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Comparison of the Volatile Compounds Generated from Palm Oil by Heat and Irradiation as Analyzed by Gas Chromatography.

7. Methods of Identifying Irradiation in Foods

Reliable methodology for the detection of irradiation treatment in food is important to enforce compliance, enhance consumer confidence and facilitate international trade. A good detection method should meet certain criteria. These include specificity, reproducibility, sensitivity, accuracy, discrimination, dose dependence, simplicity, speed of measurement and low cost.

A number of methods based on the measurement of chemical, physical or biological changes which can be detected in irradiated foods have been studied (Anon, 1991a). To date no single method is available which can be universally applied to all foods. This is to be expected since most of the detectable effects are often extremely small and are not necessarily unique to the irradiation treatment. Radiolytic changes in food are influenced by other factors, e.g., food composition and physical structure and processing parameters. Another difficultly is caused by the lack of uniformity among agricultural products and their handling practices.

At present three techniques, the lipid-derived volatiles, electron spin resonance and thermoluminescence, have reached the stage of practical application. At various stages of development are several other detection methods. Only a brief listing of these and/or the principles upon which they are based, is given here.

7.1. Physical Methods

7.1.1. Electron Spin Resonance (ESR)

This method involves the detection by ESR of paramagnetic centers produced by irradiation in rigid matrices, e.g., bones, shells, seeds (Derosiers, 1989; Derosiers and McLaughlin, 1989; Gray, 1995; Gray and Stevenson, 1989; Raffi and Angel, 1989). The resulting asymmetric signal is believed to be derived from the hydroxyapatite mineralized component of bone and is different from the endogenous symmetric signal commonly detected in the unirradiated product and is assumed to originate from the organic component of the bone tissue.

In addition to whole bones of fresh and frozen meats, the method is applicable to submillimeter bone fragments recovered from mechanically deboned meat; fish bones, teeth, fins and scales; and the exoskeleton of shrimp, lobster and crab. The ESR method can also be applied to spices, and dry fruits and vegetables in which case the free radicals derived from cellulose or sugar are measured, and to egg shells where ESR signals have been attributed to carbonate ion radicals.

The ESR signal is specific, stable and dose-related. The measurement is sensitive, rapid, and non-destructive. However, it requires specialized analysts and relatively expensive instrumentation.

7.1.2. Thermoluminescence (TL)

The TL method is based on the emission of light when energy trapped in crystalline lattices during the irradation is released by heat. The measurement of a TL curve is simple and rapid. It is mostly applicable to spices and dry foods. Several investigations, however, confirmed that the principal origin of the signal is the minerals adhering to the food (Sanderson, et al 1989). Techniques have been described to improve the reproducibility and discriminating power of the method by TL measurement of these minerals after their isolation from the irradiated food.

Thermoluminescence analysis has been performed on the minerals isolated from the intestine of shrimp and silicates extracted from fruits and vegetables.

More recently photostimulated luminescene techniques (PSL), in which the energy to release trapped charge carriers is provided optically, were developed. This is highly radiation specific since energy conservation principles require that the quantum energy differences between stimulation and luminescence are balanced by energy stored in the form of trapped charge carriers. The method can be used for fast screening of irradiated spice and shrimp products. A PSL instrument is now commercially available and can be easily used by non-scientific personnel.

7.1.3. Impedance

The ratio of magnitude and phase angle of electrical impedance at two different frequencies can be used as parameters for the detection of irradiated potatoes (Hayashi, et al 1982). The method is only applicable to fresh potato tubers.

7.1.4. Viscosity

The viscosity of some spices is lowered by irradiation. Although irradiated black and white pepper were clearly distinguishable from unirradiated controls, the applicability of the method for other spices requires more study (Farkas, 1987; Farkas, et al 1990).

7.1.5. Near Infrared Analysis

The near infrared spectrum of spices is influenced by irradiation. However, in some cases the effects of storage on the spectra were significant. The applications of the method have been studied with black and white pepper, paprika, allspice and cinnamon (Barbassy, et al 1992).

7.2. Chemical Methods

7.2.1. The Lipid-Derived Volatile Method

This method relies on the measurement of "key" volatile radiolytic compounds which result form the preferential cleavage in the carbonyl region of fatty acids (Nawar and Balboni, 1970; Nawar, et al 1990). In particular, two hydrocarbons from each fatty acid, are especially useful as markers. One has one carbon atom less, and the other has two carbons less than the substrate fatty acid and an extra double bond. The formation of these compounds increases linearly with dose. The technique involves solvent extraction of the lipids, collection of the volatiles and quantitative measurement of the hydrocarbons by gas chromatography.

The reliability of this method was extensively studied with chicken, beef and pork with excellent results (Nawar, et al 1990). Its application to seafood, spices, egg powder and other lipid-containing foods is promising (Anon, 1994).

7.2.2. Ortho-Tyrosine

This method is based on the measurement of o-tyrosine produced by irradiation via hydroxyl radical attack on the phenylalanine component of proteins. Since o-tyrosine has been found in the fluid, but not in the fiber of unirradiated meat, measurement of o-tyrosine in the water insoluble fraction, i.e., muscle fiber, can be used to detect irradiation treatment (Karam and Simic, 1988; Meir, et al 1988). In meats, improvements in the separation and measurement of o-tyrosine by GC, HPLC have been recently described.

Various techniques based on DNA fragmentation, or modifications of pyrimidine or purine bases and sugar moieties are being developed and tested. These techniques include the use of conventional agarose electrophoresis of mitochondrial DNA, micro-gel eletrophoresis (Delincee, 1993) and the development of antibodies to dihydrothymine, a reductive radiolytic product of thymine. Application of these techniques has been tested on meat, fish, fruits and vegetables (Anon. 1994).

7.2.4. Gas Evolution

Some of the hydrogen and carbon monoxide produced by irradiation is trapped in frozen meats and dry grains. The gases can be released by heating and detected by gas chromatography or other gas sensors (Dohmaru, et al 1989). Research is needed to assess the effect of storage on the applicability of the method to specific foods.

7.3. Biological Methods

7.3.1. Shift in Microbiological Composition

Irradiation de-activates microorganisms but does not affect their ability to be stained. The difference between the viable bacterial count, obtained by conventional aerobic plating (APC), and the total count (dead and alive), obtained by the direct epifluorescent filter technique (DEF) or by Limulus Amoebocyte Lysate test, indicates the number of organisms deactivated by irradiation.

The method has been applied to the detection of irradiation in spices (Betts, et al 1988, Anon, 1994). Conclusive evidence of irradiation treatment, however, can not be obtained if the spice has been treated by fumigation or heat.

7.3.2. Half-Embryo Test

This test is based on the inhibiting effect of irradiation on the ability of seeds to germinate (Kawamura, et al 1989).

The shell of the seed is removed and half-embryos are incubated at optimum conditions of temperatures and humidity. Samples are judged as irradiated if shooting is less than 40%. The method is applicable to citrus fruit, apples and cherries.

7.3.3. Immunochemical Detection

Fragments resulting from radiolytic decomposition of egg white proteins can be separated and detected by immunoblotting using specific antibodies. (Bugyaki and van der Strichelon-Rogier, 1970).

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