Course:FNH200/Lessons/Lesson 10/Page 10.7

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10.7 Factors affecting Food Irradiation

The factors that need to be considered and controlled during food irradiation include:

  1. Safety and wholesomeness of the foods
  2. Resistance of food to irradiation
  3. Resistance of microorganisms to ionizing energy
  4. Resistance of enzymes to ionizing energy
  5. Cost

10.7.1. Safety and Wholesomeness of Irradiated Foods

In Canada, the Health Products and Food Branch of Health Canada considers issues about safety and wholesomeness of irradiated foods, in addition to the safety and wholesomeness of foods preserved by other food preservation methods. The Canadian Food Inspection Agency considers aspects related to labelling of irradiated foods. The issues of safety and wholesomeness of irradiated foods revolve around criteria of the following four principles:

  • Radiological safety: ensuring that foods do not become radioactive during irradiation;
  • Toxicological safety: ensuring that production of toxic and possibly carcinogenic substances does not occur;
  • Microbiological safety: ensuring the efficacy of the radiation process with respect to the ability of the prescribed absorbed dose to kill disease-causing microorganisms that could be in the food.
  • Nutritional adequacy: ensuring that undue losses of nutrients do not occur as a consequence of treatment of food with ionizing energy; and

The conclusions drawn by the Canadian and international regulatory agencies about food irradiation are that foods treated such that the absorbed dose is below 10 kGy do not contain toxicants at undesirable levels. That is, the irradiated foods which have absorbed a dose of less than 10 kGy are wholesome and safe for long-term consumption.

In 1997, a Study Group was convened by the World Health Organization (WHO), the Food and Agriculture Organization (FAO), and the International Atomic Energy Agency (IAEA), to evaluate wholesomeness of food irradiated with doses above 10 kGy. In a report published in 1999, "The Study Group concluded that food irradiated to any dose appropriate to achieve the intended technological objective is both safe to consume and nutritionally adequate ... Accordingly, irradiated foods are deemed wholesome throughout the technologically useful dose range from below 10 kGy to envisioned doses above 10 kGy" (WHO Technical Report Series 890). Applying the concept of "substantial equivalence", even high-dose irradiated foods are considered to be as safe as foods sterilized by conventional thermal processing, such as canning of low-acid foods.

10.7.2. Resistance of Foods to Ionizing Energy

Not all foods are amenable to preservation by treatment with ionizing energy. The same can be said for thermal processing, freezing and dehydration as methods of food preservation. The quality of some foods may be adversely affected by irradiation, depending on the dose, temperature and conditions during irradiation. For example, colour, flavour or textural changes may result after exposure of food components to ionizing energy. Lipids or fats are particularly susceptible to oxidative reactions triggered by the radiolytic reactions and presence of free radicals. Losses of some vitamins may also occur; vitamins A, C, E and B1 (thiamine) are the most sensitive, particularly at higher doses and in foods packaged in air.

As mentioned above, these changes may be minimized by irradiating foods in the frozen state, in a vacuum, and/or with the addition of radical scavengers such as ascorbic acid. Additional strategies include applying the lowest effective irradiation dose and choosing appropriate packaging in terms of moisture and oxygen barriers.

10.7.3. Resistance of Microorganisms to Ionizing Energy

As in the case of thermal processing (pasteurization, commercial sterilization), microorganisms vary in their resistance to the killing effects of ionizing energy. Analogous to thermal processing where Clostridium botulinum is the most heat resistant pathogen, C. botulinum spores are the most radiation resistant forms of pathogenic bacteria.

You may recall that we discussed the decimal reduction time (D-value) in Lesson 6 in conjunction with thermal processing of foods. Similarly, it is possible to determine the dose of ionizing energy necessary to effect a 90% destruction of the particular microorganism in question. When we calculated the D-value in the lesson on thermal processing, we referred to the length of time at a constant temperature required to create a 90% decrease in the population of the microorganisms or spores in question. In irradiation concept, the time at a constant temperature could be converted to an absorbed dose of thermal energy.

With food irradiation, we calculate the absorbed dose of ionizing energy that produces a 90% decrease in the microbial population (D10 values). To achieve an appropriate margin of safety, a 5D or a 12D radiation treatment would have to be applied to acid and low acid foods, respectively. Although the source of the energy and the mechanisms by which microorganisms and spores are killed are different, the same concept (decimal reduction value) is applied during determination of the efficacy of thermal processing and preservation of food with ionizing energy.

Is it true that irradiation can mask food spoilage?

Irradiation cannot be effectively used to mask or cover up food spoilage since the microorganisms can be easily killed but the spoilage odours, off-flavours and colour changes caused by the spoilage microorganisms can not be changed or eliminated by ionizing radiation.

Thus, claims that ionizing radiation can be used to mask signs of poor quality in food are untrue.

What about Microbiological safety?

There has been concern about the creation of "superbugs" or mutants that are more dangerous but this is not a significant issue at the doses of ionizing radiation used in food processing.

Want to learn more?
  • Take a look at D10-values of some important foodborne pathogens, listed in Table 1 of the IFT Scientific Status Summary on "Irradiation and Food Safety" by Smith and Pillai:
  • The D10-values for E. coli 0157:H7 (in ground beef patties, at 5oC) and for Salmonella spp. (in turkey breast meat at 5oC) are 0.27-0.38 and 0.71 kGy, respectively.
    • What dose would be required for a 5D process for each of these pathogens under those conditions?
    • What dose would be required for a 12D process for each of these pathogens under those conditions?

10.7.4. Resistance of Enzymes to Ionizing Energy

The majority of food enzymes are more resistant to ionizing energy than spores of C. botulinum. The term DE (D-enzyme) is used to determine the radiation dose that produces a 90% reduction of enzyme activity. The DE values are of the order of 5 Mrad. Four DE values (5 x 4 = 20x106 rad1 or 200 kGy) would produce nearly total enzyme destruction; however, 200 KGy would also destroy many food constituents!

1Remember that 100 rad is equivalent to 1 Gray of absorbed ionizing energy and 1000 Gray equals 1 KGy.

From this calculation, you will have noted that enzymes cannot be easily inactivated by treatment with ionizing radiation. Ionizing energy could never be used for blanching vegetables. One of the concerns expressed by groups opposed to food irradiation is that enzymes in food are destroyed by exposure to ionizing energy. Clearly, from the example above, that is not the case especially if you consider the maximum dose permitted in Division 26 of the Food Regulations of Canada is 10 kGy.

If vegetables were to be preserved with ionizing energy, they would first have to be blanched with heat followed by treatment with ionizing energy to inactivate the microorganisms of concern.

10.7.5. Costs

After the issues of safety and wholesomeness have been satisfied, economic factors must be considered in evaluating the feasibility of an application of food irradiation. Food irradiation may be economically viable if it results in substantial increases in storage life and therefore marketing time and decreases in post-harvest or catching losses. This may be the case in terms of radicidation (discussed below) treatments of fresh fish or some fresh fruits. In cases where the process does not offer advantages (such as nutrition retention, technological advantages, economic advantages) it would not be economically viable.