Food Additives & Contaminants
ISSN: 0265-203X (Print) (Online) Journal homepage: http://www.tandfonline.com/loi/tfac19
Detection of o‐tyrosine in irradiated chicken by reverse‐phase HPLC and fluorescence detection F. I. Ibe , R. Grinter , R. Massey & R. Homer To cite this article: F. I. Ibe , R. Grinter , R. Massey & R. Homer (1991) Detection of o‐tyrosine in irradiated chicken by reverse‐phase HPLC and fluorescence detection, Food Additives & Contaminants, 8:6, 787-792, DOI: 10.1080/02652039109374037 To link to this article: http://dx.doi.org/10.1080/02652039109374037
Published online: 10 Jan 2009.
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Date: 08 November 2015, At: 12:19
FOOD ADDITIVES AND CONTAMINANTS, 1991, VOL. 8, NO. 6, 7 8 7 - 7 9 2
Detection of o-tyrosine in irradiated chicken by reverse-phase HPLC and fluorescence detection
Downloaded by [NUS National University of Singapore] at 12:19 08 November 2015
F. I. IBE,† R. GRINTER,† R. MASSEY‡ and the late R. HOMER† †School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, UK; ‡ Ministry of Agriculture, Fisheries and Food, Food Safety Directorate, Food Science Laboratory, Colney Lane, Norwich NR4 7UQ, UK (Received 23 October 1991, accepted 13 November 1991) A method for the measurement of o-tyrosine in irradiated chicken has been developed. The procedure involves the solvent extraction and removal of free o-tyrosine, which is present in unirradiated tissue, followed by acid hydrolysis of bound o-tyrosine in the proteinaceous residue and measurement of the cleaved residues by HPLC with fluorescence detection. Bound o-tyrosine was not detected above 0.01 mg/kg in unirradiated tissue but was observed, in increasing amounts of up to 5.18 mg/kg, when the tissue was irradiated at doses of between 2.5 and 20 kGy. The precision of the analysis was assessed by duplicate determinations, the agreement between duplicates and their respective means averaged 1.7% as defined by the term [(a- b)/(a + b)] × 100% where a and b are the repeat determination values.
Keywords: Irradiation, food, chicken, o-tyrosine
Introduction
Food irradiation may be employed for a range of purposes including inhibiting sprouting of vegetables, delaying the ripening of fruits, insect disinfestation of crops and reduction of the microbial load of products such as spices and chicken. A joint FAO/IAEA/WHO Expert Committee (1981) concluded that the irradiation of food up to an overall average dose of 10 kGy presents no toxicological hazard and introduces no special nutritional or microbiological problems. In the UK the independent Advisory Committee on Irradiated and Novel Foods (ACINF 1986) reviewed the safety of the process and reached the same conclusion as the international committee. In 1988 the UK Government accepted the advice of ACINF and announced its intention to legalize the process. Subsequently the Food (Control of) Irradiation Regulations 1990 were laid before Parliament and came into force on 1 January 1991. The strict licensing system requires operators to apply for prior approval through a central licensing authority with licences only granted for the treatment of specific foods or foods at a specific plant. The first licence, for the irradiation of spices and condiments, has now been issued (Ministry of Agriculture, Fisheries and Foods 1991). Much research has been devoted to the development of tests that can be employed to check whether or not a food item has been irradiated and to what dose. The unambiguous detection of irradiated foods is far from easy and whilst many compounds are formed when foodstuffs are irradiated very few of them have 0265-203X/91 $3.00 © 1991 Taylor & Francis Ltd.
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F. Ibe et al.
proved to be unique to the process and are of limited value as dosimeters. Several approaches have been investigated including enzymatic changes in irradiated mushrooms (Munzner 1973), protein changes in meat and poultry (Dizdaroglu and Simic 1981), volatiles from fatty acids in meat and fish (Merritt and Vajdi 1982, Boyd et al. 1991), ESR detection of trapped radicals (Dodd et al. 1985, Lea et al. 1988, Gray and Stevenson 1990), thermoluminescence and chemiluminescence measurements on spices (Heide and Bogl 1987, Sanderson et al. 1989) and viscosity measurements on spices (Heide et al. 1990). The presence of radiation-induced o-tyrosine (Karam and Simic 1988a,b) has been proposed as a method for detecting irradiation of chicken meat. Its formation during radiolysis arises from the reaction of radiation-generated hydroxy radicals with protein-bound residues of phenylalanine. During analysis the protein-bond o-tyrosine is released from the protein matrix by acid hydrolysis, prior to derivatization and quantification by gas chromatography/mass spectrometry. Karam and Simic (1988a) reported that free o-tyrosine was present in the unirradiated sample they examined, at a level of 12 mg/kg, and removed it from the protein fraction, prior to the acid hydrolysis step, by extraction with chloroform. The use of this, and certain other organic solvents, has also however been reported to induce the formation of o-tyrosine on occasion (Karam and Simic 1988b). Hart et al. (1988) investigated the use of successive washes of the proteinaceous matrix with distilled water and concluded that the o-tyrosine detected in unirradiated chicken (0-06 mg/kg) could not be removed by this means. Meier et al. (1988) used the method of Karam and Simic (1988a) but were unable to detect o-tyrosine in chicken even at doses of up to 40 kGy. However, by modifying a high performance liquid chromatography (HPLC) procedure, originally developed by Ishimitsu and co-workers (1986) for the measurement of o-tyrosine in serum, Meier et al. (1988) devised an alternative method for the measurement of o-tyrosine in the acid hydrolysate of chicken meat. These authors did not however employ a prior solvent extraction step as levels of free o-tyrosine in unirradiated chicken were apparently all at or below 0-05 mg/kg (Meier et al. 1988, 1990). We have examined the method of Meier et al. (1988) and wish to report our findings on the levels of o-tyrosine in unirradiated chicken tissue and also the precision of the results obtained when the procedure is applied to irradiated samples. Modifications to the assay are described and their effects on the precision of the assay are discussed.
Experimental
Materials Three whole, unfrozen, uncooked chickens were purchased at separate retail food outlets in Norwich during February 1989. The muscle tissue from the breast of two of the birds was removed and each of the two samples divided into five approximately equal pieces of ca 110 cm3. The leg muscle tissue (drumstick) was removed from the remaining bird and similarly divided into five sub-samples. The 15 tissue samples were sent to Isotron (Swindon, UK) and one sub-sample from each of the three portions was irradiated (60Cobalt source, 10 kGy/h, 21 °C) at 2-5, 5-0, 10-0 and 20*0 kGy. One sub-sample from each portion remained unirradiated. The samples were then all returned to Norwich and stored at -22°C until analysis.
O-tyrosine in irradiated chicken
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Methods Defatting and freeze drying. The chicken tissue was allowed to thaw for 2 h and a weighed portion (ca 10 g) minced for about 30 s using a hand-operated mincer. The homogenate was transferred to an electric mixer and blended with chloroform/methanol (2:1; 50 ml) for 3 min. The resulting suspension was filtered, using a water pump, and the proteinaceous residue removed and blended with ethanol/water (4:1; 150 ml) for 5 min. The suspension was again filtered and the residue washed successively with ethanol/water (4:1; 100 ml; used previously to rinse the blender), acetone (100 ml, Distol grade) and water (100 ml). The sample was quantitatively transferred to a pre-weighed, round bottom flask and freeze dried for at least 24 h. The ratio of [(chicken tissue):(defatted and freeze dried material)] was calculated and used to express the results for o-tyrosine in terms of original chicken tissue. Acid hydrolysis. A weighed portion of the defatted, freeze-dried sample (ca 50 mg) was placed in a hydrolysis tube (15 x 160 mm) together with alpha-methyl tyosine (internal standard, 40 /d, 26-2 /*g/ml) and methane sulphonic acid (0-9 ml, 4-0 M). The contents were frozen in liquid nitrogen and the tube evacuated on a water pump for 5 min prior to sealing it. On warming to ambient temperature the tube was heated at 110°C for 24 h in a heating block. The hydrolysate was adjusted to pH 5*9 ± 0 - 0 3 with sodium hydroxide (4-0 M) and filtered using a 0-45 ^m PTFE membrane filter (Anachem, UK). HPLC analysis. A 20 /d aliquot of the neutralized hydrolysate was analysed using a Hypersil H - 5 - O D S column (250 mm x 4-6 mm i.d.) connected to a Perkin Elmer 3000 fluorescence spectrometer (excitation: wavelength 270 nm, slit 15 nm; emission: wavelength 303 nm, slit 20 nm). The chromatograph was operated at room temperature, flow rate 1-0 ml/min, and solvent programmed as follows: 0 to 17 min, 100% solvent A (acetonitrile + 1-0% w/w aqueous sodium chloride; 9:991, v/v); 17 to 21 min, linear gradient to 100% solvent B (acetonitrile+ 0-5% w/w aqueous sodium chloride; 640:360, v/v). Under these conditions o-tyrosine and alpha-methyl tyrosine eluted after 16-3 and 12-8 min respectively. The column was reconditioned with 14 ml of solvent A prior to the next injection and stored overnight in acetonitrile/water (800/200). Each sample was injected onto the HPLC column five times and the mean peak area used to quantify the content of o-tyrosine. Results and Discussion The irradiated and unirradiated chicken tissue samples were examined using the method devised by Meier et al. (1988) and the results are shown in Table 1. oTyrosine was detected in all three of the unirradiated samples examined, i.e. the leg muscle tissue and both of the breast muscle tissue samples, in levels of between 0*5 mg/kg and 0-9 mg/kg. Higher concentrations of o-tyrosine, up to 3-4 mg/kg, were observed when the samples were irradiated at doses of 2-5, 5-0, 10-0 and 20-0 kGy. However, the precision of the data was poor such that there seemed little prospect of using the method for dosimetric purposes. In order to improve the potential of the method for discriminating between irradiated and unirradiated chicken tissue, a solvent extraction step was developed as described in the Experimental section, to remove any free o-tyrosine that might be present in sample. Analysis of the two unirradiated chicken breast and the leg muscle tissue samples showed that the levels of o-tyrosine in the analytical samples,
Downloaded by [NUS National University of Singapore] at 12:19 08 November 2015
790
F. Ibe et al.
i.e. the defatted, dried, proteinaceous residue, were reduced to less than the 0-01 mg/kg detection limit of the method. To improve the precision of the analysis an internal standard, alpha-methyl tyrosine, was introduced. In addition, the pH of the hydrolysate was adjusted to 5 • 9 to avoid the high acidity problems associated with the original method in which the acid hydrolysate was diluted five-fold with water and injected directly onto the column. Chromatograms of unirradiated chicken and samples irradiated at 2*5 kGy and 20 kGy are shown in figure l(a-c). The recovery of o-tyrosine added to chicken tissue at a level of 2-2 mg/kg averaged 103% (standard deviation 3%, five determinations). The modified method was applied to the irradiated and unirradiated chicken tissue samples and the results are shown in Table 2. Each sample was analysed in two separate analytical batches. It is evident that there is a reasonably close correspondence between the duplicate analyses, for each tissue sample, at the five dose levels examined. The agreement between duplicate determinations and their respective means averaged ± 1-7% as defined by the term [(a - b)j(a + b)] x 100% where a and b are the duplicate determination values. The results also show that for each of the three samples the amount of o-tyrosine detected increases as the irradiation dose is raised. It is of interest that the levels of o-tyrosine formed are very similar for both the two samples of chicken breast tissue and the leg muscle tissue. In conclusion the procedures that have been developed enable o-tyrosine to be quantified with precision in irradiated chicken tissue. The solvent extraction step permits any free o-tyrosine present in the unirradiated sample to be removed, Table 1. Mean levels of o-tyrosine (mg/kg) in chicken tissue samples using method of Meier el al. (1988). Dose (kGy) 0 2-5 5-0
10-0 20-0
Breast tissue (sample 1) 0-5(0-4, 0-6) 0-8(1-0,0-6) 0-9(1-0,0-8) 3-3(5-2,1-3) 3-4(4-7,2-1)
Breast tissue (sample 2) a
0-9(1-0,0-8) NA NA NA NA
Leg tissue 0-6(0-7, 0-5) 1-9(3-1,0-6) 1-3(1-8,0-8) 1-8(2-3,1-2) 2-9(3-7,2-0)
a
Results of individual analysis (whole procedure) in parenthesis. NA, not analysed.
Table 2. Mean levels of o-tyrosine (mg/kg) in chicken tissue samples using current method. Dose (kGy)
Breast tissue (sample 1)
Breast tissue (sample 2)
Leg tissue
0 2-5 5-0
ISSN: 0265-203X (Print) (Online) Journal homepage: http://www.tandfonline.com/loi/tfac19
Detection of o‐tyrosine in irradiated chicken by reverse‐phase HPLC and fluorescence detection F. I. Ibe , R. Grinter , R. Massey & R. Homer To cite this article: F. I. Ibe , R. Grinter , R. Massey & R. Homer (1991) Detection of o‐tyrosine in irradiated chicken by reverse‐phase HPLC and fluorescence detection, Food Additives & Contaminants, 8:6, 787-792, DOI: 10.1080/02652039109374037 To link to this article: http://dx.doi.org/10.1080/02652039109374037
Published online: 10 Jan 2009.
Submit your article to this journal
Article views: 7
View related articles
Citing articles: 1 View citing articles
Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=tfac20 Download by: [NUS National University of Singapore]
Date: 08 November 2015, At: 12:19
FOOD ADDITIVES AND CONTAMINANTS, 1991, VOL. 8, NO. 6, 7 8 7 - 7 9 2
Detection of o-tyrosine in irradiated chicken by reverse-phase HPLC and fluorescence detection
Downloaded by [NUS National University of Singapore] at 12:19 08 November 2015
F. I. IBE,† R. GRINTER,† R. MASSEY‡ and the late R. HOMER† †School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, UK; ‡ Ministry of Agriculture, Fisheries and Food, Food Safety Directorate, Food Science Laboratory, Colney Lane, Norwich NR4 7UQ, UK (Received 23 October 1991, accepted 13 November 1991) A method for the measurement of o-tyrosine in irradiated chicken has been developed. The procedure involves the solvent extraction and removal of free o-tyrosine, which is present in unirradiated tissue, followed by acid hydrolysis of bound o-tyrosine in the proteinaceous residue and measurement of the cleaved residues by HPLC with fluorescence detection. Bound o-tyrosine was not detected above 0.01 mg/kg in unirradiated tissue but was observed, in increasing amounts of up to 5.18 mg/kg, when the tissue was irradiated at doses of between 2.5 and 20 kGy. The precision of the analysis was assessed by duplicate determinations, the agreement between duplicates and their respective means averaged 1.7% as defined by the term [(a- b)/(a + b)] × 100% where a and b are the repeat determination values.
Keywords: Irradiation, food, chicken, o-tyrosine
Introduction
Food irradiation may be employed for a range of purposes including inhibiting sprouting of vegetables, delaying the ripening of fruits, insect disinfestation of crops and reduction of the microbial load of products such as spices and chicken. A joint FAO/IAEA/WHO Expert Committee (1981) concluded that the irradiation of food up to an overall average dose of 10 kGy presents no toxicological hazard and introduces no special nutritional or microbiological problems. In the UK the independent Advisory Committee on Irradiated and Novel Foods (ACINF 1986) reviewed the safety of the process and reached the same conclusion as the international committee. In 1988 the UK Government accepted the advice of ACINF and announced its intention to legalize the process. Subsequently the Food (Control of) Irradiation Regulations 1990 were laid before Parliament and came into force on 1 January 1991. The strict licensing system requires operators to apply for prior approval through a central licensing authority with licences only granted for the treatment of specific foods or foods at a specific plant. The first licence, for the irradiation of spices and condiments, has now been issued (Ministry of Agriculture, Fisheries and Foods 1991). Much research has been devoted to the development of tests that can be employed to check whether or not a food item has been irradiated and to what dose. The unambiguous detection of irradiated foods is far from easy and whilst many compounds are formed when foodstuffs are irradiated very few of them have 0265-203X/91 $3.00 © 1991 Taylor & Francis Ltd.
Downloaded by [NUS National University of Singapore] at 12:19 08 November 2015
788
F. Ibe et al.
proved to be unique to the process and are of limited value as dosimeters. Several approaches have been investigated including enzymatic changes in irradiated mushrooms (Munzner 1973), protein changes in meat and poultry (Dizdaroglu and Simic 1981), volatiles from fatty acids in meat and fish (Merritt and Vajdi 1982, Boyd et al. 1991), ESR detection of trapped radicals (Dodd et al. 1985, Lea et al. 1988, Gray and Stevenson 1990), thermoluminescence and chemiluminescence measurements on spices (Heide and Bogl 1987, Sanderson et al. 1989) and viscosity measurements on spices (Heide et al. 1990). The presence of radiation-induced o-tyrosine (Karam and Simic 1988a,b) has been proposed as a method for detecting irradiation of chicken meat. Its formation during radiolysis arises from the reaction of radiation-generated hydroxy radicals with protein-bound residues of phenylalanine. During analysis the protein-bond o-tyrosine is released from the protein matrix by acid hydrolysis, prior to derivatization and quantification by gas chromatography/mass spectrometry. Karam and Simic (1988a) reported that free o-tyrosine was present in the unirradiated sample they examined, at a level of 12 mg/kg, and removed it from the protein fraction, prior to the acid hydrolysis step, by extraction with chloroform. The use of this, and certain other organic solvents, has also however been reported to induce the formation of o-tyrosine on occasion (Karam and Simic 1988b). Hart et al. (1988) investigated the use of successive washes of the proteinaceous matrix with distilled water and concluded that the o-tyrosine detected in unirradiated chicken (0-06 mg/kg) could not be removed by this means. Meier et al. (1988) used the method of Karam and Simic (1988a) but were unable to detect o-tyrosine in chicken even at doses of up to 40 kGy. However, by modifying a high performance liquid chromatography (HPLC) procedure, originally developed by Ishimitsu and co-workers (1986) for the measurement of o-tyrosine in serum, Meier et al. (1988) devised an alternative method for the measurement of o-tyrosine in the acid hydrolysate of chicken meat. These authors did not however employ a prior solvent extraction step as levels of free o-tyrosine in unirradiated chicken were apparently all at or below 0-05 mg/kg (Meier et al. 1988, 1990). We have examined the method of Meier et al. (1988) and wish to report our findings on the levels of o-tyrosine in unirradiated chicken tissue and also the precision of the results obtained when the procedure is applied to irradiated samples. Modifications to the assay are described and their effects on the precision of the assay are discussed.
Experimental
Materials Three whole, unfrozen, uncooked chickens were purchased at separate retail food outlets in Norwich during February 1989. The muscle tissue from the breast of two of the birds was removed and each of the two samples divided into five approximately equal pieces of ca 110 cm3. The leg muscle tissue (drumstick) was removed from the remaining bird and similarly divided into five sub-samples. The 15 tissue samples were sent to Isotron (Swindon, UK) and one sub-sample from each of the three portions was irradiated (60Cobalt source, 10 kGy/h, 21 °C) at 2-5, 5-0, 10-0 and 20*0 kGy. One sub-sample from each portion remained unirradiated. The samples were then all returned to Norwich and stored at -22°C until analysis.
O-tyrosine in irradiated chicken
789
Downloaded by [NUS National University of Singapore] at 12:19 08 November 2015
Methods Defatting and freeze drying. The chicken tissue was allowed to thaw for 2 h and a weighed portion (ca 10 g) minced for about 30 s using a hand-operated mincer. The homogenate was transferred to an electric mixer and blended with chloroform/methanol (2:1; 50 ml) for 3 min. The resulting suspension was filtered, using a water pump, and the proteinaceous residue removed and blended with ethanol/water (4:1; 150 ml) for 5 min. The suspension was again filtered and the residue washed successively with ethanol/water (4:1; 100 ml; used previously to rinse the blender), acetone (100 ml, Distol grade) and water (100 ml). The sample was quantitatively transferred to a pre-weighed, round bottom flask and freeze dried for at least 24 h. The ratio of [(chicken tissue):(defatted and freeze dried material)] was calculated and used to express the results for o-tyrosine in terms of original chicken tissue. Acid hydrolysis. A weighed portion of the defatted, freeze-dried sample (ca 50 mg) was placed in a hydrolysis tube (15 x 160 mm) together with alpha-methyl tyosine (internal standard, 40 /d, 26-2 /*g/ml) and methane sulphonic acid (0-9 ml, 4-0 M). The contents were frozen in liquid nitrogen and the tube evacuated on a water pump for 5 min prior to sealing it. On warming to ambient temperature the tube was heated at 110°C for 24 h in a heating block. The hydrolysate was adjusted to pH 5*9 ± 0 - 0 3 with sodium hydroxide (4-0 M) and filtered using a 0-45 ^m PTFE membrane filter (Anachem, UK). HPLC analysis. A 20 /d aliquot of the neutralized hydrolysate was analysed using a Hypersil H - 5 - O D S column (250 mm x 4-6 mm i.d.) connected to a Perkin Elmer 3000 fluorescence spectrometer (excitation: wavelength 270 nm, slit 15 nm; emission: wavelength 303 nm, slit 20 nm). The chromatograph was operated at room temperature, flow rate 1-0 ml/min, and solvent programmed as follows: 0 to 17 min, 100% solvent A (acetonitrile + 1-0% w/w aqueous sodium chloride; 9:991, v/v); 17 to 21 min, linear gradient to 100% solvent B (acetonitrile+ 0-5% w/w aqueous sodium chloride; 640:360, v/v). Under these conditions o-tyrosine and alpha-methyl tyrosine eluted after 16-3 and 12-8 min respectively. The column was reconditioned with 14 ml of solvent A prior to the next injection and stored overnight in acetonitrile/water (800/200). Each sample was injected onto the HPLC column five times and the mean peak area used to quantify the content of o-tyrosine. Results and Discussion The irradiated and unirradiated chicken tissue samples were examined using the method devised by Meier et al. (1988) and the results are shown in Table 1. oTyrosine was detected in all three of the unirradiated samples examined, i.e. the leg muscle tissue and both of the breast muscle tissue samples, in levels of between 0*5 mg/kg and 0-9 mg/kg. Higher concentrations of o-tyrosine, up to 3-4 mg/kg, were observed when the samples were irradiated at doses of 2-5, 5-0, 10-0 and 20-0 kGy. However, the precision of the data was poor such that there seemed little prospect of using the method for dosimetric purposes. In order to improve the potential of the method for discriminating between irradiated and unirradiated chicken tissue, a solvent extraction step was developed as described in the Experimental section, to remove any free o-tyrosine that might be present in sample. Analysis of the two unirradiated chicken breast and the leg muscle tissue samples showed that the levels of o-tyrosine in the analytical samples,
Downloaded by [NUS National University of Singapore] at 12:19 08 November 2015
790
F. Ibe et al.
i.e. the defatted, dried, proteinaceous residue, were reduced to less than the 0-01 mg/kg detection limit of the method. To improve the precision of the analysis an internal standard, alpha-methyl tyrosine, was introduced. In addition, the pH of the hydrolysate was adjusted to 5 • 9 to avoid the high acidity problems associated with the original method in which the acid hydrolysate was diluted five-fold with water and injected directly onto the column. Chromatograms of unirradiated chicken and samples irradiated at 2*5 kGy and 20 kGy are shown in figure l(a-c). The recovery of o-tyrosine added to chicken tissue at a level of 2-2 mg/kg averaged 103% (standard deviation 3%, five determinations). The modified method was applied to the irradiated and unirradiated chicken tissue samples and the results are shown in Table 2. Each sample was analysed in two separate analytical batches. It is evident that there is a reasonably close correspondence between the duplicate analyses, for each tissue sample, at the five dose levels examined. The agreement between duplicate determinations and their respective means averaged ± 1-7% as defined by the term [(a - b)j(a + b)] x 100% where a and b are the duplicate determination values. The results also show that for each of the three samples the amount of o-tyrosine detected increases as the irradiation dose is raised. It is of interest that the levels of o-tyrosine formed are very similar for both the two samples of chicken breast tissue and the leg muscle tissue. In conclusion the procedures that have been developed enable o-tyrosine to be quantified with precision in irradiated chicken tissue. The solvent extraction step permits any free o-tyrosine present in the unirradiated sample to be removed, Table 1. Mean levels of o-tyrosine (mg/kg) in chicken tissue samples using method of Meier el al. (1988). Dose (kGy) 0 2-5 5-0
10-0 20-0
Breast tissue (sample 1) 0-5(0-4, 0-6) 0-8(1-0,0-6) 0-9(1-0,0-8) 3-3(5-2,1-3) 3-4(4-7,2-1)
Breast tissue (sample 2) a
0-9(1-0,0-8) NA NA NA NA
Leg tissue 0-6(0-7, 0-5) 1-9(3-1,0-6) 1-3(1-8,0-8) 1-8(2-3,1-2) 2-9(3-7,2-0)
a
Results of individual analysis (whole procedure) in parenthesis. NA, not analysed.
Table 2. Mean levels of o-tyrosine (mg/kg) in chicken tissue samples using current method. Dose (kGy)
Breast tissue (sample 1)
Breast tissue (sample 2)
Leg tissue
0 2-5 5-0
