Food irradiation is a process by which food is exposed to ionizing radiation for the purpose of preservation.

 Principles:-The energy employed in food irradiation technology is referred to as ionizing irradiation. Ionizing radiation occurs when one or more electrons are removed from an atom. Electrons orbiting at minimum energy level or ground state can be raised to higher levels, becoming electronically excited (excitation). If enough energy is transferred to an orbital electron, the excited electron may be ejected from the atom (ionization). A minimum amount of absorbed energy, called ionization potential, exists for each electron energy level necessary to exit the atom domain. If the energy absorbed by the electron is greater than its ionization potential, the excess energy enters a kinetic state, enabling the electron to leave the atom domain. Although each electron behaves individually, electrons can be used in large numbers, called electron beams, to irradiate food. Electron beams are produced from commercial electron accelerators. One advantage of electron beam radiation is that the electron accelerators can be switched off when not in use, leaving no radiation hazard; however, the penetration of electron beams into foods is limited. Electrostatic forces tend to attract charged particles such as electrons (negatively charged), limiting the electron beam penetration into foods. Therefore, electron beam radiation is limited to small food items, such as grains, or to the removal of surface contamination of prepared meals.

Photons of electromagnetic radiation such as those created by x-rays and gamma rays travel without charge; thus, they do not interact with electrostatic forces while traveling through the food, and can penetrate deeper than electron beams.

 One mode of producing x-rays is through the use of electron beams. If the highly accelerated electrons penetrate a thin foil of certain metals, such as tungsten, tantalum, or any other material capable of withstanding high heat, x-rays are produced. Radioactive isotopes such as cobalt 60 or cesium 137 are used to produce gamma rays. Although radiation isotopes cannot be switched off, they have the advantage of producing gamma rays, which offer the largest penetration among all ionization sources. Dose is the most important parameter in food irradiation. The quantity of energy absorbed by the food is measured in grays (Gy). One gray equals one joule per kilogram of matter. A gray (equal to 100 rad) is a very small quantity; therefore, the dose is expressed in kilograys (kGy). Up to 10 kGy is still a very small amount of energy, equal to the amount of heat required to raise the temperature of water 2.4ºC. For this reason irradiation is considered a “cold” or nonthermal technology.

Effects of Irradiation on Microbial Inactivation

Irradiation is currently used as a tool to control naturally occurring processes   such as ripening or senescence of raw fruits and vegetables and is an effective way to inactivate spoilage and pathogenic microorganisms. Irradiation causes microbial death by inhibiting DNA synthesis. Other mechanisms involved in irradiation microbial inactivation are cell membrane alteration, denaturation of enzymes, alterations in ribonucleic acid (RNA) synthesis, effects on phosphorylation, and DNA compositional changes. According to the dose used and the goal of the treatment, food irradiation can be classified into three categories:

1. Radurization is a process comparable with thermal pasteurization. The goal of radurization is to reduce the number of spoilage microorganisms, using doses generally below 10 kGy.

2. Radicidation is a process in which the irradiation dose is enough to reduce specific non-spore-forming microbial pathogens. Doses generally range from 2.5 to 10 kGy, depending on the food being treated.

3. Radappertization is a process designed to inactivate spore-forming pathogenic bacteria, similar to thermal sterilization. Irradiation doses must be between 10 and 50 kGy.

Molds and Gram-positive vegetative bacteria are more tolerant than Gram-negative bacteria.

Resistance of Salmonella typhimurium increased at reduced irradiation temperatures. Chemical compounds with nutritional or flavor functions can also be affected by ionizing irradiation; At higher irradiation doses, such as those required for food sterilization, some vitamins such as A, B1, C, E, and K can degrade to some extent. Irradiation may cause some changes in the sensory characteristics of food and the functional properties of food components. Irradiation initiates the autoxidation of fats, which gives rise to rancid off flavors. The extent of irradiation-induced lipid oxidation depends on factors associated with oxidation such as temperature, oxygen availability, fat composition, and pro-oxidants.