Low-energy electron beam (LEEB) technology was validated in 2020, demonstrating a high level of reduction of Salmonella spp. from spices at industrially relevant throughputs. A letter of no objection from the U.S. Food and Drug Administration (FDA) was recently granted to the breakthrough application. What is it and how does it solve the rising issues of microbial contamination in dry foods?

Microbiological Quality of Dry Foods

Globalization and new consumer trends, demanding fresh, high-quality, minimally processed, ready-to-eat (RTE) foods, have resulted in high industry standards and strict regulations for the food safety of low-moisture foods.¹ Fatal foodborne-illness outbreaks, costly lawsuits, and recalls are an issue not only for perishable, high-water-activity RTE products but also for their ingredients, such as spices, herbs, seeds, and grains. The microbial levels on dry food matrices are often extremely high due to potentially low hygiene levels during their growth, harvest, storage, and processing, especially if they include steps such as fermentation or sun drying. Salmonella, Bacillus spp. (including Bacillus cereus), Clostridium perfringens, Shigella, Staphylococcus aureus, as well as insect parts and excrement are among the main pathogens and filth adulterants found in spices.² 

Technologies that traditionally tackle such contamination in dry foods are based on treatments using steam (thermal treatment), chemicals (e.g., ethylene oxide), and irradiation. These technologies can sufficiently reduce vegetative pathogens but may be associated with drawbacks such as deterioration of food quality, low efficiency of spore inactivation, high costs, and potential risks to the environment. Thermal treatment introduces water to the dry food facility and to the product, which may cause issues such as increased risk of growth of Listeria or mold after the treatment has been applied. Chemical treatment may leave harmful residues on the end product, alter food quality, and pose a hazard for the operators’ health and safety. Gamma irradiation treatment is outsourced due to the strict safety measures and large footprint of the equipment, which results in lack of control over the product, treatment capacities, and costs. To control bacterial spores, the food industry has to apply more intensive decontamination processing steps (e.g., thermal sterilization) alone or in combination with water activity and pH reduction. Many novel microbial reduction technologies, such as high-pressure processing or UV-C light, are suitable for use on high-moisture foods but may not be effective enough in reducing pathogens on low-moisture foods.³,⁴

“Globalization and new consumer trends…have resulted in higher industry standards and stricter regulations for the food safety of low-moisture foods.”

“Irradiation treatment has been used in food processing for decades to eliminate harmful microorganisms, control infestation by insects, or delay ripening and prevent sprouting of produce.”

Industrial Application of LEEB 

According to the Food Safety Modernization Act, every process when introduced into the production as a process preventive control step must be validated and incorporated into a food safety plan at the facility. This means that compelling evidence must be provided that the process constantly delivers the treatment target, for example, the delivery of an irradiation dose that causes a 5-log₁₀ reduction of Salmonella spp. or equivalent in black peppercorns at industrial throughput. LEEB technology was, until recently, studied mainly in regard to the reduction of natural contamination, which does not provide sufficient evidence for validation.¹⁷ In addition, the approaches of measuring the absorbed dose were not provided. Without evidence to link the achieved inactivation of the target pathogen to precise dosimetric values, there was no way to validate the process. Recent developments in both the research of the behavior of vegetative pathogens and thermoresistant bacterial spores in LEEB¹⁸⁻²⁰ and the establishment of approaches toward the dosimetry measurements enabled this technology to be utilized as a process preventive control for whole spices. 

When introducing a new pathogen-reduction technology to the market, the confirmation and validation of food safety is paramount, followed by considerations about microbial stability, process capability, and product chemistry. Regulatory agencies must be provided with evidence that the product is meeting safety requirements with a high level of confidence. In the recently validated LEEB processing unit, loose granular product is exposed to a beam of low-energy electrons without protective packaging. To achieve industrially relevant throughputs, it is treated in free fall, between two electron beam lamps (Figure 1). The spice seeds fall spatially separated from one another to provide for a homogenous and sufficient treatment during the short exposure. The irradiation dose that the food surface receives within several milliseconds is around 10–15 kGy. This is sufficient to cause a 5-log₁₀ reduction of Salmonella or equivalent. After the treatment, the material can be fed directly to the final packaging to prevent cross-contamination. Critical processing parameters, such as beam energy and beam current, are recorded, assessed against their limits, and logged at frequent intervals on the device. This information is immediately accessible for the quality manager and regulatory bodies. The in-house solution with continuous process monitoring minimizes potential for food waste, food safety risk, and recalls.²¹,²²

Measurement of Dose

Irradiation dose is one of the critical control parameters in LEEB. A dose that is too small means that the foodstuff is undertreated, while a dose that is too large would exceed the regulatory limits for food irradiation.²³ The absorbed dose is measured by radiochromic B3 film, following the guidelines and practices for irradiation dosimetry applications according to ISO/ASTM 51818²⁴ and ISO/ASTM 51275:2013.²⁵ B3 film is a preferred dosimetry system for LEEB due to its thinness, which can record the penetrability of the beams. A dosimeter the size of a grain was developed. A special scanner can quantify and relate the absorbance in the optical spectrum of the film to the absorbed dose in the simulated grain.²⁶ Grain speed measurements together with Monte Carlo simulations and statistical analysis are used to quantify the absorbed doses.

Effects of LEEB on Microbial Control 

The  efficacy of the LEEB technology was tested on black peppercorns with three types of contamination: natural bio-burden, nonpathogenic surrogate for spores of B. cereus (thermoresistant spores of Geobacillus stearothermophilus ATCC 7953), and nonpathogenic surrogate for Salmonella (Enterococcus faecium NRRL B-2354). The tests revealed that natural contamination can be reduced from 8-log₁₀ to 4-log₁₀ in LEEB-treated samples. Naturally present aerobic spores were reduced more than 3-logs and anaerobes more than 4-logs. Yeasts and molds were reduced from 3-log₁₀ initially to less than 1-log₁₀. LEEB-treated pepper was microbiologically stable due to the absence of water and chemicals during the process. LEEB treatment of black peppercorns inoculated with thermoresistant spores of G. stearothermophilus and B. subtilis resulted in noticeable reductions of 4-log₁₀ and 3.5-log₁₀, respectively. Moreover, the extensive validation study has proven that the technology can deliver the doses that reduce Salmonella by 5-log₁₀ (>99.999%) in black peppercorns. For the validation experiments, E. faecium was selected as a nonpathogenic surrogate organism to avoid contamination risks in industrial plants.²¹ 

Effects of LEEB on Food Quality Parameters

Every decontamination process has an impact on the food material it treats. What differs among the technologies is the magnitude of the changes. Exposure to temperature and oxidizing agents is an important consideration in production of fresher, minimally processed products. Thermally sensitive vitamins, enzyme activity, and other bioactive constituents are among the most sensitive indicators of the decontamination process. As a nonthermal and surface treatment, LEEB has a minimal impact on the quality attributes of foods, such as aroma and taste. Kotilainen et al.²⁷ studied the impact of different treatments on the quality properties of spices. The study compared the effects of LEEB and the effects of steam treatment on the moisture, color, essential oil yield, and volatile compounds in black peppercorn and coriander seeds. It was observed that the treatment with LEEB can preserve all studied parameters better than steam treatment. 

Profound evidence of quality preservation was obtained with mung beans (Vigna radiata), specifically for the preservation of embryo and enzymes responsible for sprouting (Figure 2). Mung bean seeds were treated with HEEB and LEEB prior to germination. Since the low-energy electrons dissipated before they reached the embryo of the seed, the embryo remained undamaged, and the seed was able to germinate. Seeds treated with HEEB were unable to germinate, as the electrons penetrated through the seed and negatively impacted the embryo.²⁷ In 2003, Kikuchi et al.¹⁴ recommended LEEB treatment over gamma irradiation for soybean decontamination because it induced minimal or no quality deterioration, since the electrons do not reach the internal matrix.

Effects of LEEB on Pest Control

LEEB treatment is also a viable alternative to fumigation as a pre- or poststorage disinfestation treatment for control of grain pests. Low doses of irradiation must reach the part of the grain where the immature life stages (eggs, larvae, pupae) reside. Tests where rice grains were artificially infested with two rice pests (Rhyzopertha dominica and Sitophilus oryzae) resulted in a promising outcome. While a significant number of adults developed in the untreated transport and processing of control samples, no development was observed in any of the LEEB-treated samples. This indicates that the applied doses were sufficient to reduce the two target insects developing inside rice grains.¹⁶

Conclusion

LEEB technology is a validated microbial control solution that sufficiently eliminates human pathogenic bacteria in dry foodstuffs, including thermoresistant bacterial spores, while preserving the natural richness of the food. LEEB offers benefits such as being chemical-free, water-free, radioactive material-free, in-house, compact, and agile, with 24/7 availability. The letter of no objection from FDA has confirmed the readiness of this technology to provide an attractive alternative to conventional solutions with a state-of-the-art pathogen contamination control approach for the 21st century.