Irradiation Sources And Facilities

Two types of radiation sources are used in food irradiation, radionuclide and machine radiation. Radionuclides are radioactive elements which upon decay release deeply penetrating gamma rays. Cobalt-60, produced by exposing Cobalt-59 to neutrons, and cesium-137, a by-product of nuclear reactors, are the only radionuclides permitted in food processing. However, cesium-137 is less widely used because of its limited availability. The machine sources are electron beam generators or electron accelerators and x-ray machines. In an electron accelerator, a hot filament emits electrons into an evacuated chamber. The electrons are attracted by a positive electrode and focused into a narrow beam. The high-energy electrons penetrate the "window", a very thin metal foil, which scatters the beam focus allowing a more uniform exposure of the food surface. In contrast to gamma rays, accelerated electrons have a limited penetrating capability, less than 8 cm and are therefore only useful for the treatment of foods which can be processed in thin layers e.g., grain and thin slices of meat. X-rays are generated by high speed electrons from accelerators, which are made to hit a metal target causing it to emit x-rays. Like gamma rays, x-rays are deeply penetrating. However, their use in food processing is more expensive and impractical. Recent advances indicate that new types of x-ray generators may be developed which would be better suited for food irradiation.

Both radionuclide and machine sources must be provided with adequate shielding to protect personnel. In case of radionuclide sources, the source is usually stored in a deep pool of water, and raised above the water during irradiation of food.

The irradiation treatment can be conducted in either a batch or continuous mode. The batch method is simpler to design, easier to operate and is more flexible. The continuous operation is more suitable for large volumes. It may involve a single-pass or a multiple pattern, the design of which allows a more uniform exposure of the food and a more efficient use of the source. Mobile irradiators are also available. These are compact units which are best suited for certain foods where season, location, transport and interval between harvest and processing, may be limiting factors, e.g., seafoods.

5. GENERAL MECHANISM

Ions and excited molecules are the first species formed when ionizing radiation is absorbed by matter. A high-energy charged particle loses energy, in a series of small steps, by electric interaction with electrons in the absorbing material. Such interaction involves a large number of atoms or molecules, distributed along the particle's track, raising them to excited levels. If these levels are above the ionization potential of the atom or molecule, ions are formed. The positive charge may be distributed over a part of the molecule or localized in one atom or group of atoms. If the excited levels are below the ionization potential, excited atoms or molecules are produced. Excited molecules could also result indirectly by neutralization of the ions formed. The protons of electromagnetic radiation, unlike charged particles, tend to lose a large amount of energy upon interaction with matter and the ionization or excitation occurs largely via secondary electrons. The primary effect of a single gamma ray, for example, may be to produce a 1 Mev electron (and a positive ion). Such an electron may produce 30,000 - 40,000 additional ionization processes and 45,000 - 80,000 excitations. The secondary electrons produced in this process are called "6-rays" when they have sufficient energy ( 100 ev) to produce further ionizations. Only about 5% of the primary fast-electron interactions result in 6-rays, but 8-rays cause about half the total ionization from fast electrons. Primary processes, which result in the formation of ions and excited molecules, occur in the first 10"14 sec on the time scale for events in radiation chemistry.

Chemical breakdown is brought about by the decomposition of these primary species (excited molecules and ions), or by their reaction with neighboring molecules. The free radicals formed by the dissociation of excited molecules and by ion reactions are largely responsible for the observed chemical changes and generally dominate the mechanisms postulated for the formation, upon irradiation, of stable radiolytic products. They may combine with each other in regions of high radical concentrations, or may diffuse into the bulk of the medium and react with other molecules.

The ability of ionizing energy to break certain bonds in organic molecules is the reason behind its special value. The splitting of critical bonds in microorganisms or other pests will result in their death or dysfunction.

6. EFFECTS ON FOOD COMPONENTS

Radiation chemistry of the major food components has been reviewed in great detail in the literature (Diehl, 1982; Dauphin and St. Lebe, 1977; Delincee, 1983a, b; Nawar, 1977). Only a brief summary is given here.

6.1. Water

In food, or parts of food containing little water, direct action of the radiation on the organic molecules is the major source of chemical change. In many foods, however, water is the major constituent. Since fast electrons interact indiscriminately with molecules along their track, much more excitations and ionizations of the water molecules will result than of the other components. The products formed in pure water and dilute aqueous solutions by irradiation can be summarized in the following general equation:

H20-> 2.7 'OH + 2.7 e"aq + 0.55 'H + 0.45 H2 + 0.71 H202 + 2.7 H30+

The primary radiolysis products of water disappear in fractions of a second by reacting with each other or with other food components. The hydroxyl radical, 'OH, is a powerful oxidizing agent. It can add to aromatic and olefmic compounds and abstract hydrogen atoms from carbon-hydrogen and sulfur-hydrogen bonds. The hydrated electron, e", is also highly reactive. It adds rapidly to most aromatic compounds, carboxylic acids, ketones, aldehydes and thiols. H202 is formed by recombination but in the absence of oxygen its formation is extremely low. Hydrogen atoms can abstract hydrogen from C-H bonds or add to olefinic compounds. They are produced in a relatively low yield.

6.2. Proteins

The radiolysis of proteins can be largely ascribed to reactions of their constituent amino acids, with the sulfur and aromatic amino acids being the most sensitive. In addition, the structural and conformational features of the proteins influence their response to radiation. Reactions of the electrons lead to deamination, reduction of disulfide and peptide linkages, addition to aromatic groups (Delincee, 1983a; Garrison, 1981; Taub, et al 1979). The OH radical reacts readily with aromatic heterocyclic and sulfur-containing residues. Volatile products resulting from radiolytic decomposition of protein include ammonia, fatty acids, ketoacids, aromatic compounds, amides and mercaptans. Sulfur-containing compounds are important because of their off-flavor characteristics. In addition to degradation, the protein may undergo unfolding and aggregation, particularly in the case of globular proteins.

In cases where meats are irradiated at sub-freezing temperatures, the radiolytic effects on the proteins are generally very small. At -40° C, the proteins are fixed in a rigid matrix and indirect effects by radical diffusion are minimized. Radicals from direct effects undergo recombination reactions with little or no degradation or aggregation detected. Consequently, under such conditions, changes in amino acid profiles are found to be negligible and the amount of volatile compounds produced markedly reduced. Very little structural alterations occur in the protein molecules and no free radicals persist in the meat.

The effects of irradiation on enzymes is responsible for certain distortions of physiological processes during the storage of irradiated fruits and vegetables. The doses normally used for the irradiation of meat are not sufficient to inactivate enzymes which cause autolysis during storage. Heat inactivation before irradiation is thus necessary for long-term stability.

6.3. Carbohydrates

Although the major products formed by irradiation in many pure sugars and saccharides have been studied (Dauphin and St. Lebe, 1977), little research has been conducted on the radiolytic products derived from the carbohydrate portion in complex foodstuffs. However, reaction mechanisms in simple bound sugars are similar to those in the more complicated polymeric materials containing these subunits.

In aqueous systems, radiolysis of carbohydrates occurs mainly by indirect action of OH radicals which react primarily with C-H bonds. The carbohydrate radicals thus formed react further by dismutation, dimerization and dehydration. The deoxycarbonyl radical can undergo further reactions of dimerization, dismutation and saturation.

Radiolytic products of glucose, for example, include gluconic acid, glucuronic acid, D-glucono-l,5-lactone, saccharic acid, D-arabinose, D-xylose, D-erythrose, glyoxal, dihydroxyacetone, formol and H202, if irradiated in oxygen atmosphere, and deoxygluconic acid, erythritol, deoxymannitol, deoxyarabinohexitol, deoxyribohexitol, and D-mannitol, if irradiated under nitrogen.

Polysaccharides such as starch, cellulose, pectin, and glycogen undergo cleavage of the glycosidic linkage producing lower molecular weight fractions such as glucose, maltose and dextrin. Further decomposition gives rise to radiolytic products of smaller molecular weights including formic, gluconic, acetic, glyoxylic, pyruvic, malic and oxalic acids, acetone, acetaldehyde, malonaldehyde, glyoxal, glyceraldehyde, dihydroxyacetone, hydroxymalonaldehyde, furfural, hydroxymethylfurfural, diacetyl, acetoin, methyl formate and methyl alcohol.

Detailed quantitative data for the products from various sugars and saccharides irradiated under different conditions can be found in the comprehensive review by Dauphin and Saint-Lebe (1977). Obviously, the amounts of radiolytic products from carbohydrates vary with composition, dose, and irradiation parameters. When com starch, with a water content of 1213%, was irradiated under 02, the concentration of the malondialdehyde produced was 0.2 pg/g/kGy. The concentrations of formol, acetaldehyde, acetone, glyoxal, methyl alcohol, glucose, and ribose were 2, 4, 0.21, 0.35, 0.28, 0.58 and 0.06 |ig/g/kGy, respectively. The presence of other nutrients in complex foods, e.g., proteins and lipids, are known to provide protection against radiation damage in carbohydrates. Diehl (1982) calculated that in a model food consisting of 80% water and 6.6% each of carbohydrates, proteins and fats, the maximum concentration of the total products produced from the carbohydrate component by irradiation at 5 kGy would be 0.5 mg/lOOg.

Although the possible cytotoxic effects of malondialdehyde have received particular attention, the yields of this product are minimal under the normal pH encountered in foods.

6.4. Lipids

Fatty-acid-containing compounds typically undergo preferential cleavage at locations in the vicinity of the carbonyl group and randomly at the other carbon-carbon bonds (Delincee, 1983b, Nawar, 1977). Primary ionization in oxygen-containing compounds involves the loss

CHaOOCR

of a non-bonding electron with the result that the unpaired electron is highly localized on the oxygen. Preferential bond cleavages near the carbonyl group are facilitated by the tendency of the oxygen atom to complete its valence shell of electrons. Thus, in a triacylglycerol molecule-ion, preferential cleavages (solid lines) result in the formation of alkyl, acyl, acyloxy, and acyloxymethylene free radicals and, in addition, the free radicals representing the corresponding glyceryl residues. Free radical termination may occur by hydrogen abstraction, and to a lesser degree by loss of hydrogen with the formation of an unsaturated linkage. Thus, depending on fatty acid composition, specific compounds of smaller molecular weight than their substrate, e.g., hydrocarbons, aldehydes, esters, are produced.

Alternatively, the free radicals may recombine, giving rise to a variety of radiolytic products of longer-chain or dimeric nature. Table 2 provides a list of radiolytic compounds identified in model systems of triacylglycerols. Most of these compounds were also identified in irradiated meats.

Irradiation and subsequent storage in the presence of oxygen accelerate lipid oxidation probably by enhancing the formation of free radicals which can combine with oxygen, the breakdown of the hydroperoxides and/or the destruction of antioxidants. The products thus formed are identical to those usually present in unirradiated but oxidized lipids.

6.5. Vitamins

The radiation stability of vitamins has been studied and reviewed (Tobback, 1977; Basson, 1983). Much information on the radiosensitivity of individual vitamins has been obtained from experiments with pure model systems in simple solutions. In foods, however, the effects of irradiation depend, to a large extent, on a number of factors as for example composition of the food and its physical structure, water activity, temperature, gas atmosphere, etc.

In general, vitamin B, has been reported to be the most radiation-sensitive among the water-soluble vitamins. Of the fat-soluble vitamins, vitamin E is recognized as the most sensitive. Whole milk irradiated at approximately 1.5 kGy caused a loss of 35% in vitamin B, but no losses were observed in the thiamin content of milk powder irradiated at doses 28 and 56 kGy. Thiamin loss of 20% in wheat flour irradiated at 0.35 kGy and 35% in oat flakes irradiated at 0.25 kGy, were reported. No significant loss of vitamin B, was found in egg powder irradiated at doses below 5 kGy and stored for 15 months. The observation that vitamin B, exhibits much more resistance to radiation when present in food has been

Table 2

Radiolytic Products of Triacylglycerols

A. Breakdown products of smaller molecular weight than substrate

C02, CO, H2 Propanediol diesters

Carboxylic acids n-Alkanes Propenediol diesters

2-Alkylcyclobutanones

1-Alkenes Oxopropanediol diesters Monoacylglycerols

Alkadienes Aldehydes

Diacylglycerols

Alkynes

Ketones

Triacylglycerols

Esters

Lactones

Ethanediol diesters

B. Adduct Products Glyceryl ether diesters Propanedioldiester dimers Propanedioldiester-triacylglycerol adducts Triacylglycerol dimers attributed to the fact that it is usually bound to proteins which protect prosthetic groups against radiation. The sensitivity of vitamin C to irradiation treatment is believed to be similar to its sensitivity to heating or oxidation. Under practical conditions of irradiation, losses rarely exceed 30%. Niacin is known to be very resistant to radiation.

The radiosensitivity of vitamin E is influenced by its initial concentration and varies markedly from one food to another. It can be significantly minimized by exclusion of oxygen. Vitamin D in food is relatively stable to irradiation.

Although some reports concerning the degradation products of some vitamins are available, the characterization of these compounds and their significance in irradiated food have not been fully explored. Since some of these products may play a significant role with regard to food quality and wholesomeness, this aspect merits further research.

6.6. Complex Foods

In general, the products formed by irradiation in a complex food such as meat are qualitatively the same as those which arise from the radiolysis of its constituent components, i.e., lipids, proteins, and carbohydrates. However, there is a quantitative difference. While dilute solutions of pure sugars, amino acids, vitamins, and enzymes are relatively sensitive to radiation, these compounds are more stable when exposed as constituents of food. This is attributed to the distribution of radiation damage among the many constituents in food and to the protective effects of certain constituents. For example, amino acids were found to protect trehalose against radiolytic decomposition. This phenomenon was explained by their "OH scavenging properties and/or their ability to act as hydrogen donors (Adam, 1977 a,b; Diehl, et al 1975, 1978; Dizdaroglu, et al 1977; Taub, et al 1979).

6.7. Comparison with Heat

Heat processing of food is an accepted practice. The energy absorbed by food when exposed to irradiation at legal doses is much less than that absorbed when the food is heated. For example, an absorbed dose of 1 kGy would increase the temperature of aqueous foods by 0.24° C.

It is not surprising, therefore, that for most practical applications, the chemical and physical changes which occur in food by irradiation are often less than those observed by cooking, frying, canning, baking, etc. (Fig. 4).

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