Food Irradiation and Radioactivity
- Page ID
- 418944
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I. F. 3. b. Radioactivity may occur via alpha, beta, gamma or other decay events and the type of radioactivity observed is often an important component of identifying the decaying isotope.
VI. I. 1. b. Mass deficits in nuclear reactions arise from the conversion of mass to energy and are calculated via the equation: E = mc2.
VI. I. 1. c. Nuclear reactions give off radiation that may have biological effects.
Have you ever opened a box of strawberries to find that half of them are covered with mold? Well, you probably wouldn’t expect this unlikely hero to keep pests out of your food—radioactivity! Radioactivity is applicable to the field of food science in the useful, yet controversial, process of food irradiation.
Numerous health organizations across the world have deemed food irradiation, the process of exposing food and food packaging to ionizing radiation, to be fairly safe. Despite this, irradiating food using nuclear technology remains a controversial topic. Many people perceive irradiated food to be contaminated, dangerous, and unsuitable for consumption.1
Why is irradiation still used if it’s still controversial?
Irradiation is incredibly useful in many different contexts. In addition to eliminating or reducing pests, irradiation can reduce the risk of food-borne illnesses, and it can prevent or slow spoilage and plant maturation or sprouting. Compared to other preservation processes, irradiation can preserve sensory and chemical qualities that are more similar to unprocessed food.2
So, how does food irradiation work?
Irradiation uses radioactive decay to reduce or eliminate pathogens. To understand food irradiation, one must first understand radioactive decay.
Radioactive decay is the process by which an unstable atomic nucleus loses energy by emitting energetic waves or particles. When a food is undergoing food irradiation, it is exposed to a source of radiation. The radiation source emits energetic particles or waves, and these waves and particles enter the target material and collide with other particles. For example, Cobalt – 60 is often used in food irradiation. An unstable Cobalt – 60 atom undergoes beta decay as its specific form of nuclear disintegration.3 Beta decay is the process of emitting an energetic electron to form a new element. The overall equation of the nuclear reaction is \[^{60}_{28}\text{Co} \rightarrow ^{60}_{29}\text{Ni} + ^{0}_{\text{-1}}\beta \]
When these high energy particles collide with molecules in the food, chemical bonds are broken, and radicals (atoms, molecules, or ions with unpaired electrons) are formed. These radicals further cause other chemical changes in cells that slow cell division. Therefore, food matures more slowly, and bacterial cells are killed.3
Fig. 1. Comparison of irradiated strawberries with untreated strawberries after 15 days of storage. Reprinted from ref 4. Copyright 1988 World Health Organization.
It is possible to calculate the exact amount of energy released in a radioactive process. During a nuclear reaction, mass is not conserved. The mass of the products is usually less than the mass of the reactants. Some of the mass of the original particle is converted to energy. The amount of energy released nuclear decay is given by \[ \Delta E = \Delta m c^2\] where m is the mass deficit of the reaction, and c is the speed of light (2.998 \(\cdot\) 108 m/s).
So, it is clear that irradiating food is effective, but just how effective is it? It was previously mentioned that the creation of free radicals is what slows the growth of bacteria and other microorganisms in food. Let’s see how well this works.
The majority of radiation energy goes into the creation of hydrogen and hydroxyl radicals from water molecules.5 Since the foods exposed to irradiation energy are usually hydrated, the radiolysis of the water in food is responsible for the inactivation effects of microorganisms.
Consider the reaction given by Cobalt – 60 undergoing beta decay to produce Nickel – 60. Given that the mass of Cobalt – 60 is 59.9338222 amu,6 the mass of Nickel – 60 is 59.930785 amu,7 and the bond energy O – H in water is 461.5 kJ/mol.
How many moles of water can 5g of Cobalt – 60 cause to undergo radiolysis?
Solution
For a nuclear equation, \(\Delta E = \Delta mc^2\). The mass defect of this reaction is the difference in the mass of the reactants and the mass of the products. Since electrons are assumed to be approximately massless, this is the difference between the mass of Nickel - 60 and the mass of Cobalt - 60: 59.9338222 amu - 59.930785 = 0.0030372 amu.
In order to use the equation, this value must be converted to kilograms. One amu is equivalent to 1.66 \(\cdot\) 10^{-27} kg, and this gives that \(0.0030372 \text{amu} \cdot \tfrac {1.66 \cdot 10^{-27} \text{kg}}{\text{amu}} \) = 5.0448 
\(\cdot\) 10-30 kg. By plugging the speed of light into the equation, we have that the energy released with one atom decaying into nickel and an electron is 5.0448 \(\cdot\) 10-30 kg \(\cdot\) (2.998 \(\cdot\) 108 m/s)2 = 4.5342 \(\cdot\)10-13 J.
Next, the amount of energy released for one mole of this reaction should be calculated. One mole of 60Co releases 6.022 \(\cdot 10^{23}\) electrons, so the mole reaction releases \(\tfrac {4.5342 \cdot 10^{-13} \text{J}}{\text{electron}} \cdot \tfrac {6.022 \cdot 10^{23} \text{electrons}}{\text{mol reaction}} \) =2.7305 \(\cdot\)1011 J/mol rxn.
One mole of Cobalt – 60 has a mass of 59.933822 g, and so 5g of Cobalt – 60 is 0.083425 mol of 60Co. This means that 5g of 60Co releases \(\tfrac {2.7305 \cdot 10^{11} \text{J}}{\text{mol rxn}} \cdot \text{0.083425} \) = 2.277 \(\cdot\) 1010 J of energy.
Since it takes 461.5kJ to break 1 mol of O – H double bonds, this is \( 2.277 \cdot 10^{10} \text{J} \cdot \tfrac {1 mol H_2O}{461500J} \) = 4.9339 \(\cdot 10^4\) mol \(H_2O\). Therefore, 5g of 60Co would be able to lyse 4.9339 \(\cdot\)104 moles of water.
Miller and Jensen8 found that no radioactivity was detected when beef was irradiated with electrons with energies less than 13.5 MeV. Is irradiating beef with Cobalt – 60 sufficient to make food radioactive? Why or why not?
Solution
As calculated in problem 1, the energies of the electrons released in the decay of 60Co is 4.5342 \(\cdot\)10-13 J. However, the threshold electrons must reach to create radioactive meat is \(13.5 \text{MeV} \cdot \tfrac {1.60218 \cdot 10^{-13} \text{J}}{\text{MeV}} \) = 2.16294 \(\cdot 10^{-12}\) J, which is higher than the energies of electrons produced from the decay of 60Co. Therefore, irradiating beef with Cobalt – 60 does not produce electrons with enough energy to make the food radioactive.
In order for food to become radioactive, radiation must be absorbed into the nucleus of the cells that compose the foods. As shown by the energy calculation, using Cobalt does not have enough energy to do this, so using Cobalt for the irradiation process is perfectly safe.
References
1. Castell-Perez, M. E.; Moreira, R. G. Irradiation and Consumers Acceptance. Innovative Food Processing Technologies. 2021, 122–135. DOI:10.1016/B978-0-12-815781-7.00015-9.
2. Nutrition, C. for F. S. and A. Food Irradiation: What You Need to Know. FDA 2022.
3. US EPA, O. Radionuclide Basics: Cobalt-60. https://www.epa.gov/radiation/radion...sics-cobalt-60 (accessed 2022-11-11).
4. World Health Organization. Food irradiation: A technique for preserving and improving the safety of food, 1988. https://apps.who.int/iris/handle/10665/38544 (accessed 2022-10-28).
5. Munir, M. T.; Federighi, M. Control of Foodborne Biological Hazards by Ionizing Radiations. Foods. 2020, 9 (7), 878. DOI: 10.3390/foods9070878 (accessed 2022-10-28 from MDPI Open Access Journals).
6. National Institute of Health, National Library of Medicine, National Center for Biotechnology Information. PubChem Compound Summary for CID 61492, Cobalt-60, 2022. https://pubchem.ncbi.nlm.nih.gov/compound/Cobalt-60 (accessed 2022-10-28).
7. National Institute of Health, National Library of Medicine, National Center for Biotechnology Information. PubChem Compound Summary for CID 44152085, Nickel-60, 2022. https://pubchem.ncbi.nlm.nih.gov/compound/44152085 (accessed 2022-10-28).
8. Miller, A.; Jensen, P. H. Measurements of Induced Radioactivity in Electron- and Photon-Irradiated Beef. International Journal of Radiation Applications and Instrumentation. Part A, Applied Radiation and Isotopes. 1987, 38 (7), 507–512. DOI: 10.1016/0883-2889(87)90196-1.
9. World Nuclear Organization. Radioisotope Uses for Food and Agriculture, 2021. https://world-nuclear.org/informatio...riculture.aspx (accessed 2022-10-28).