Consumers are becoming more aware of the importance of eating fresh and fresh-cut fruits and vegetables. However, these minimally processed foods have repeatedly become a source of foodborne illnesses in the United States. Escherichia coli O157:H7, Salmonella spp. and Listeria spp. are a public health concern, since these microorganisms have been associated with foodborne outbreaks from consumption of cantaloupes, sprouts, spinach, lettuce and bagged salad mixes.

Recent studies indicate that pathogenic microorganisms may internalize into the core of leafy vegetables rather than contaminate the exposed surface only. This bacterial mobility makes surface treatments to reduce pathogens very ineffective. Most of the commercially used interventions employ chemical agents, such as washing with 2% chlorinated water, which cannot either wash these pathogens off the produce or inactivate them. Another side effect of this treatment is detrimental effects on the organoleptic properties of the food.

Since current production and processing practices cannot ensure pathogen-free fresh and fresh-cut produce and with almost 25% of food production after harvest in the United States lost due to damage caused by bacteria, mold, insects and contamination with spoilage microorganisms, effective food safety interventions are needed for implementation throughout the production, processing and distribution of these food items. Development of alternative pathogen decontamination technologies would certainly improve the safety of ready-to-eat and fresh agricultural products.

Because thermal processing of fresh produce is not an option, non-thermal interventions are the only means to include as a lethality step in processing and handling of fresh fruits and vegetables. Technologies include high-pressure processing, radio frequency, pulsed-electric fields, ultrasound, irradiation and others. Treatment of fresh produce using ionizing radiation has a significant strategic importance for the future of food safety worldwide. This is simply because it is the most researched non-thermal food process technology and has been proven that it is safe (when done properly). Yet, despite the impressive advances in irradiation methods available, the technology must still be optimized for application to all types of fresh and/or minimally processed fruits and vegetables.

Proper irradiation of fresh produce requires an engineering approach. In short, it involves an understanding and quantification of how great a dose is applied to the food, how uniform is the distribution of dose with respect to the food and what is the effect (if any) on the quality of the food when a target inactivation dose is applied. For instance, if irradiation treatment is not carried out properly, a bag of baby spinach leaves or lettuce may not receive the dose in a uniform manner, leaving some parts of the food untreated. The parts of the food exposed to doses lower than the inactivation dose may allow the pathogens to grow with the potential of foodborne illness if consumed. An engineering approach deals with understanding, modeling and predicting the irradiation dose distribution for food safety applications, that is, effective pathogen decontamination while ensuring product integrity and quality. This approach provides reliable methods to predict dose delivery to ensure that fresh fruits and vegetables are exposed to the dose necessary to inactivate the pathogenic microorganisms.

Food Irradiation
Food irradiation is the exposure of the food, either packaged or in bulk, to controlled amounts of ionizing radiation for a specific time to achieve a specific amount of inactivation of pathogens. The word “controlled” is very important because the irradiation treatment requires rigorous process control to ensure that the dose delivered to all parts of the food falls within some target range. Irradiation can be accomplished using gamma rays, X-rays and high-energy electrons (e-beams). Gamma rays are produced from a nuclear source (cobalt-60 and cesium-137) and have excellent penetration capabilities. X-rays and electron beams are produced from accelerators, which are powered by electricity. X-rays are electromagnetic radiation produced when energetic electrons hit a target and are emitted by a heated cathode whose potential may be approximately 30 to 50 kV above the target (made of a material such as tungsten or molybdenum). Although X-rays are photons with better penetration capabilities, electron beams provide high efficiency (higher dose rate) and high throughput, and the system has switch-on/switch-off capability. A linear accelerator consists of a conveyor or cart system where the product to be irradiated moves through the electron beam at a predetermined speed to obtain the desired dosage. This article focuses on the recent advances in electron beam irradiation of fresh produce achieved using an engineering approach.

How Does Irradiation Work?
Living cells lose their biological function when exposed to ionizing radiation mainly by breaking or damaging their DNA or by interactions with active radicals such as the products of water radiolysis. One way to quantify the effectiveness of a specific irradiation treatment is the D{10} value, the amount of radiation necessary to achieve a 90% (one-log) reduction of the initial population for a target pathogen in a particular product. The D{10} value varies with the specific pathogen, food temperature, other environmental factors and knowledge of the dose. In general, food irradiation treatments are designed for a five-log reduction of the initial population of pathogens.

Dose and Dosimetry
The primary physical quantity used in dosimetry is the absorbed dose, defined as the energy absorbed per unit mass from any kind of ionizing radiation in any target. The unit dose commonly used is the Gray (Gy) or Joules per kilogram. The older unit radian (rad) is defined as 100 erg/g, and 1.0 Gy is equal to 100 rads.

Food irradiators must be designed to yield an absorbed dose in the product within the minimum and maximum limits in accordance with process specifications and government regulatory requirements. The actual minimum dose, D[min], and maximum dose, D[max], as measured in the product, must be within these limits. The dose uniformity ratio is defined as D[max]/D[min], a concept used by irradiator designers and food scientists and engineers. For research applications, this ratio should be as close to 1 as possible, that is, the dose should be very uniform in a small sample so that experimental results can clearly demonstrate the dose-effect relationship. For industrial applications where large process loads are irradiated, wider dose variation is unavoidable, and a vast portion of the food item will receive significantly greater than the minimum absorbed dose required for safety, sometimes as much as twice the inactivation dose. Although a low dose uniformity ratio less or equal to 1.5 is the main goal in food irradiation facilities, many food product applications can tolerate a higher dose uniformity ratio of 2 or even 3.

Understanding how the electrons transport within the foods enables us to predict where the radiation deposits energy in the food. The correct procedure for food irradiation processing depends largely on accurate and reproducible measurement of radiation quantities. Consequently, developments of accurate dose calculation methodology are critical for the production of more high quality and safe irradiated fresh produce.

Dosimeters, in radiation research and commercial processing, are used for quality and process control. In food irradiation applications, doses are measured either with alanine or radiochromic film dosimeters placed at the sample’s surface. However, these dosimeters do not properly fit the irregular and complex shapes of some fresh produce and can alter the absorption of the mono-energetic radiation energy, possibly introducing variability in the absorbed dose. The energy distribution in a fresh product is strongly related to the electron’s entrance region at the surface of the product and the sample’s density.

Thus, inaccurate interpretation of the measured dose can result in incorrect D[10] values. To make things a little more complicated, there are no dosimeters available for measuring doses in the internal regions of the produce. The engineering approach allows for an accurate calculation of the dose distribution data within the food product using Monte Carlo methods.

Dose Calculation
Monte Carlo techniques use random input to obtain a result. The random nature of electron beams makes them very suitable for this purpose. Even though several radiation transport computer codes have been developed and adapted for dose distribution calculations in radiation processing, they do not properly account for the complex three-dimensional (3D) structure and non-homogeneity of foods (e.g., density differences including air pockets). The main difficulty in applying those codes for complex-shaped foods lies in obtaining the actual product geometry and density values, which are critical factors in the evaluation of electron/photon interactions. The combination of computed tomography (CT) scans, which yield the geometrical and density information of the food item, and Monte Carlo simulation provides detailed and high-resolution dose maps for complex-shaped foods. This methodology has been extensively validated with experimental measurements and has proven to be an excellent tool to design irradiation protocols for fresh produce.

Fresh produce is generally very susceptible to overdosage that can result in tissue damage and affect its quality attributes, such as color, flavor, aroma and texture. Therefore, its dose distribution should be as uniform as possible and high enough to inactivate pathogens without deteriorating quality.

Figure 1,[1] for example, illustrates the procedure used to calculate the dose distribution in a whole cantaloupe.

The CT scan of the product reveals the different densities in the fruit, including the air cavity. The number of image slices produced by the CT scan is used to construct a 3D image of the cantaloupe. Finally, a computer simulation is used to calculate the dose distribution using photon and electron transport codes based on nuclear physics (e.g., Monte Carlo).

Because of its shape (round) and composition (air cavity and peel versus flesh), a precise design of the irradiation process is required to effectively treat the cantaloupe. Electron scattering, dose build-up and/or overdoses are some examples of problems encountered when irradiating whole cantaloupes. Particle transport codes like Monte Carlo help in visualizing the dose distribution from different irradiation sources so that the best irradiation protocol can be designed. Rotation of the samples around the beam, dual-mode radiation sources or high-efficiency X-ray sources are examples of solutions to make the process more efficient.

Another challenging situation is irradiating produce in bulk. For instance, irradiation of a tray of blueberries (Figure 2)[2] can be very complicated because of the large differences in densities inside of the trays, that is, in the air pockets and the fruits themselves. The random distribution of the berries around the air pockets can result in a large dose uniformity ratio inside the tray (remember that the goal is a uniformity ratio ≤ 1.5). This aspect of the dose distribution is not detected using traditional dosimeters, which are generally placed outside the tray (bottom and top) and can result in incorrect irradiation planning with the consequent effect of shorter produce shelf life. Product shelf life can be defined as the period of time when the product has acceptable nutritional and quality attributes. By the end of this time, the product is either spoiled or of poor quality.

Even more challenging is the irradiation of leafy vegetables (lettuce, spinach, cilantro, arugula, etc.) that requires very precise dosimetry to reduce tissue damage. Most of the high-energy (10 MeV) electron beam accelerators commercially available are not suitable for fresh produce because these units were designed for irradiation of meat (particularly ground beef patties). A hamburger patty has a simple shape (very short cylinder) and is more uniform than a bag of lettuce. In addition, the hamburger patty requires higher doses (up to 7 kGy) to process. The maximum approved dose for processing of lettuce is 4 kGy. Thus, irradiation treatment of the lettuce using the 10-MeV accelerators is complicated, and passing the bagged lettuce in front of the accelerator beam through a conveyor at a specified speed (which is what is done with the box of hamburger patties) will not be sufficient to uniformly treat the entire bag. Thus, careful placement and rotation of the bag should be taken into account to ensure that all the produce has received the same dose. Gamma-ray facilities for irradiation of fresh produce may be very expensive because of low throughput.

This technology can be easily applied to other foods such as fresh eggs. Just recently, a large nationwide recall of fresh shell eggs occurred due to contamination with Salmonella. A recent study evaluated the possibility of irradiating fresh eggs using electron beam accelerators and the simulation approach. Interesting enough, it was found that to irradiate the entire egg, two-sided, high-energy e-beam (10 MeV) sources are required for an efficient (i.e., uniform dose throughout the entire egg) treatment. Unless the egg is rotated in front of the source (the electron beam), the dose uniformity ratio for gamma or X-rays is inadequate for shell egg treatment for pathogen decontamination purposes.

Overall, the best treatment planning by irradiation for fresh and fresh-cut fruits and vegetables will require not only the proper irradiation source (electron beam vs. X-rays vs. gamma rays), but also innovative packaging systems that could help provide radiosensitization to enhance the killing effect of irradiation without degrading their organoleptic characteristics. Radiosensitization strategies refer to means to increase the sensitivity of a specific pathogen to ionizing radiation. This approach is significant because it allows for reduction of the applied dose that enhances the probability of minimizing the effect of irradiation on the quality of the product. Radiosensitization strategies include the use of antimicrobials in films, modified atmosphere packaging, ozonation and others. The engineering approach allows for development of models to determine, for example, the rate of delivery of an antimicrobial when exposed to ionizing radiation; the duration of ozone exposure; the synergistic effect of temperature and ionizing radiation; and so on. Many engineering parameters must still be investigated, and this is the current focus of many research laboratories.

Conclusions
Through good irradiation planning based on an engineering approach, improved shelf life and quality of fresh produce will significantly reduce economic losses from spoilage. Additionally, if consumers begin to realize that the safety of their fresh produce has increased because of the use of accurate dose calculations in irradiation treatments, their perception of irradiated food may change in a positive way.

Irradiation technology will help ensure that the U.S. will have a safe and plentiful supply of food. The contribution of engineers to this technology has been significant because the prediction of absorption and the actual visualization of the dose distribution within the treated produce has reduced the uncertainty in the treatment of a vast majority of products. In addition, research on radiosensitization strategies may help reduce the dose required to inactivate a pathogen with obvious benefits to the quality of fresh produce.

Elena Castell-Perez, Ph.D. is a professor of food engineering at the Department of Biological Engineering at Texas A&M University. She has experience in the areas of produce quality, shelf life testing and use of packaging materials to ensure the safety of irradiated fresh produce.

Rosana G. Moreira, Ph.D. is a professor of food engineering at the Department of Biological Engineering at Texas A&M University. She has worked in the area of food safety of fresh produce since 2002 with emphasis on dosimetry, Monte Carlo simulation applications to food safety and process control. Other aspects of her research include evaluation of different frying methods and their effect on acrylamide content.

References
1. Kim, J., R. G. Moreira and M. E. Castell-Perez. 2010. Simulation of Pathogen Inactivation in Whole and Fresh-cut Cantaloupe (Cucumis melo) Using Electron Beam Treatment. J Food Eng 97:425-433.

2. Moreno, M. A., M. E. Castell-Perez, C. Gomes, P. F. Da Silva, J. Kim and R. G. Moreira. 2008. Treatment of Cultivated Highbush Blueberries (Vaccinium corymbosum L.) with Electron Beam Irradiation: Dosimetry and Product Quality. J Food Proc Eng 31:155-172.

Suggested Reading
Gomes, C., P. Da Silva, R. G. Moreira, E. Castell-Perez, E. A. Ellis and M. Pendleton. 2009. Understanding E. coli Internalization in Lettuce Leaves for Optimization of Irradiation Treatment. Int J Food Microbiol 135:238-247.

Gomes, C., P. Da Silva, E. Chimbombi, J. Kim, M. E. Castell-Perez and R. G. Moreira. 2008. Electron-beam Irradiation of Fresh Broccoli Heads (Brassica oleracea L. Italica). Food Sci Technol 41:1828-1833.

Gomes, C., R. G. Moreira, M. E. Castell-Perez, J. Kim, P. Da Silva and A. Castillo. 2007. E-beam Irradiation of Bagged Ready-to-eat Spinach Leaves (Spinacea oleracea): An Engineering Approach. J Food Sci 73:E95-E102.

Kim, J., R. G. Moreira and M. E. Castell-Perez. 2010. Optimizing Irradiation Treatment of Shell Eggs Using Simulation. J Food Sci in press.

Kim, K., R. G. Moreira, Y. Huang and M.E. Castell-Perez. 2007. 3-D Dose Distributions for Optimum Radiation Treatment Planning of Complex Foods. J Food Eng 79:312-321.

Kim, J., R. G. Moreira, R. Rivadeneira and M. E. Castell-Perez. 2006. Monte Carlo-based Food Irradiation Simulator. J Food Proc Eng 29:72-88.

Kim, J., R. Rivadeneira, M. E. Castell-Perez and R. G. Moreira. 2006. Development and Validation of a Methodology for Dose Calculation in Electron beam Irradiation of Complex-shaped Foods. J Food Eng 74:359-369.