Spices impart desirable characteristics to foods: bright colors, tantalizing tastes, and mouthwatering aromas. The increased use of these dehydrated herbs, seeds, and vegetables as food flavorings is trending worldwide because they enhance a meal without contributing sugar, fat, sodium, or unwanted calories. In the United States, the majority of spices are imported since many grow in tropical or subtropical regions. A simplified definition of “spice” by the Food and Drug Administration (FDA) is any dried aromatic vegetable substance whose significant function in food is seasoning rather than nutrition (FDA 2017).
Plant materials such as spices originate from the soil and are exposed to microbial contamination and other adulterants all along the farm-to-fork continuum of planting, cultivation, harvesting, processing, storage, and transport. While spices have low water activities (aw), which can limit microbial growth, pathogens such as Salmonella, Bacillus cereus, Clostridium perfringens, Cronobacter, Shigella, and Staphylococcus aureus have been found in spices (FDA 2017). Filth adulterants such as live and dead insects and their parts, animal excrement, human and animal hair, feathers, stones, twigs, staples, wood slivers, plastic pieces, synthetic fibers, and rubber bands have also been reported in spices.
The microbiological risk to public health occurs when contaminated spices are added to raw foods, ready-to-eat (RTE) foods, processed, or cooked foods after pathogen kill steps are employed. Spices and powders are commonly used in meals prepared at home as RTE flavorings to recipes. When introduced to the nutritionally rich food matrix of a finished product with higher aw, microorganisms present in the spices can not only survive, but grow and multiply to infectious levels.
From 1973 to 2010, there were 14 illness outbreaks reported worldwide due to microbial contaminants in spices (Van Doren et al. 2013). A total of 1,946 human illnesses were reported, which included 128 hospitalizations and two deaths. Seventy percent of the illnesses were due to the consumption of RTE foods prepared with spices that were applied after the final pathogen reduction step. Salmonella enterica and Bacillus spp. were identified as the causative agents in 71% and 29% of the outbreaks, respectively. No pathogen reduction step had been applied to the spices themselves in 75% of the outbreaks.
“To be in compliance with the Food and Drug Administration’s Preventive Controls for Human Foods rule, any treatment method must be validated to a 5-log reduction of Salmonella, which is the primary target microorganism of public health significance for spices,” according to Laura Shumow, executive director of the American Spice Trade Association (ASTA).
Due to pathogens such as Salmonella, decontamination or a “kill step” is required for spices to reduce the risk of human infection and product recalls. Steam sterilization, irradiation, or fumigation are the main industry methods used to reduce microbial loads in spices. Each of these methods has both benefits and drawbacks, so manufacturers choose the approach that best fits their needs.
Thermal treatments such as steam sterilization have broad consumer acceptance. But, because the essential oils and polyphenols in spices are highly volatile, thermal treatments can negatively affect the color and flavor of certain spices. Studies have found that heat treatment is less effective with low-moisture foods like spices and powders (Nyhan et al. 2021). Also, soil-originating spores, such as Bacillus and Clostridia, can withstand even prolonged periods of high temperature. Bacillus cereus can tolerate 100⁰C for 3.4 min (Yehia et al. 2022).
Irradiation technology has been used successfully for many years to decontaminate spices. But consumers are wary of the process due to the unfortunate fear of the word radiation. Irradiation, like heat, has also been shown to impact sensory characteristics (Nyhan et al. 2021). Fumigation with ethylene oxide (EtO) can reduce the microbial load but may produce carcinogenic compounds (Yehia et al. 2022). Conversely, according to the U.S. Environmental Protection Agency’s (EPA) ethylene oxide website, “No EtO residues remain on spices at the time they are consumed” (EPA 2022).
To address the risks identified by the FDA survey, ASTA, its stakeholders, and others support and evaluate research into mitigation technologies. Food scientists continue to assess nonthermal treatments such as ultraviolet-C light emitting diode (UVC-LED), cold plasma, and ozone technologies for use as commercial hurdles. “However, it is important to recognize that each of these processes has drawbacks and must be validated to ensure that a 5-log reduction of Salmonella can be achieved without damaging the sensory attributes of the spices. There is no silver bullet, one-size fits all solution,” Shumow says.
UV-LEDs don’t require mercury and have the advantages of lower cost, better efficiency, and less physical hazard to users when compared to traditional UV lamps used in the food industry. Composed of layers of semiconductor materials, LEDs emit light instantaneously when an electrical current is applied. Despite their current use to decontaminate fruit juices, cheese, produce, tuna, and chicken, UV-LEDs are just now being investigated for use in powdered ingredients such as spices (Nyhan et al. 2021). A recent study concluded that UVC-LED emission at a wavelength of 270 nm was as effective or more effective than a traditional 254 nm mercury lamp for surface decontamination of Listeria monocytogenes, Escherichia coli, Bacillus subtilis, and Salmonella Typhimurium. Powdered seasonings (onion, garlic, cheese, and chili) had lower bacterial reductions than surfaces, probably due to the shielding effect of particles, which create crevices and cavities that hide the microbes from the UVC light. Longer exposure times (40 sec) resulted in reductions of 0.75–3.0 log colony forming units in powdered seasonings. Gram-positive pathogens such as L. monocytogenes were found to be more resistant. They required higher UV dosages for inactivation than the Gram-negative bacteria. Organoleptic properties were not evaluated in this study.
Plasma is the fourth state of matter. We view it in nature as the northern lights and lightning, or at the local bar as a neon sign. “Cold atmospheric pressure plasma is generated when high energy is applied to a gas, leading to the partial ionization of the gas at atmospheric pressure,” according to Deepti Salvi, assistant professor of food engineering at North Carolina State University’s Food, Bioprocessing and Nutrition Sciences Department. “The antimicrobial action of cold plasma is due to the creation of highly reactive oxygen-based and nitrogen-based species. These are strong oxidizing agents, which because they are short-lived, are effective for pathogen inactivation while less detrimental to food quality and the environment.”
Studies using cold plasma for microbial decontamination of spices all indicate that microbial loads can be significantly reduced with a lower impact on organoleptic properties and functional ingredients when compared to other methods (Kim et al. 2013, Bagheri & Abbaszadeh, 2021, Wiktor et al. 2020). One study reported a 2.3-log reduction of aerobic bacteria in black and red pepper powder after a cold plasma treatment of 900 W for 10 min (Kim et al. 2013). Native microflora on pepper seeds and paprika were reduced by more than 3 logs after a 60-minute treatment (Hertwig, et al. 2015). Treatment times in the literature varied widely, from 15–60 sec to 60 min. It is evident that cold plasma treatment duration and strength should be optimized by ingredient, initial microbial load, and a variety of other factors. Sensory attributes should not be overlooked. In the case of red paprika powder treatment, there was a considerable loss of redness after a treatment time in excess of 5 min. For successful adoption of cold plasma in the food industry, more systemic research is needed (Charoux et al. 2020).
Spices and dried herbs are also susceptible to contamination by fungi such as Aspergillus, Penicillium, Fusarium, and Rhizopus, which may produce mycotoxins, a human health concern (Ouf and Ali 2021). A pale blue gas with a distinctively pungent smell, ozone or trioxygen (O3) is formed from dioxygen (O2) by interactions with UV light and electrical charges in the Earth’s atmosphere. It is common to smell ozone after a violent electrical storm. Like cold plasma, ozone can be created in the lab, is an antimicrobial oxidizing agent, has a short half-life, and leaves no lingering residue.
Powdered, sun-dried herbs (licorice root, peppermint leaves, chamomile, black cumin, and fennel) were analyzed for fungal contamination before and after ozone treatment (3 ppm, Ouf and Ali 2021). After 280 min of ozone exposure, the reduction of fungal count ranged from 96% to 98%, or about a 2 log reduction (Ouf and Ali 2021). The ozone exposure also reduced the total volume of essential oils in chamomile and peppermint by 57% and 27%, respectively. While a suitable method for decreasing the fungal load on spices, ozone’s effects on active ingredients such as essential oils and phenols should be considered.
In a survey of imported spices in the United States, an average prevalence of 6.6% was reported for contamination with Salmonella bacteria (FDA 2017). However, when spices at retail were tested, the prevalence dropped to nearly zero, indicating that a kill step is typically applied prior to the point of sale to the consumer. Nonetheless, a rigorous testing program is an important part of the verification that treatment was effectively applied.
Third-party analytical tests should be used to monitor the microbiological and chemical safety of spices and dried powders. Chemical testing of spices for heavy metals or pesticides can be straightforward, but detection of pathogens in spices is complicated and problematic due to the presence of antimicrobial compounds such as essential oils and phenolic compounds. The FDA BAM method for Salmonella detection in spices relies on 100 times or greater dilutions to lessen the effect of these antimicrobial compounds and to allow growth and detection of the pathogen.
Large volumes of media are problematic for laboratories as they require huge resources of media and incubator space. A recent FDA study looked at the adsorbent beta zeolite as an additive in the pre-enrichment step to increase detection while reducing the volume needed. Zeolites have microporous crystalline structures that can trap molecules in a three-dimensional structure (Babu et al. 2021). When phenolic compounds are removed from the spice matrix, the environment is more favorable for bacterial growth. The FDA study found that the use of the adsorbent, beta zeolite, in the enrichment media allowed researchers to reduce media volumes without compromising on the sensitivity of the efficiency of Salmonella detection in either cinnamon bark or oregano leaves. When compared to the BAM method, there was no significant difference in the minimum level of detection between the two methods.
ASTA has summarized best practices related to testing of Salmonella in spices. Further research with beta zeolite is needed to adhere to best practices. This would include evaluation of the many other Salmonella serovars in combination with different spices and their cultivars. Third-party analytical labs may run validation studies on a case-by-case basis at the customer’s request.
According to experts on ASTA’s Microbiology Task Force, alternative methods have been studied for decades but they note “inherent limitations such as chemical incompatibility and lack of penetration cannot be engineered away.” Rather than surrender, the task force continues to advocate for research, validation, and education for a safe and spicy tomorrow. For more information on spice microbial safety, refer to the ASTA guidance.