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Initial set of physical, Altered set of physical, chemical, and biological chemical, and biological properties properties

Figure 1. Illustration of principle of radiation processing. Prior to irradiation, the commodity has an initial set of properties. Following treatment, the processed commodity has an altered set of properties.

Figure 2. Cutaway view of typical -/-radiation facility, such as could be used for processing food.

Figure 2 illustrates a typical industrial irradiation plant, which can be used for treating food. The illustration shows the essential elements of such a facility, including the warehouse area, the biological shield, the maze (required to allow transfer of product through the shield while not permitting escape of radiation to the outside), the product carriers, the cobalt-60 source assembly, and the storage pool (needed for safe storage of the cobalt-60 when product is not being treated). It should be noted that, unlike the case with microwave cooking, food irradiation is an industrial process and cannot be miniaturized for use in home kitchens.

Mechanism of Action

Exposure of the food product to the field of ionizing energy within the radiation chamber results in a specified, desired amount of energy being absorbed by the food material. The amount of energy absorbed per unit mass of the absorbing material is termed the absorbed dose of radiation and is measured in units called grays. The practical unit for measuring radiation dose used in food processing is called the kilogray. It is the absorbed energy that gives rise to the effects ascribed to the treatment. Energy that passes through the product without being absorbed does not affect the exposed material. A central question is how the absorbed energy leads to the observed effects, which constitute the benefits and detriments of the process.

Thermal Effects Are Negligible. Table 1 presents data on the theoretical maximum temperature rise associated with irradiation of food products treated to effect the specified technical end points. This illustrates that, for all practical purposes, the maximum temperature rise is generally negligible. From this it follows that the benefits of the treatment are effected by nonthermal mechanisms.

Action Cascade as Mechanism. Food is a biological system, and the effects of irradiation on food constitute a particular subset of the more general effects of irradiation on any such system. Biological effects of irradiation result from a complex sequence of reactions (3) that are well de scribed by the term action cascade. Figure 3 illustrates the essential features of this sequence. The action cascade consists of a series of stages, beginning with energy absorption (a physical, discrete process that occurs at random throughout the irradiated material). Through ionization and excitation pathways the absorbed energy generates a variety of primary reactive species (free radicals, ions, excited molecules) that serve to propagate the effects through the subsequent chemical, biochemical, physiological, and biological stages. The final result consists of physiological and biological effects that constitute the observable benefits and detriments deriving from the treatment. Note that each successive stage operates on a longer time scale than the preceding one.

Basis for Beneficial Effects. Given that the initiating event (energy absorption) of the action cascade is unavoidably random in its occurrence within the treated material, it is instructive to examine the basis for beneficial effects of food irradiation. Intuitively, it is not obvious how random acts of molecular damage can give rise to a net benefit. The secret lies in the fact that different functional entities within biological systems exhibit large intrinsic differences with respect to their sensitivity to inactivation by ionizing radiation. Empirical characterization of the individual dose response curves for inactivation of specific biological functions of interest allows exploitation of these differential dose responses for our benefit, in favorable cases. Thus, differential dose response underlies the benefits of the process. This is illustrated schematically in Figure 4. With reference to Figure 4, it can be seen that, because the individual dose response curves are separated along the dose axis, it is generally possible to select a treatment dose suitable for effecting a desired technical end point (such as insect killing) without at the same time inducing significant detriment to the nutritional value, taste, or texture of the treated food.

Benefits

Technical End Points of Irradiation. Empirical observation has demonstrated a variety of possible technical

Unloading area

Source hoists

Source pass mechanism Radiation room

Radiation shield

Figure 2. Cutaway view of typical -/-radiation facility, such as could be used for processing food.

Loading area

Control console

Source in storage pool

Unloading area

Loading area

Source hoists

Control console

Source in storage pool

Source pass mechanism Radiation room

Radiation shield

Table 1. Temperature Rise Associated with Food Irradiation

Dose (kGy)

Typical application

Energy Absorbed (J/kg)

Temperature rise (°C.; water)

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