Food irradiation involves the use of either high-speed electron beams or high-energy radiation with wavelengths smaller than 200 nanometres, or 2000 angstroms (e.g., X rays and gamma rays). These rays contain sufficient energy to break chemical bonds and ionize molecules that lie in their path. The two most common sources of high-energy radiation used in the food industry are cobalt-60 (60Co) and cesium-137 (137Cs).
Gamma rays have a greater penetrating power into foods than high-speed electrons. Food irradiation is used to increase the shelf life of foodstuffs like potatoes.
Effects of Irradiation :
Irradiation has both direct and indirect effects on biological materials. The collision of radiation with atoms causes an ejection of electrons from the atoms. The indirect effects are due to the formation of free radicals during the radiolysis (radiation-induced splitting) of water molecules. The radiolysis of water molecules produces hydroxyl radicals, that are highly reactive and interact with the organic molecules present in foods.
Positive Effects
The bactericidal effect of ionizing radiation is due to damage of the biomolecules of bacterial cells. The free radicals produced during irradiation may destroy or change the structure of cellular membranes. Radiation hampers the growth of bacterial cells by causing irreversible changes to their nucleic acid molecules (i.e., DNA and RNA). In poultry, meat and seafood, radiation dose of 3 - 10 kilograys is enough to eliminate the pathogenic bacteria that are unable to produce resistant endospores. Presence of oxygen during irradiation, helps in effective damage of the bacteria. To stop the reproduction of the parasitic worm that causes trichinosis in pork, doses in the range of 0.2 to 0.36 kilograys are required. Meat requires a higher dose.
Negative Effects
In the absence of oxygen, radiolysis of lipids leads to cleavage of the interatomic bonds in the fat molecules, producing compounds such as carbon dioxide, alkanes, alkenes, and aldehydes. Peroxides, carbonyl compounds, alcohols, and lactones are formed due to oxidation by free radicals, to which lipids are highly vulnerable. This leads to rancidity in certain foodstuffs. That is why, fatty foods are vacuum-packed and held at subfreezing temperatures during irradiation. Low doses of radiation do not significantly degrade proteins. Hence, enzymes involved in food spoilage are not inactivated, as most enzymes survive doses of up to 10 kilograys. On the other hand, the large carbohydrate molecules that provide structure to foods are depolymerized by irradiation which reduces the gelling power of the long chains of structural carbohydrates. Many times, internal protection against this is available in the form of certain other substances. Vitamins A, E, and B1 (thiamine) are also sensitive to irradiation.
Safety concerns
To utilise the beneficial effects of irradiation on certain foods, several countries have permitted its use for specific purposes, such as the prevention of sprouting of potatoes, onions, and garlic through the use of gamma rays. Similarly, use of irradiation for extension of shelf life of strawberries, mangoes, pears, grapes, cherries, red currants, and cod and haddock fillets; and the insect disinfestation of pulses, peanuts, dried fruits, papayas, wheat, and ground-wheat products are also allowed. Precautions like lining of walls of processing centers with lead or thick concrete walls to prevent radiation from escaping are taken. The energy source, such as a radioactive element or a machine source of electrons, is placed inside the room. Radioactive elements such as 60Co are stored in stainless steel tubes. Because an isotope cannot be switched on or off, when not in use it is, placed into a large reservoir of water. Humans are not allowed inside the room while the process is on. The food to be irradiated is then conveyed by remote means exposed to the radiation source for a specific time. The time of exposure and the distance between the radiation source and the food are the factors on which irradiation treatment is dependent. The irradiated food is conveyed out of the room, and the radioactive element is again lowered into the water reservoir. Large-scale studies conducted around the world have concluded that irradiation does not cause harmful reactions in foods. In 1980 a joint committee of the Food and Agriculture Organization (FAO), the International Atomic Energy Agency (IAEA), and the World Health Organization (WHO) opined that a dose of 10 kilograys is safe for food products. The maximum energy emitted by 60Co and 137Cs is too low to induce radioactivity in food. The energy output of electron-beam generators can be controlled.