiz, 2013;Ha et al., 2012). Infrared decontamination efficiency depends on parameters such as decontamination temperature, infrared power, distance from the source, etc. (Rifna et al., 2019). Infrared heating has a penetration depth of 0.31-4.76 mm depending on the product and the wavelength used, so infrared heating is considered as a surface heating technology (Eliasson et al., 2015). However, infrared radiation has not been widely used as an alone energy source.

| Microwave heating
Microwaves are a type of nonionizing radiation of electromagnetic waves with wavelengths and frequencies in the range of 1 mm-1 m and 300 MHz-30 GHz, respectively. In general, two frequencies, 915 and 2450 MHz, have been employed for medical, industrial, and scientific applications . The wavelength related to these frequencies is in the range of 12-24 cm. A schematic of a microwave heating system is shown in Figure (6). A conventional microwave heating system consists of three main components: a magnetron, a waveguide, and a sample chamber (Ştefănoiu et al., 2016). The efficacy of microwave treatment depends on factors such as treatment time, product geometry, type of microorganism, power, and frequency range hired (Jiang et al., 2018). However, microwave heating has disadvantages including nonuniform heating and the formation of cold spots inside food, which has limited the industrial use of this method for disinfection.

F I G U R E 6
Components of an industrial microwave heating system

| Radiofrequency heating
Radiofrequency is a type of dielectric heating, which has the potential for uniform and rapid heating of solid and semisolid samples.
Radiofrequency is part of electromagnetic waves with a frequency in the range of 30 kHz-300 MHz. Usually, three frequencies of 13.56, 27.27, and 40.68 MHz are used in food processing (Ştefănoiu et al., 2016). In this method, unlike microwave heating, ionic conduction is the main mechanism of heat produced inside the product (Dev et al., 2012). The displacement of ions with opposite charges in the presence of an alternating electric field leads to an increase in the kinetic energy of the molecules and thus to an increase in the temperature of the product. In radiofrequency heating, the heat generated inside the food as a result of radiofrequency radiation is absorbed by the DNA of microorganisms, and subsequently it leads to a change in their physical structure and reduced function (Rifna et al., 2019).
A schematic of the radiofrequency heating system is shown in Figure (7). A conventional radiofrequency system consists of a radiofrequency generator, an adapter, and an applicator (Dev et al., 2012). The applicator consists of two metal plates between which the product is placed. These plates (electrodes) form a capacitor with the food. In radiofrequency heating, the electrodes do not come into contact with food, so they can easily be used for solid and liquid foods. The structure of the radiofrequency heating system is simpler compared to the microwave, in addition to being able to penetrate deeper into the food because of its longer wavelengths and more uniform field patterns. Radiofrequency heating is now generally used on an industrial scale for drying processes in the textile, paper, and biscuit industries (Orsat & Raghavan, 2014). Recently, this method has been employed to decontaminate solid foods.
The following is a summary of the research carried out in the field of decontamination hiring a radiofrequency heating system. respectively. However, radiofrequency treatment led to a significant reduction in color and flavor of the product. Finally, it was reported that the combination of these two methods can effectively control the microorganisms of the product, but a remarkable effect will be observed in the quality parameters of the product.
Decontamination efficiency using radiofrequency treatment depends on factors such as radiofrequency temperature, sample geometry, sample moisture, equipment capacity, and the desired microorganisms (Rifna et al., 2019). However, the dielectric properties of food change continuously with temperature, and this should be considered in the design of dielectric heating systems as it might affect the processing time and uniformity of heating. Therefore, applying this technology to food decontamination on an industrial scale requires computer simulation of temperature dependence on the dielectric properties of materials.

| EFFEC T OF DECONTAMINATION TECHNI QUE S ON THE QUALITATIVE AND MICROB IAL CHAR AC TERIS TIC S OF MAPS
MAPs which are produced in different regions, besides their physical and chemical properties, should be evaluated for their microbial contamination. Table (1) provides an overview of the most prominent studies on the decontamination of MAPs and the impact of these methods on the qualitative and microbial characteristics of processed plants. In addition to reducing the microbial load, the qualitative characteristics of the product are affected, so selecting the most appropriate technology is of great importance for the type of product. However, an optimal relationship between the qualitative and microbial characteristics of the product is required. Therefore, F I G U R E 7 Components of a radiofrequency heating system

Treatment condition
Processed sample

Reference
Gamma irradiation: at 5 and 10 kGy Dried lotus pollen

Gnetum gnemon, Khaya senegalensis and Euodia malayana in two different forms (leaf extracts and dried leaves)
Bacteria, fungus, spores, total phenolic content and antioxidant activity The results showed that the appropriate doses for extract and dried leaves were 6-12 and 9-13 kGy, respectively. Enhanced total phenolic content and antioxidant activity.

Phenolic compounds
Increase in the extractability phenolic compounds was observed in highest dose (10 kGy).  Gamma rays and Electron beam: at 0, 1 and 10 kGy

Menthapiperita, Aloysia citrodora, Melissa officinalis, and Melittis melissophyllum
Chemical and bioactive properties The effect of gamma rays and electron beams varied according to the type of plant. Electron beam is more effective than gamma rays.  Steam: Ozone: times of 10 and 30 min in dose of 0.3, 0.6 and 0.9 ml/L Peppermint, summer savory, Indian valerian, lemon balm and Iranian thyme Microbial Load and essential oil Concentration of 0.9 ppm for 30 min was the most effective in reducing the microbial load (reduction 1.12, 1.79, 3.5, and 4 log CFU/g in peppermint, summer savory, lemon balm and Iranian thyme, respectively, no effect on essential oil content.

TA B L E 1 (Continued)
along with the reduction of the microbial load, the minimization of the damage to the qualitative characteristics of the product is achieved subsequently.

| COMPARISON OF TECHNOLOG IE S
In 2012, Germany, Turkey, Sweden, and Spain launched a joint project called GreenFoodec to overcome the limitation of conventional technologies for MAPs' decontamination. In this project, whole black pepper (seed), paprika (powder), and oregano (herb) were decontaminated using four technologies: High pressure CO 2 + ultrasound, cold plasma, microwave, and infrared radiation. Then, the most ap- spectively. There was no significant difference between the color of the samples in the control sample and the samples treated with the combined system. Eventually, it was reported that the combined system is more effective than ultraviolet irradiation alone for decontaminating microorganisms without degrading the quality of the powder, and has been suggested as an alternative to conventional methods (superheated steam). Choi et al. (2018) examined the effect of the combined radiofrequency treatments (500, 1000, and 1500 W for 2 min) and cold plasma (700, 1000, and 1500 W for 2 min) on qualitative parameters and microbial characteristics of red pepper powder. In the combined system, the samples were first exposed to different radiofrequency powers and instantly placed in the refrigerator for 5 min. Finally, the samples were transferred into a cold plasma system. The results revealed that the combined radiofrequency system and cold plasma were more effective in reducing microbial load rather than individual treatments. In addition, the combined system led to minor changes in the color parameters of the samples. Eventually, it was reported that the best condition to reduce the microbial load (S. aureus counts and E. coli O157: H7 to 3.19 and 3.73 log CFU/g, respectively.) without significant changes in color parameter or antioxidant activity of red pepper powder was when using the power of 1500 W in the radiofrequency system and a power of 1000 W in the cold plasma system. Watson et al. (2020) examined the combination of ozone, ultraviolet, infrared, and fluidized bed systems for decontamination of chili flakes. Treatment with each method effectively reduced the microbial load to 6 logs (CFU/g) in ≤20 min for ozone, 7 logs (CFU/g) in ≤40 min for UV, and 7 logs (CFU/g) in ≤20 min for IR. The combination of infrared and ultraviolet treatments also improved performance compared to individual treatments. Finally, the combination of infrared and ultraviolet treatments followed by ozonation (UV and IR for 10 min and ozone 10 min) was reported to be an effective way to decrease the contamination in chili flakes; this is due to more effective reduction in microbial load for infrared and ultraviolet (0.80 log (CFU/g)) compared to ozone first (0.13 log (CFU/g)).

| CON CLUS ION
MAPs, like other agricultural products, are prone to contamination with germs, insects, etc. at any step in the production chain.
Therefore, decontamination of MAPs with the aim of reducing adverse effects on raw material compounds, less waste, and more added value along with consumer health is necessitated. Fumigation with ethylene oxide and methyl bromide, heat treatment with steam, and gamma irradiation are common technologies that have been used on a commercial scale for the decontamination of MAPs. These technologies have disadvantages such as the formation of toxic byproducts and carcinogenicity, poor acceptance among consumers, and also the impact on the quality of the product. New technologies such as ozonation, cold plasma, ultraviolet, infrared, microwave, and radiofrequency have been used in order for decontamination of MAPs to overcome these limitations. These technologies have improved product quality parameters compared to conventional methods. However, these technologies are limited to a specific product (powder, seed, and leaf) and some of them have been used on a laboratory scale. Combined technologies have recently been studied for the decontamination of MAPs. It is to be hoped that with the development of such a system, a major step will be taken to ameliorate the quality control of MAPs and thus increase the added value of the product. Further research is required to develop the system, by taking into consideration environmental issues, the optimal relationship between product quality and microbial properties, energy consumption and continuous process. In addition, after the development of such a system, the obstacles that cause the transfer of this technology should be examined from the laboratory to the industrial scale, as well as the equipment required on an industrial scale and the initial economic evaluation of the process.

ACK N OWLED G EM ENT
We thank the Department of Biosystem Engineering, Tarbiat Modares University, Iran.

CO N FLI C T O F I NTE R E S T
The authors declare that they have no conflict of interest.

E TH I C A L A PPROVA L
This study did not involve any human or animal testing.

DATA AVA I L A B I L I T Y S TAT E M E N T
Data sharing is not applicable to this article, as no datasets were generated or analyzed during the current study.