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Simulation of pathogen inactivation in whole and fresh-cut cantaloupe (Cucumis melo) using electron beam treatment
Journal of Food Engineering 97 (2010) 425–433
Contents lists available at ScienceDirect
Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Simulation of pathogen inactivation in whole and fresh-cut cantaloupe (Cucumis melo) using electron beam treatment Jongsoon Kim, Rosana Moreira *, Elena Castell-Perez Department of Biological and Agricultural Engineering, Texas A&M University, College Station, TX 77843-2117, United States
a r t i c l e
i n f o
Article history: Received 7 August 2009 Received in revised form 10 October 2009 Accepted 25 October 2009 Available online 29 October 2009 Keywords: Dosimetry Safety e-Beam Irradiation
a b s t r a c t Cantaloupes (Cucumis melo) have been implicated in several recent Salmonella outbreaks. Electron beam irradiation at the required dose can effectively inhibit pathogens while maintaining produce quality. The objective of this study was to evaluate pathogen inactivation in cantaloupes for optimization of electron beam treatment using dose distributions from Monte Carlo simulation and computed tomography (CT) scan data. 3D geometry and component densities of whole cantaloupes and fresh-cut cantaloupes packed in plastic trays, extracted from CT-scan data, were entered into a radiation transport code (MCNP5) to simulate dose distributions. Lucite (4.2 cm) was used as an electron absorber when simulating high energy (10 MeV) irradiation to make the penetration depth similar to the lower energy (1.35 MeV) electron simulations. For surface irradiation of the whole cantaloupe with a 1.35 MeV e-beam source, the penetration depth was 0.7 cm, well beyond the cantaloupe rind. For an entrance dose of 1 kGy, the log-reductions for Salmonella (D10-value of 0.359) were 3.30 ± 0.43 and 3.58 ± 0.58 at the depths of 0.2 and 0.4 cm, respectively. The dose uniformity ratio (Dmax/Dmin) up to 0.2 cm was improved from 1.81 to 1.15 with one beam rotation around the fruit. Surface irradiation using a 10 MeV e-beam source showed far less microbial reduction, only 1.58 ± 0.93 and 1.32 ± 0.84 at 0.2 and 0.4 cm, respectively. Irradiation of fresh-cut cantaloupe with a 10 MeV e-beam source showed that the maximum dose of 1.1 kGy occurred at the 3.5 cm depth and the minimum dose of 0.81 kGy took place at 5.5 cm depth, with an average 2.99 ± 0.79 log reduction around the whole package. These results provide valuable information for optimizing irradiation treatment planning for fresh produce. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Since current production and processing practices cannot ensure pathogen-free fresh and fresh-cut produce, effective food safety interventions are needed for implementation throughout the production, processing, and distribution of these foods (Doyle and Erickson, 2008). Non-thermal interventions with minimal effects in quality deterioration and capable of maintaining the intrinsic organoleptic profiles of produce are the only alternatives currently available to include as a lethality step in fresh and fresh-cut produce processing. On August 22, 2008, the Food and Drug Administration (FDA) published a final rule that allows the use of ionizing radiation (IR) to make fresh iceberg lettuce and fresh spinach safer and last longer without spoiling. Therefore, proper treatment protocols are needed for effective inactivation of pathogens in fresh and fresh-cut produce.
* Corresponding author. Tel.: +1 979 847 8794; fax: +1 979 845 3932. E-mail address: [email protected] (R. Moreira). 0260-8774/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2009.10.038
The main assets of IR include its high lethality, unmatched penetration into the product core, cold application, capabilities to be applied in a conveyor belt, and it has been proven as safe for treating food products (Miller, 2005). Whenever irradiation is used to treat a fresh produce, a compromise must be achieved between the quality of the treatment (i.e., decontamination, surface pasteurization, disinfestation) and the radiation dose received by the produce. However, not all microorganisms are equally sensitive to radiation. The challenge is how to reduce the damage to produce quality while applying sufficient doses of radiation to inactivate the pathogen. Correct food irradiation depends on accurate and reproducible measurement of dose (Kim et al., 2005, 2006, 2007, 2008; Gomes et al., 2008a,b). Problems with dosimetry for fresh produce include accuracy and uniformity of dose measured all over the product; and lack of standards to validate the measurements. Inaccurate interpretation of dose measurements can result in misleading D10-values for a target pathogen in a particular produce. Overdosing the produce is costly and under-dosing has serious safety implications.
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Palekar et al. (2004) showed that e-beam irradiation of cantaloupe slices resulted in a reduction in the total aerobic microbial counts with increasing doses. Across all doses of irradiation, counts were consistently lower for cantaloupe pieces obtained from melons that had been subjected to chlorine rinse in comparison with those washed with water without chlorine. Castell-Perez et al. (2004) showed that irradiation of cantaloupes, as whole fruits with dose up to 1.0 kGy, caused no significant changes on the fruit’s physical and nutritional quality attributes. Irradiating at higher doses had an undesirable effect on product quality. The authors indicated that the fresh-cut packaged cantaloupe may be irradiated up to 1.5 kGy without worsening the product quality attributes.
Fig. 1. Simulation set-up for e-beam irradiation of a whole cantaloupe with a 7.3 cm radius (a) 1.35 MeV radiation source (single upper mode); (b) 10 MeV radiation source (single upper mode) with a 4.2 cm thick Lucite shield (14.6 cm 14.6 cm 4.2 cm).
A combination of hot-water pasteurization of whole cantaloupe and low-dose irradiation of packaged fresh-cut melon showed reduced population of native microflora while maintaining their quality (Fan et al., 2006). Boyton et al. (2006) studied the combination of e-beam irradiation of fresh-cut cantaloupe in modified atmosphere packaging (MAP) and showed that the produce shelf life extended to about 2–3 weeks. The main assets of e-beam technology for treating fresh produce include its high lethality, low quality detrimental effects, unmatched penetration into the product core, cold application, capabilities to be applied in a conveyor belt, and tested safety of the intervention technology (Miller, 2005). However, whenever e-beams are used to irradiate a food product, a compromise must be achieved between the quality of the treatment (i.e., decontamination, surface pasteurization) and the radiation dose received by the food. Additionally, not all pathogens are equally sensitive to radiation. A microorganism that is radio-resistant may be treated with higher dose and the consequent side effects are unavoidable. At the optimal point, the lowest dose is applied that can still have
Fig. 2. Simulation set-up for e-beam irradiation of fresh-cut cantaloupe in a plastic tray (a) 10 MeV radiation source (single lower mode); (b) product sample configuration used in this study (container size – 13.5 cm 11.5 cm 6.0 cm).
Fig. 3. CT-scan and imaging technique to reconstruct a 3D image of a whole cantaloupe.
J. Kim et al. / Journal of Food Engineering 97 (2010) 425–433
the same effect: maximizing the dose to the pathogens while minimizing the loss of produce quality. The correct food irradiation process depends to a large extent on accurate and reproducible measurement of radiation dose, i.e., the amount of energy absorbed per unit of mass. However, the methods used to measure dose distribution in food materials have limitations, including the assumptions that the product is uniform and that the reading of a finite size dosimeter is representative of dose in the product. Therefore, if dosimetry is faulty, the resulting dose will be wrong. An alternative to dose measurement is the calculation of absorbed dose distribution using mathematical models coupled with computer simulation (Kim et al., 2005, 2006, 2007, 2008). Nevertheless, the methodology must be optimized for use with many of the fresh fruits and vegetables, which have complex shapes and are highly non-homogeneous with respect to their 3D mass distribution. In the irradiation of very heterogeneous objects, particularly fresh-cut items, one will encounter air pockets within the package as a matter of course. These pockets will cause variations in density of up to three orders of magnitude (Tutt, 2007). The objective of this work was to evaluate pathogen inactivation in whole and fresh-cut cantaloupe for optimization of electron beam treatment using dose distributions from Monte Carlo simulation and computed tomography (CT) scan data.
in packed plastic tray (polypropylene clam-shell type – 13.5 cm 11.5 cm 6.0 cm) with (a) a 10-MeV e-beam linear accelerator (LINAC) and (b) a lower e energy source (Van der Graaf-1.35 MeV) at a dose level of 1 kGy. For the whole fruit, a Lucite block (14.6 cm 14.6 cm 4.2 cm) was used as an electron absorber to simulate the high energy (LINAC-10 MeV) irradiation to make the penetration depth similar to the lower energy source (Van der Graaf-1.35 MeV) electron simulations (Fig. 1). For the fresh-cut fruit, the radiation source (LINAC-10 MeV) was simulated
2. Materials and methods Monte Carlo simulation: the MCNP-5 software (Monte Carlo NParticle – Version 5) used in this study was developed at the Los Alamos National Laboratory (Radiation Safety Information Computational Center (RSICC), Oak Ridge National Laboratory, Oak Ridge, TN). The simulator was run in parallel computer platform (DellTM PowerEdgeTM 6650, 4 CPU) located at the Department of Biological and Agricultural Engineering, Texas A&M University. Irradiation studies: simulation was done of irradiation treatment of a whole cantaloupe (7.2 cm radius) and fresh-cut cantaloupes
427
Fig. 5. CT-scan of a whole cantaloupe showing density distribution.
Fig. 4. CT-scan and imaging technique to reconstruct a 3D image of fresh-cut cantaloupe pieces in a plastic tray.
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Fig. 6. Monte Carlo simulation of dose distribution at the surface of whole cantaloupe irradiated with e-beam source (single upper mode) for (a) 1.35 MeV; and (b) 10 MeV.
Fig. 7. Average dose distribution simulation from entrance versus penetration depth from e-beam source (single top beam mode) energies of (a) 1.35 MeV; and (b) 10 MeV.
Fig. 8. Monte Carlo simulation of energy distribution in a whole cantaloupe (10 MeV e-beam source; single bottom mode).
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Fig. 11. Average dose distribution simulation from entrance (bottom of tray) to the top of a fresh-cut tray for a 10 MeV e-beam radiation source.
Fig. 9. Monte Carlo simulation of dose distribution at the surface of a whole cantaloupe for 1.35 MeV e-beam source (single upper mode), showing results for one rotation of the target in front of the beam.
as a single beam from the bottom (Fig. 2). The e-beam was described as a parallel plane large enough to cover the target. In the linear accelerator, the electrons were emitted in a plane and distributed evenly within the scan area. A total of 107 histories were used to reduce the statistical uncertainty to about 5% or less (Brown, 2003). A technique using CT-scan and imaging processing was used to create the 3D image of the products (Kim et al., 2007). A 3D geometry and component densities of whole (Fig. 3) and fresh-cut cantaloupes in a plastic tray (Fig. 4), extracted from CT-scan data, were entered into a radiation transport code (MCNP5) to simulate dose distributions. Electron simulation in MCNP: Monte Carlo radiation transport is a simulated track consisting of the geometrical positions of all energy deposition events and the amount of energy deposited at each interaction point. Each electron track starts at a given position, with initial direction and energy. The state of a particle after interaction is defined by its position coordinates, energy, and direction cosines of the direction of flight. An electron path is broken into
many steps to follow an electron through a significant energy loss. These steps are chosen (1) to be long enough to include many collisions so that multiple-scattering theories are valid (major steps) and, (2) to be short enough that mean energy loss in any step is small (sub-steps). The energy loss and angular deflection (Goudsmit and Saunderson, 1940) of the electron during each of steps can then be sampled using probability distributions on the appropriate multiple-scattering theories. It is impractical to model all individual interactions. Instead, well-established statistical theories (condensed history) are used to describe those interactions into single steps. The electron tracks end either by when the electrons leave the target material or when their energy becomes smaller than an energy cutoff (i.e., the energy the particles are assumed to be absorbed in the medium). Log-reduction calculation: we used the experimental D10-value for Salmonella of 0.359 kGy, from Niemira et al. (2006), for the most resistant strain. The microbial survival distribution within samples was simulated using both the ‘‘target theory” approach initially described by Lea (1955) and the single-hit inactivation model to solve the general survival equation (Alpen, 1998), a classical first-order relationship of logarithmic survival on dose. The assumptions for the development of the single-hit inactivation model are (1) the deposition of energy as ionizing or excitation in the critical volume
Fig. 10. Log-reduction of Salmonella for 1.35 MeV e-beam radiation of whole cantaloupe surface (a) at 2 mm depth; (b) at 4 mm depth.
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Fig. 12. 2D view of simulated dose distribution in a fresh-cut cantaloupe tray irradiated with 10 MeV e-beam source (single bottom mode) – slices represent the target depth.
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431
Fig. 12 (continued)
leads to the production of molecular lesions in the cell and thus inactivation of the microorganisms and, (2) a cell will survive only if it has received no hits at all and that it will always die if it has received one or more hits. Thus, the microorganisms survival was calculated as described in Gomes et al. (2008b) as:
S ¼ eD=Do ¼ eD=0:156 ¼ e6:415D
ð1Þ
with
Do ¼
D10 2:303
ð2Þ
with D the dose (kGy), Do the mean lethal dose, defined as the dose required to reduce the survival fraction S to 1/e (i.e., 37%) of the initial value, and D10 = 0.359 kGy. 3. Results For electrons or photons interaction with matter, the density of the product is very important. Energy deposition is very much a function of the product density. The change in densities in a product can affect the way energy is distributed in the product thus affecting quality and safety aspect of the process. For a non-uniform and/or complex shaped produces, if the dose is not accurately predicted around and within each point in the product, changes in density can result is lower energy absorption resulting higher pathogens survival. Fig. 5 shows a CT-scan slice of the whole cantaloupe showing the rind, flesh, and the seed pulp. The density of flesh is almost uniform around 750–900 kg/m3. The seeds and the pulp show slightly lighter density than the flesh. The cavity is showing in black (very low density).
The MCNP simulation of the dose distribution at the surface of the whole cantaloupe irradiated with the 1.35 and 10 MeV e-beam sources is shown in Fig. 6a and b, respectively. The penetration depth was around 0.7 cm, well beyond the cantaloupe rind. The dose distribution covered a larger surface with the 1.35 MeV source (Fig. 6a) than with the 10 MeV e-beam source (Fig. 6b). This is because the Lucite blocks are used to absorb the income energy from the e-beam source when working with higher-energy accelerators so the penetration depth is reduced. Fig. 7 shows the average dose–depth curves for 1.35 MeV ebeam (Fig. 7a) and 10 MeV e-beam (Fig. 7b) where the Lucite absorbed the incoming energy and the energy that reached the cantaloupe surface only penetrated it up to 1 cm in depth (the irradiated area of cantaloupe shows a decreasing slope region at the dose–depth curve). Note that Fig. 7a also shows a buildup region. Generally, in the case of incident electron beams, the buildup region in food materials is due to the progressive cascading of secondary electrons by collisional energy losses (Attix, 1986). This buildup region extends up to a depth of approximately one third to two thirds of the electron range, except when the angle of incidence is greatly increased (away from the normal) or when a scattering material is placed between the source and the irradiated material (IAEA, 2002). If the Lucite block was not used, the results would be much different, as illustrated in Fig. 8. This simulation was conducted with a 10-MeV electron beam source (single bottom mode) to the 3D geometry of the whole cantaloupe in the voxel resolution of 5 0.3 0.3 mm (Kim et al., 2007). Unlike the 1.35-MeV electrons source that provided only surface treatment, the absorbed energy (Fig. 8) covered almost half of the cantaloupe. The largest vertical distance the electrons penetrated in the cantaloupe was 5 cm, significantly deeper than the
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Fig. 13. 3D view of simulated log-reduction distribution of Salmonella in a fresh-cut cantaloupe tray irradiated with 10 MeV e-beam source from bottom (z axis represents the penetration depth of the e-beam source).
penetration depth (0.7 cm) achieved with the 1.35-MeV e-beam source. The energy distribution in the whole cantaloupe is strongly related to the electron’s entrance region at the surface of the product and the sample’s density (Fig. 8, upper left). As the electrons penetrate, their energy is absorbed, even though electrons inherently scatter easily. At the maximum penetration depth, the relative error is relatively large (Fig. 8, upper right) because the number of particles reaching that area is very small. The energy deposition plot (Fig. 8, bottom) represents the energy distribution around the bottom of the cantaloupe. The pixel number at the xaxis was counted from the bottom to the top. Simulation of dose distribution for the whole cantaloupe irradiated with a 1.35 MeV e-beam source, with one rotation of the fruit in front of the beam, improved the dose distribution around the entire fruit surface (Fig. 9). For an entrance dose of 1 kGy, the dose uniformity ratio (Dmax/Dmin) up to the 0.2 cm depth improved from 1.81 to 1.15 with one beam rotation. It was assumed surface contamination of a whole cantaloupe, which means microorganisms are on or just under the rind. Therefore, radiation up to 0.2 cm depth would be enough for surface decontamination of pathogens in a whole cantaloupe. The log-reductions for Salmonella were about 3.30 ± 0.43 and 3.58 ± 0.58 at the depths of 0.2 and 0.4 cm, respectively (Fig. 10). However, when the cantaloupe was irradiated with a 10 MeV source, the pathogen population log-reduction was only 1.58 ± 0.93 and 1.32 ± 0.84, respectively. The dose uniformity ratio of cantaloupe up to 0.2 cm for the 10 MeV source was larger (>2), due to the presence of the Lucite absorber.
Irradiation of fresh-cut cantaloupe with a 10 MeV electron beam source showed a maximum dose of 1.1 kGy at 3.5 cm depth and a minimum dose of 0.8 kGy at 5.5 cm depth into the cubic shaped samples (Fig. 11). Fig. 12 shows a 2D view of the dose distribution within the fresh-cut cantaloupe tray. This clearly illustrates the non-uniformity of the energy deposited in the samples in the x, y, and z directions. Due to the large variation in the dose distribution within the sample, the log-reduction for Salmonella was about 2.99 ± 0.79. The doses values at the edge of the cantaloupe pieces were 19.4% higher than at the middle region because of electron scattering (Fig. 13). Because of the non-uniformity in dose distribution in all directions around the samples, it would be better to irradiate fresh-cut produces with spherical or even conical shape than cubic or cylindrical cuts, to reduce electron scattering around the sample edges. 4. Conclusions Because of the difficulty of measuring dose inside of fresh produce, the only approach possible is to use mathematical simulation. Monte Carlo method is currently the most accurate procedure for dose calculation in electron beams. When comparing irradiation treatment of whole cantaloupes with 1.35 MeV to a 10 MeV e-beam sources, the treatment with the lower energy can more effectively inactivate pathogens located at the surface (by or right below the rind) than the higher-energy treatment. Around 3.3–3.6 logs of a resistant strain of Salmonella can be reduced with a 1.35 MeV source compared to only 1.3–
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1.6 logs reduction achieved using the 10 MeV e-beam. Therefore, when using high energy accelerators to treat the surfaces of fresh produces, careful planning is needed since energy absorbers are required to reduce the entrance e-beam energy to the produce. This requires a fundamental understanding of e-beam interaction with the target produce. Irradiation of fresh-cut cantaloupes in trays with a 10 MeV ebeam source showed that the maximum dose of 1.1 kGy occurred at 3.5 cm depth and the minimum dose of 0.81 kGy took place at 5.5 cm depth, with an average 2.99 ± 0.79 log reduction around the whole package. The dose distribution was very affected by the product shapes, showing large energy absorbed at the product edges than at the middle. Therefore, to reduce electrons scattering, it would be better to irradiate fresh-cut produces with spherical or even conical shape than cubic or cylindrical cuts.
References Alpen, E.L., 1998. Radiation Biophysics, second ed. Academic Press, New York. Attix, F.H., 1986. Introduction to Radiological Physics and Radiation Dosimetry. Wiley-Interscience Publication, New York. Boyton, B.B., Welt, B.A., Sims, C.A., Balaban, M.A., Brecht, J.K., Marshall, M.R., 2006. Effects of low-dose electron beam irradiation on respiration, microbiology, texture, color, and sensory characteristics of fresh-cut cantaloupe stored in modified-atmosphere packages. Journal of Food Science 71, S149–S155. Brown, F.B., 2003. MCNP – A General Monte Carlo N-Particle Transport Code, version 5, LA-UR-03-1987. Los Alamos National Laboratory, Los Alamos, New Mexico. Castell-Perez, M.E., Moreno, M., Rodriguez, O., Moreira, R.G., 2004. Electron beam irradiation treatment of cantaloupes: effect on product quality. Food Science and Technology International 10, 383–390. Doyle, M.P., Erickson, M.C., 2008. Summer meeting 2007 – the problems with fresh produce: an overview. Journal of Applied Microbiology 105, 317–330.
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Fan, X., Annous, B.A., Sokorai, K.J.B., Burke, A., Mattheis, J.P., 2006. Combination of hot-water surface pasteurization of whole fruit and low-dose gamma irradiation of fresh-cut cantaloupe. Journal of Food Protection 69, 912–919. Gomes, C., Da Silva, P., Chimbombi, E., Kim, J., Castell-Perez, M.E., Moreira, R.G., 2008a. Electron-beam irradiation of fresh broccoli heads (Brassica oleracea L. italica). LWT – Food Science and Technology 41, 1828–1833. Gomes, C., Moreira, R.G., Castell-Perez, M.E., Kim, J., Da Silva, P., Castillo, A., 2008b. E-beam irradiation of bagged, ready-to-eat spinach leaves (Spinacea oleracea): an engineering approach. Journal of Food Science 73, E95–E102. Goudsmit, S., Saunderson, J.L., 1940. Multiple scattering electrons. Physical Review 57, 24–29. IAEA, 2002. Dosimetry for Food Irradiation. Technical Reports Series 409. International Atomic Energy Agency, Vienna, Austria. Kim, J., Moreira, R.G., Rivadeneira, R., Castell-Perez, M.E., 2005. Monte Carlo based food irradiation simulator. Journal of Food Processing Engineering 29 (1), 72– 88. Kim, J., Rivadeneira, R., Castell-Perez, M.E., Moreira, R.G., 2006. Development and validation of a methodology for dose calculation in electron beam irradiation of complex-shaped foods. Journal of Food Engineering 74, 359–369. Kim, J., Moreira, R.G., Huang, Y., Castell-Perez, M.E., 2007. 3-D dose distributions for optimum radiation treatment planning of complex foods. Journal of Food Engineering 79, 312–321. Kim, J., Moreira, R.G., Castell-Perez, M.E., 2008. Validation of irradiation of Broccoli with a 10 MeV electron beam accelerator. Journal of Food Engineering 4, 595– 603. Lea, D.E., 1955. Actions of Radiation on Living Cells, second ed. Cambridge University Press, New York. Miller, R., 2005. Electronic Irradiation of Foods: An Introduction to the Technology. Springer. 1995. Niemira, B.A., Lonczynski, K.A., Sommers, C.K., 2006. Radiation sensitivity of Salmonella isolates relative to resistance to ampicillin, chloramphenicol or gentamicin. Radiation Physics and Chemistry 75, 1080–1086. Palekar, M.P., Cabrera-Diaz, E., Kalbasi-Ashtari, A., Maxim, J.E., Miller, R.K., CisnerosZevallos, L., Castillo, A., 2004. Effect of electron beam irradiation on the bacterial load and sensorial quality of sliced cantaloupe. Journal of Food Science 69, M267–M273. Tutt, T., 2007. Experimental Validation and Evaluation of Uncertainty in the Monte Carlo Modeling of Electron Irradiation of Complex Objects. PhD Dissertation, Department of Nuclear Engineering, Texas A&M University, College Station, TX 77843.
Contents lists available at ScienceDirect
Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Simulation of pathogen inactivation in whole and fresh-cut cantaloupe (Cucumis melo) using electron beam treatment Jongsoon Kim, Rosana Moreira *, Elena Castell-Perez Department of Biological and Agricultural Engineering, Texas A&M University, College Station, TX 77843-2117, United States
a r t i c l e
i n f o
Article history: Received 7 August 2009 Received in revised form 10 October 2009 Accepted 25 October 2009 Available online 29 October 2009 Keywords: Dosimetry Safety e-Beam Irradiation
a b s t r a c t Cantaloupes (Cucumis melo) have been implicated in several recent Salmonella outbreaks. Electron beam irradiation at the required dose can effectively inhibit pathogens while maintaining produce quality. The objective of this study was to evaluate pathogen inactivation in cantaloupes for optimization of electron beam treatment using dose distributions from Monte Carlo simulation and computed tomography (CT) scan data. 3D geometry and component densities of whole cantaloupes and fresh-cut cantaloupes packed in plastic trays, extracted from CT-scan data, were entered into a radiation transport code (MCNP5) to simulate dose distributions. Lucite (4.2 cm) was used as an electron absorber when simulating high energy (10 MeV) irradiation to make the penetration depth similar to the lower energy (1.35 MeV) electron simulations. For surface irradiation of the whole cantaloupe with a 1.35 MeV e-beam source, the penetration depth was 0.7 cm, well beyond the cantaloupe rind. For an entrance dose of 1 kGy, the log-reductions for Salmonella (D10-value of 0.359) were 3.30 ± 0.43 and 3.58 ± 0.58 at the depths of 0.2 and 0.4 cm, respectively. The dose uniformity ratio (Dmax/Dmin) up to 0.2 cm was improved from 1.81 to 1.15 with one beam rotation around the fruit. Surface irradiation using a 10 MeV e-beam source showed far less microbial reduction, only 1.58 ± 0.93 and 1.32 ± 0.84 at 0.2 and 0.4 cm, respectively. Irradiation of fresh-cut cantaloupe with a 10 MeV e-beam source showed that the maximum dose of 1.1 kGy occurred at the 3.5 cm depth and the minimum dose of 0.81 kGy took place at 5.5 cm depth, with an average 2.99 ± 0.79 log reduction around the whole package. These results provide valuable information for optimizing irradiation treatment planning for fresh produce. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Since current production and processing practices cannot ensure pathogen-free fresh and fresh-cut produce, effective food safety interventions are needed for implementation throughout the production, processing, and distribution of these foods (Doyle and Erickson, 2008). Non-thermal interventions with minimal effects in quality deterioration and capable of maintaining the intrinsic organoleptic profiles of produce are the only alternatives currently available to include as a lethality step in fresh and fresh-cut produce processing. On August 22, 2008, the Food and Drug Administration (FDA) published a final rule that allows the use of ionizing radiation (IR) to make fresh iceberg lettuce and fresh spinach safer and last longer without spoiling. Therefore, proper treatment protocols are needed for effective inactivation of pathogens in fresh and fresh-cut produce.
* Corresponding author. Tel.: +1 979 847 8794; fax: +1 979 845 3932. E-mail address: [email protected] (R. Moreira). 0260-8774/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2009.10.038
The main assets of IR include its high lethality, unmatched penetration into the product core, cold application, capabilities to be applied in a conveyor belt, and it has been proven as safe for treating food products (Miller, 2005). Whenever irradiation is used to treat a fresh produce, a compromise must be achieved between the quality of the treatment (i.e., decontamination, surface pasteurization, disinfestation) and the radiation dose received by the produce. However, not all microorganisms are equally sensitive to radiation. The challenge is how to reduce the damage to produce quality while applying sufficient doses of radiation to inactivate the pathogen. Correct food irradiation depends on accurate and reproducible measurement of dose (Kim et al., 2005, 2006, 2007, 2008; Gomes et al., 2008a,b). Problems with dosimetry for fresh produce include accuracy and uniformity of dose measured all over the product; and lack of standards to validate the measurements. Inaccurate interpretation of dose measurements can result in misleading D10-values for a target pathogen in a particular produce. Overdosing the produce is costly and under-dosing has serious safety implications.
426
J. Kim et al. / Journal of Food Engineering 97 (2010) 425–433
Palekar et al. (2004) showed that e-beam irradiation of cantaloupe slices resulted in a reduction in the total aerobic microbial counts with increasing doses. Across all doses of irradiation, counts were consistently lower for cantaloupe pieces obtained from melons that had been subjected to chlorine rinse in comparison with those washed with water without chlorine. Castell-Perez et al. (2004) showed that irradiation of cantaloupes, as whole fruits with dose up to 1.0 kGy, caused no significant changes on the fruit’s physical and nutritional quality attributes. Irradiating at higher doses had an undesirable effect on product quality. The authors indicated that the fresh-cut packaged cantaloupe may be irradiated up to 1.5 kGy without worsening the product quality attributes.
Fig. 1. Simulation set-up for e-beam irradiation of a whole cantaloupe with a 7.3 cm radius (a) 1.35 MeV radiation source (single upper mode); (b) 10 MeV radiation source (single upper mode) with a 4.2 cm thick Lucite shield (14.6 cm 14.6 cm 4.2 cm).
A combination of hot-water pasteurization of whole cantaloupe and low-dose irradiation of packaged fresh-cut melon showed reduced population of native microflora while maintaining their quality (Fan et al., 2006). Boyton et al. (2006) studied the combination of e-beam irradiation of fresh-cut cantaloupe in modified atmosphere packaging (MAP) and showed that the produce shelf life extended to about 2–3 weeks. The main assets of e-beam technology for treating fresh produce include its high lethality, low quality detrimental effects, unmatched penetration into the product core, cold application, capabilities to be applied in a conveyor belt, and tested safety of the intervention technology (Miller, 2005). However, whenever e-beams are used to irradiate a food product, a compromise must be achieved between the quality of the treatment (i.e., decontamination, surface pasteurization) and the radiation dose received by the food. Additionally, not all pathogens are equally sensitive to radiation. A microorganism that is radio-resistant may be treated with higher dose and the consequent side effects are unavoidable. At the optimal point, the lowest dose is applied that can still have
Fig. 2. Simulation set-up for e-beam irradiation of fresh-cut cantaloupe in a plastic tray (a) 10 MeV radiation source (single lower mode); (b) product sample configuration used in this study (container size – 13.5 cm 11.5 cm 6.0 cm).
Fig. 3. CT-scan and imaging technique to reconstruct a 3D image of a whole cantaloupe.
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the same effect: maximizing the dose to the pathogens while minimizing the loss of produce quality. The correct food irradiation process depends to a large extent on accurate and reproducible measurement of radiation dose, i.e., the amount of energy absorbed per unit of mass. However, the methods used to measure dose distribution in food materials have limitations, including the assumptions that the product is uniform and that the reading of a finite size dosimeter is representative of dose in the product. Therefore, if dosimetry is faulty, the resulting dose will be wrong. An alternative to dose measurement is the calculation of absorbed dose distribution using mathematical models coupled with computer simulation (Kim et al., 2005, 2006, 2007, 2008). Nevertheless, the methodology must be optimized for use with many of the fresh fruits and vegetables, which have complex shapes and are highly non-homogeneous with respect to their 3D mass distribution. In the irradiation of very heterogeneous objects, particularly fresh-cut items, one will encounter air pockets within the package as a matter of course. These pockets will cause variations in density of up to three orders of magnitude (Tutt, 2007). The objective of this work was to evaluate pathogen inactivation in whole and fresh-cut cantaloupe for optimization of electron beam treatment using dose distributions from Monte Carlo simulation and computed tomography (CT) scan data.
in packed plastic tray (polypropylene clam-shell type – 13.5 cm 11.5 cm 6.0 cm) with (a) a 10-MeV e-beam linear accelerator (LINAC) and (b) a lower e energy source (Van der Graaf-1.35 MeV) at a dose level of 1 kGy. For the whole fruit, a Lucite block (14.6 cm 14.6 cm 4.2 cm) was used as an electron absorber to simulate the high energy (LINAC-10 MeV) irradiation to make the penetration depth similar to the lower energy source (Van der Graaf-1.35 MeV) electron simulations (Fig. 1). For the fresh-cut fruit, the radiation source (LINAC-10 MeV) was simulated
2. Materials and methods Monte Carlo simulation: the MCNP-5 software (Monte Carlo NParticle – Version 5) used in this study was developed at the Los Alamos National Laboratory (Radiation Safety Information Computational Center (RSICC), Oak Ridge National Laboratory, Oak Ridge, TN). The simulator was run in parallel computer platform (DellTM PowerEdgeTM 6650, 4 CPU) located at the Department of Biological and Agricultural Engineering, Texas A&M University. Irradiation studies: simulation was done of irradiation treatment of a whole cantaloupe (7.2 cm radius) and fresh-cut cantaloupes
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Fig. 5. CT-scan of a whole cantaloupe showing density distribution.
Fig. 4. CT-scan and imaging technique to reconstruct a 3D image of fresh-cut cantaloupe pieces in a plastic tray.
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Fig. 6. Monte Carlo simulation of dose distribution at the surface of whole cantaloupe irradiated with e-beam source (single upper mode) for (a) 1.35 MeV; and (b) 10 MeV.
Fig. 7. Average dose distribution simulation from entrance versus penetration depth from e-beam source (single top beam mode) energies of (a) 1.35 MeV; and (b) 10 MeV.
Fig. 8. Monte Carlo simulation of energy distribution in a whole cantaloupe (10 MeV e-beam source; single bottom mode).
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Fig. 11. Average dose distribution simulation from entrance (bottom of tray) to the top of a fresh-cut tray for a 10 MeV e-beam radiation source.
Fig. 9. Monte Carlo simulation of dose distribution at the surface of a whole cantaloupe for 1.35 MeV e-beam source (single upper mode), showing results for one rotation of the target in front of the beam.
as a single beam from the bottom (Fig. 2). The e-beam was described as a parallel plane large enough to cover the target. In the linear accelerator, the electrons were emitted in a plane and distributed evenly within the scan area. A total of 107 histories were used to reduce the statistical uncertainty to about 5% or less (Brown, 2003). A technique using CT-scan and imaging processing was used to create the 3D image of the products (Kim et al., 2007). A 3D geometry and component densities of whole (Fig. 3) and fresh-cut cantaloupes in a plastic tray (Fig. 4), extracted from CT-scan data, were entered into a radiation transport code (MCNP5) to simulate dose distributions. Electron simulation in MCNP: Monte Carlo radiation transport is a simulated track consisting of the geometrical positions of all energy deposition events and the amount of energy deposited at each interaction point. Each electron track starts at a given position, with initial direction and energy. The state of a particle after interaction is defined by its position coordinates, energy, and direction cosines of the direction of flight. An electron path is broken into
many steps to follow an electron through a significant energy loss. These steps are chosen (1) to be long enough to include many collisions so that multiple-scattering theories are valid (major steps) and, (2) to be short enough that mean energy loss in any step is small (sub-steps). The energy loss and angular deflection (Goudsmit and Saunderson, 1940) of the electron during each of steps can then be sampled using probability distributions on the appropriate multiple-scattering theories. It is impractical to model all individual interactions. Instead, well-established statistical theories (condensed history) are used to describe those interactions into single steps. The electron tracks end either by when the electrons leave the target material or when their energy becomes smaller than an energy cutoff (i.e., the energy the particles are assumed to be absorbed in the medium). Log-reduction calculation: we used the experimental D10-value for Salmonella of 0.359 kGy, from Niemira et al. (2006), for the most resistant strain. The microbial survival distribution within samples was simulated using both the ‘‘target theory” approach initially described by Lea (1955) and the single-hit inactivation model to solve the general survival equation (Alpen, 1998), a classical first-order relationship of logarithmic survival on dose. The assumptions for the development of the single-hit inactivation model are (1) the deposition of energy as ionizing or excitation in the critical volume
Fig. 10. Log-reduction of Salmonella for 1.35 MeV e-beam radiation of whole cantaloupe surface (a) at 2 mm depth; (b) at 4 mm depth.
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Fig. 12. 2D view of simulated dose distribution in a fresh-cut cantaloupe tray irradiated with 10 MeV e-beam source (single bottom mode) – slices represent the target depth.
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Fig. 12 (continued)
leads to the production of molecular lesions in the cell and thus inactivation of the microorganisms and, (2) a cell will survive only if it has received no hits at all and that it will always die if it has received one or more hits. Thus, the microorganisms survival was calculated as described in Gomes et al. (2008b) as:
S ¼ eD=Do ¼ eD=0:156 ¼ e6:415D
ð1Þ
with
Do ¼
D10 2:303
ð2Þ
with D the dose (kGy), Do the mean lethal dose, defined as the dose required to reduce the survival fraction S to 1/e (i.e., 37%) of the initial value, and D10 = 0.359 kGy. 3. Results For electrons or photons interaction with matter, the density of the product is very important. Energy deposition is very much a function of the product density. The change in densities in a product can affect the way energy is distributed in the product thus affecting quality and safety aspect of the process. For a non-uniform and/or complex shaped produces, if the dose is not accurately predicted around and within each point in the product, changes in density can result is lower energy absorption resulting higher pathogens survival. Fig. 5 shows a CT-scan slice of the whole cantaloupe showing the rind, flesh, and the seed pulp. The density of flesh is almost uniform around 750–900 kg/m3. The seeds and the pulp show slightly lighter density than the flesh. The cavity is showing in black (very low density).
The MCNP simulation of the dose distribution at the surface of the whole cantaloupe irradiated with the 1.35 and 10 MeV e-beam sources is shown in Fig. 6a and b, respectively. The penetration depth was around 0.7 cm, well beyond the cantaloupe rind. The dose distribution covered a larger surface with the 1.35 MeV source (Fig. 6a) than with the 10 MeV e-beam source (Fig. 6b). This is because the Lucite blocks are used to absorb the income energy from the e-beam source when working with higher-energy accelerators so the penetration depth is reduced. Fig. 7 shows the average dose–depth curves for 1.35 MeV ebeam (Fig. 7a) and 10 MeV e-beam (Fig. 7b) where the Lucite absorbed the incoming energy and the energy that reached the cantaloupe surface only penetrated it up to 1 cm in depth (the irradiated area of cantaloupe shows a decreasing slope region at the dose–depth curve). Note that Fig. 7a also shows a buildup region. Generally, in the case of incident electron beams, the buildup region in food materials is due to the progressive cascading of secondary electrons by collisional energy losses (Attix, 1986). This buildup region extends up to a depth of approximately one third to two thirds of the electron range, except when the angle of incidence is greatly increased (away from the normal) or when a scattering material is placed between the source and the irradiated material (IAEA, 2002). If the Lucite block was not used, the results would be much different, as illustrated in Fig. 8. This simulation was conducted with a 10-MeV electron beam source (single bottom mode) to the 3D geometry of the whole cantaloupe in the voxel resolution of 5 0.3 0.3 mm (Kim et al., 2007). Unlike the 1.35-MeV electrons source that provided only surface treatment, the absorbed energy (Fig. 8) covered almost half of the cantaloupe. The largest vertical distance the electrons penetrated in the cantaloupe was 5 cm, significantly deeper than the
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Fig. 13. 3D view of simulated log-reduction distribution of Salmonella in a fresh-cut cantaloupe tray irradiated with 10 MeV e-beam source from bottom (z axis represents the penetration depth of the e-beam source).
penetration depth (0.7 cm) achieved with the 1.35-MeV e-beam source. The energy distribution in the whole cantaloupe is strongly related to the electron’s entrance region at the surface of the product and the sample’s density (Fig. 8, upper left). As the electrons penetrate, their energy is absorbed, even though electrons inherently scatter easily. At the maximum penetration depth, the relative error is relatively large (Fig. 8, upper right) because the number of particles reaching that area is very small. The energy deposition plot (Fig. 8, bottom) represents the energy distribution around the bottom of the cantaloupe. The pixel number at the xaxis was counted from the bottom to the top. Simulation of dose distribution for the whole cantaloupe irradiated with a 1.35 MeV e-beam source, with one rotation of the fruit in front of the beam, improved the dose distribution around the entire fruit surface (Fig. 9). For an entrance dose of 1 kGy, the dose uniformity ratio (Dmax/Dmin) up to the 0.2 cm depth improved from 1.81 to 1.15 with one beam rotation. It was assumed surface contamination of a whole cantaloupe, which means microorganisms are on or just under the rind. Therefore, radiation up to 0.2 cm depth would be enough for surface decontamination of pathogens in a whole cantaloupe. The log-reductions for Salmonella were about 3.30 ± 0.43 and 3.58 ± 0.58 at the depths of 0.2 and 0.4 cm, respectively (Fig. 10). However, when the cantaloupe was irradiated with a 10 MeV source, the pathogen population log-reduction was only 1.58 ± 0.93 and 1.32 ± 0.84, respectively. The dose uniformity ratio of cantaloupe up to 0.2 cm for the 10 MeV source was larger (>2), due to the presence of the Lucite absorber.
Irradiation of fresh-cut cantaloupe with a 10 MeV electron beam source showed a maximum dose of 1.1 kGy at 3.5 cm depth and a minimum dose of 0.8 kGy at 5.5 cm depth into the cubic shaped samples (Fig. 11). Fig. 12 shows a 2D view of the dose distribution within the fresh-cut cantaloupe tray. This clearly illustrates the non-uniformity of the energy deposited in the samples in the x, y, and z directions. Due to the large variation in the dose distribution within the sample, the log-reduction for Salmonella was about 2.99 ± 0.79. The doses values at the edge of the cantaloupe pieces were 19.4% higher than at the middle region because of electron scattering (Fig. 13). Because of the non-uniformity in dose distribution in all directions around the samples, it would be better to irradiate fresh-cut produces with spherical or even conical shape than cubic or cylindrical cuts, to reduce electron scattering around the sample edges. 4. Conclusions Because of the difficulty of measuring dose inside of fresh produce, the only approach possible is to use mathematical simulation. Monte Carlo method is currently the most accurate procedure for dose calculation in electron beams. When comparing irradiation treatment of whole cantaloupes with 1.35 MeV to a 10 MeV e-beam sources, the treatment with the lower energy can more effectively inactivate pathogens located at the surface (by or right below the rind) than the higher-energy treatment. Around 3.3–3.6 logs of a resistant strain of Salmonella can be reduced with a 1.35 MeV source compared to only 1.3–
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1.6 logs reduction achieved using the 10 MeV e-beam. Therefore, when using high energy accelerators to treat the surfaces of fresh produces, careful planning is needed since energy absorbers are required to reduce the entrance e-beam energy to the produce. This requires a fundamental understanding of e-beam interaction with the target produce. Irradiation of fresh-cut cantaloupes in trays with a 10 MeV ebeam source showed that the maximum dose of 1.1 kGy occurred at 3.5 cm depth and the minimum dose of 0.81 kGy took place at 5.5 cm depth, with an average 2.99 ± 0.79 log reduction around the whole package. The dose distribution was very affected by the product shapes, showing large energy absorbed at the product edges than at the middle. Therefore, to reduce electrons scattering, it would be better to irradiate fresh-cut produces with spherical or even conical shape than cubic or cylindrical cuts.
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