J Sci Food Agric 1998, 78, 559È564 Eþect of Irradiation Dose and Irradiation Temperature on the Thiamin Content of Raw and Cooked Chicken Breast Meat William D Graham,1* the late M Hilary Stevenson1,2 and Eileen M Stewart1,2 1 Food Science Division, The Department of Agriculture for Northern Ireland, Newforge Lane, Belfast, BT9 5PX, UK 2 The Department of Food Science, The QueenÏs University of Belfast, Newforge Lane, Belfast BT9 5PX, UK (Received 2 September 1997 ; revised version received 2 March 1998 ; accepted 6 April 1998) Abstract : The usefulness of ionising radiation for the elimination of pathogenic bacteria in poultry meat has been well documented as have the e†ects of this processing treatment on the nutritional status of the food, in particular, the vitamins. Unfortunately, much of the earlier research carried out on the e†ect of irradiation on vitamins was carried out in solution or in model systems at doses much greater than those used commercially thereby resulting in considerable destruction of these compounds. Thus, those opposed to the process of food irradiation labelled the treated food as nutritionally poor. However, in reality, due to the complexity of food systems the e†ects of irradiation on vitamins are generally not as marked and many processes, for example cooking, cause the same degree of change to the vitamins. Thiamin (vitamin B ) is the most radiation sensitive of the water-soluble vitamins and is therefore 1a good indicator of the e†ect of irradiation treatment. In this study the e†ects of irradiation at either 4¡C or [20¡C followed by cooking on the thiamin content of chicken breast meat was determined. Results showed that whilst both irradiation and cooking resulted in a decrease in thiamin concentration, the losses incurred were unlikely to be of nutritional signiÐcance and could be further minimised by irradiating the chicken meat at a low temperature. Thiamin analyses were carried out using high-performance liquid chromatography since this technique is faster and more selective than the chemical or microbiological methods more commonly employed. Total thiamin, both free and combined form, was determined following acid and enzyme hydrolysis. ( 1998 Society of Chemical Industry. J Sci Food Agric 78, 559È564 (1998) Key words : irradiation ; chicken breast meat ; thiamin ; vitamin B ; dose ; 1 cooking ; temperature ; high-performance liquid chromatography Campylobacter. Other food poisoning organisms which may be present include L isteria, Clostridia and Staphylococcus. Most foodborne disease attributed to the consumption of poultry and other meats is a consequence of inadequate cooking of the product and/or improper handling of the products after cooking (Klinger and Lapidot 1993). However, extensive research has demonstrated that exposure of fresh poultry carcasses to an irradiation dose of 2É5 kGy will signiÐcantly reduce the number of bacterial pathogens, if present (Mulder 1982 ; Kampelmacher 1983 ; Urbain 1983 ; Thayer et al 1991). INTRODUCTION One of the main uses of low-dose irradiation is to enhance food safety through the inactivation of pathogenic microorganisms such as Salmonella and Campylobacter which are known to be the most common agents of food poisoning. It has been reported (Roberts 1990) that 60È80% of poultry sold in the UK may be contaminated with Salmonella and up to 100% may contain * To whom correspondence should be addressed. 559 ( 1998 Society of Chemical Industry. J Sci Food Agric 0022È5142/98/$17.50. Printed in Great Britain 560 These Ðndings have played a part in bodies such as the Food and Drug Administration (FDA) in the USA giving approval for the irradiation of chicken (FDA 1990), and the British Medical Association (BMA 1989) identifying irradiation as the only processing technique which is likely to overcome the problem of food poisoning from chicken. Doses higher than 2É5 kGy are, however, required if the product is in the frozen state and it has been demonstrated that irradiation can be carried out with doses up to 7É5 kGy at low temperatures ([18¡C). Irradiation in the frozen state markedly reduces the organoleptic changes that would occur at the same dose if treatment was carried out at 4¡C (Coleby et al 1961 ; Mulder 1984). In 1981 the report of the Food and Agriculture Organisation/International Atomic Energy Agency/ World Health Organisation Joint Expert Committee on Irradiated Food (JECFI 1981) concluded that “irradiation of food up to an overall average dose of 10 kGy presents no toxicological hazard and introduces no special nutritional or microbiological problemsÏ. However, those opposed to the use of the irradiation process have highlighted the loss of nutritional value of foods as a major concern, especially with respect to vitamins. Reports of high vitamin losses from irradiation are largely due to the fact that many early studies were carried out using pure vitamin solutions, or by using doses higher than those which would be used to irradiate food commercially. Thus, actual losses were overestimated and bore no relevance to what happens in the actual food where the complexity of the composition of the food often protects individual vitamins from destruction by ionising radiation (Kilcast 1994). In addition, other food preservation techniques, such as those involving cooking, also destroy vitamins, so the e†ect of irradiation on these minor components is not unique (Stevenson 1994). Thiamin (vitamin B ) is recognised as the most radi1 ation sensitive of the water-soluble vitamins as well as being extremely heat labile (Josephson et al 1978). For these reasons, thiamin is considered to be a good indicator of general vitamin losses in food. However, the e†ect of irradiation on this vitamin may vary according to the type of food and the conditions during the irradiation process (Diehl 1990). As chicken meat is a signiÐcant source of thiamin in the diet it is important that the e†ect of irradiation on the concentration of this vitamin is studied along with the inÑuence of cooking post-irradiation. However, considerable variations in the thiamin concentration of irradiated chicken meat have been reported in the literature (Fox et al 1989 ; Hanis et al 1989). Therefore, this study was designed to investigate the e†ects of irradiation dose and temperature of irradiation on the thiamin content of raw and cooked chicken breast meat. In most of the work reported to date, thiamin concentrations have been determined by either chemical or microbio- W D Graham, the late M H Stevenson, E M Stewart logical methods. These methods are considered to be time consuming and may be subject to interference from other food components (Finglas and Faulks 1984). High-performance liquid chromatography (HPLC) coupled with Ñuorescence detection has the advantages of selectivity and speed of analysis, therefore, it was used as the means of thiamin analysis for this experimental work. MATERIALS AND METHODS Sample preparation and analysis Forty eight oven-ready chickens were obtained from a local chicken processing plant in Northern Ireland in order that all the samples used were from the same batch and were fresh on the day of purchase. The samples were maintained at chill temperature (4¡C) prior to use. They were then divided into two groups of 24, the breasts removed and packed together in matching pairs. Twenty four matching pairs were irradiated at an environmental temperature of 4 ^ 1¡C while the other group of 24 was frozen at [20¡C prior to being irradiated in dry ice at an environmental temperature of 1 ^ 1¡C. Four matching pairs were irradiated at doses of either 1, 2É5, 5, 7É5 and 10 kGy whilst 4 were left unirradiated and served as controls. Cobalt 60 was used as the source of ionising radiation (Gammabeam 650, Nordion International Inc, Kanata, Canada) at a dose rate of 1 kGy h~1. Two perspex dosimeters (Type 3042 B, Harwell Dosimeters Ltd, Harwell, UK) were placed one on either side of two chicken breasts at each dose. The change in absorbance of the dosimeters was measured spectrophotometrically at 603 nm and the corresponding doses calculated using calibration graphs provided by the National Physical Laboratory (Teddington, Middlesex, UK). Following irradiation, one breast from each matching pair was wrapped in aluminium foil and cooked in a convection oven at a temperature of 180¡C until the internal temperature reached 82È87¡C. Frozen breasts were allowed to thaw in a refrigerator at approximately 4¡C prior to cooking. Thiamin was determined by HPLC using a modiÐcation of the method of Finglas and Faulks (1984). Following removal of the skin and bone, the chicken meat was cut and minced in order to obtain a homogeneous sample. A 10 g sample was blended with 50 ml of 0É1 M hydrochloric acid and autoclaved for 30 min at 121¡C (103 kPa pressure). After cooling to \50¡C, 5 ml of 2 M sodium acetate bu†er and 5 ml of freshly prepared 5% (w/v) takadiastase were added to bring the pH to approximately 4É5. The mixture was incubated for 2É5 h in a shaking water bath maintained at 48¡C, cooled, diluted to 100 ml and Ðltered through Whatman No 541 Ðlter paper. The Ðltrate obtained was used for thiamin E†ect of irradiation dose, irradiation temperature and cooking on thiamin in chicken breast meat analysis by conversion of thiamin to its thiachrome derivative and subsequent injection of a 10 ll aliquot into a HPLC (Hewlett Packard HP1090 Liquid Chromatograph). The mobile phase used consisted of 60% methanol and 40% water. The Ñow rate was 0É9 ml min~1 and the HPLC column was a 25 cm ] 4É6 mm id ODS 2 column packed with 10 lm spherisorb (Phase Separations Ltd, UK). Conversion of thiamin to thiachrome was achieved by addition of 3 ml freshly prepared alkaline ferricyanide solution (15 g sodium hydroxide and 1 g potassium ferricyanide per 100 ml) to 5 ml of the extract which was left in the dark for 10 min. It is essential that the latter procedure is carried out in subdued light. Timing is also critical and injection of the sample into the HPLC must be made within 5 min of completion of the reaction. A Ñuorescence detector set at 375 nm excitation and 435 nm emission was used and identiÐcation made by comparison with a standard thiamin solution similarly derivatised. Standard solutions of thiamin prepared as described for the experimental samples, were used to check recoveries of thiamin. The dry matter content of the samples was determined after oven drying (100¡C) duplicate 10 g samples to a constant weight. Statistical analysis The experimental results were subjected to analysis of variance using a Genstat 5 package. RESULTS Statistical analysis of the vitamin results expressed on either a fresh matter or dry matter basis gave similar results ; therefore, only the concentrations based on fresh weight are referred to in this section although both are presented in Table 1. Overall, both irradiation dose and temperature had a signiÐcant e†ect (P \ 0É001) on the thiamin content of the chicken breast meat (Table 1 ; Fig 1). At an irradiation temperature of 4¡C (chilled state), the samples given a dose of 1 kGy had similar thiamin concentrations to the non-irradiated control samples. However, when the irradiation dose was increased to 2É5 kGy, which is the dose used commercially for fresh poultry, the thiamin content had decreased by approximately 20%. On the other hand, when the chicken was irradiated in the frozen state at [20¡C an approximate reduction of only 6% was observed at the same dose. The frozen samples were found to have approximately 16% more thiamin than those samples maintained at 4¡C. There was a linear decrease (P \ 0É001) in thiamin content with increasing dose up to 10É0 kGy although the diminution was less in the samples irradiated in the frozen state compared with those treated at 4¡C. At a 561 Fig 1. E†ect of irradiation temperature and dose on the thiamin concentration of uncooked and cooked chicken breast meat (Standard error of the mean \ 7É89 ; n \ 8). dose of 5É0 kGy the loss of thiamin in samples maintained in the chilled state during irradiation was approximately 30% compared to 11% at [20¡C. When samples were given 7É5 kGy, thiamin losses were 37% and 21% for 4¡C and [20¡C, respectively, whilst at 10É0 kGy they were 43% and 21%, respectively. Whilst a loss of approximately 20% thiamin occurred when the samples maintained at 4¡C during irradiation were given a dose of 2É5 kGy, a comparable loss only occurred in the samples irradiated frozen at doses of 7É5 and 10É0 kGy. As expected, cooking the chicken breasts resulted in a signiÐcant reduction (P \ 0É01) in the thiamin content of both irradiated and non-irradiated samples (Table 1 ; Fig 1). Cooking reduced the thiamin content of the nonirradiated controls by approximately 20%, for both the chilled and frozen samples. Thus, the thiamin loss sustained is equivalent to that caused by irradiation at a dose of 2É5 kGy and a temperature of 4¡C. For samples irradiated at [20¡C a combination of 2É5 kGy followed by cooking is required to give a comparable depletion. When samples irradiated in the fresh state at 4¡C were cooked, the decrease in thiamin content varied between 20 and 26% over the dose range of 1È10 kGy. The e†ect of cooking was less marked in samples irradiated at [20¡C where thiamin losses ranging from 12 to 18% were recorded. At the commercial dose of 2É5 kGy, the samples irradiated in the frozen state and cooked retained 22% more thiamin than those samples which were irradiated chilled and then cooked while the uncooked frozen samples contained approximately 12% more than those uncooked samples irradiated at 4¡C. A highly signiÐcant two-way interaction was observed between irradiation dose and cooking (P \ 0É01). This was due to the occurrence of di†erent linear trends with dose for the uncooked and cooked samples. A constant diminution in thiamin concentration with dose, approximately 12% between each dose level from 1 to 10 kGy, was observed for the cooked samples. However, in the case of the uncooked samples the reduction with dose was less pronounced as, for W D Graham, the late M H Stevenson, E M Stewart 562 TABLE 1 E†ect of irradiation dose, temperature of irradiation and cooking on the dry matter and thiamin content of chicken breast meat Irradiation dose (kGy) Uncooked Cooked Chill Frozen Chill Frozen 0 1É0 2É5 5É0 7É5 10É0 258É1 261É9 255É6 256É0 263É0 261É4 Dry matter concentration (g kg~1) 256É3 315É3 252É4 315É7 258É1 318É0 255É8 311É5 250É6 315É3 255É7 323É3 310É6 307É0 309É5 306É0 305É6 314É0 0 1É0 2É5 5É0 7É5 10É0 145É5 147É3 116É7 101É6 91É1 82É6 T hiamin concentration (lg per 100 g FW ) 140É8 116É6 135É5 118É5 132É3 91É0 125É7 75É4 110É4 68É3 110É9 65É3 113É6 113É3 116É3 106É3 91É0 94É6 0 1É0 2É5 5É0 7É5 10É0 563É0 562É7 455É9 396É6 346É6 317É1 T hiamin concentration (lg per 100 g DW ) 548É3 370É1 536É7 375É8 513É8 286É4 491É8 242É6 440É4 217É4 433É9 202É7 365É5 369É8 378É0 348É1 298É3 301É4 SEM/SigniÐcance of e†ect Dose (D) Uncooked/cooked (C) Chill/Frozen (F) D]C D]F C]F D]C]F (n \ 32) (n \ 96) (n \ 96) (n \ 16) (n \ 16) (n \ 48) (n \ 8) DM TC (FW) TC (DW) 2É35 NS 0É98*** 1É36** 2É90 NS 3É32 NS 1É68 NS 4É10 NS 5É41*** 0É78*** 3É13** 5É58** 7É66** 3É22* 7É89 NS 19É57*** 3É00*** 11É30*** 20É25*** 27É68* 11É69 NS 28É64 NS NS, not signiÐcant ; * P \ 0É05 ; ** P \ 0É01 ; *** P \ 0É001. DW, dry weight ; FW, fresh weight ; TC, thiamin concentration. example, there was a 12% decrease between the 1 and 2É5 kGy dose levels while the e†ect was less marked between 7É5 and 10 kGy with a decrease of only 4% being found. The signiÐcant interaction (P \ 0É05) observed between cooking and irradiation temperature was most likely due to the fact that samples maintained at [20¡C during irradiation and then cooked exhibited less of a decrease in thiamin concentration than those irradiated at 4¡C prior to cooking. Irradiation had the e†ect of depleting thiamin content at both irradiation temperatures employed and, as stated earlier, the diminution increased with dose. However, di†erent linear trends were found with dose for each irradiation temperature which led to a highly signiÐcant (P \ 0É01) interaction between irradiation dose and temperature. The depletion of thiamin content in samples irradiated at [20¡C was much less marked than those treated at 4¡C. Similar concentrations of the vitamin were measured in the frozen samples at both the 7É5 and 10É0 kGy dose levels while the equivalent amount in the chicken meat irradiated at chill temperature was found in samples which had received a dose of 2É5 kGy. DISCUSSION The Ðndings of this work agreed with those reported previously for bacon (Thayer et al 1989) beef liver (Williams et al 1958) chicken (Fox et al 1989 ; Hanis et al 1989), minced beef (Wilson 1959), pork (Fox et al E†ect of irradiation dose, irradiation temperature and cooking on thiamin in chicken breast meat 1989) and turkey (Thomas and Calloway 1957 ; Fox et al 1995). An increasing loss of thiamin in chicken meat was observed with increasing irradiation dose but when the irradiation treatment was carried out at freezing temperatures there was a marked reduction in the e†ects observed. Thus, the results conÐrm that destruction of the vitamin reÑects the dose applied and the conditions used during irradiation. However, as noted in the introduction, considerable variations in thiamin concentration in chicken meat following irradiation have been reported in the literature. Hanis et al (1989) recorded a loss of 43É6% thiamin when chicken meat was given an irradiation dose of 5 kGy at an environmental temperature of 10¡C. This depletion increased to 57É3% when a 10 kGy dose was used. In comparison, the thiamin destruction due to irradiation reported by Fox et al (1989) was much less marked with a 27É2% loss being observed at a dose of 5 kGy and irradiation temperature of 10¡C. The results reported in the present study clearly agree with those of Fox et al (1989) as 30É2% less thiamin was measured in samples given a dose of 5 kGy at a temperature of 10¡C. When chicken was irradiated at the same temperature with a 10 kGy dose the thiamin loss was approximately 43É2%, being equivalent to that observed by Hanis et al (1989) for a 5 kGy dose at 10¡C. Both Fox et al (1989) and Hanis et al (1989) found that less thiamin was destroyed when samples were irradiated in the frozen state which is in agreement with the Ðndings of this experimental work. Fox et al (1989) observed a loss of 8É8% at a dose of 3É3 kGy and irradiation temperature of [20¡C while Hanis et al (1989) found a 22É4% loss when samples received 2É5 kGy at [15¡C. The results reported by Fox et al (1989) compare favourably with those recorded in the present work where a diminution of 6% was measured when chicken meat was irradiated in the frozen state with a dose of 2É5 kGy. Fox et al (1989) observed further thiamin losses when irradiated chicken was cooked which was also the case in this experimental work. According to Fox et al (1995), the cookingÈdose e†ect appears to involve chemical or physical changes in the tissues or vitamin which result in increased vitamin destruction or decreased extractability. However, substantially greater thiamin losses due to cooking were observed in the present study. Fox et al (1989) found that when chicken meat was given an irradiation dose of 3É3 kGy at 0¡C and cooked the thiamin concentration was approximately 10% less than that of cooked non-irradiated samples. However, in this experimental work when chicken was given an irradiation dose of 2É5 kGy at 4¡C and cooked a thiamin loss of approximately 22% was measured, being double that recorded by Fox et al (1989). The variations observed could possibly be due to the di†erences in cooking which although small, nevertheless illustrate the e†ect of heat on thiamin. Fox et al 563 (1989) cooked chicken breasts in a convection oven at 191¡C until the samples reached an internal temperature of 82¡C. In this experimental work the samples were cooked to an internal temperature of 82È87¡C. The Ðndings of this study support those of De Groot et al (1972). These workers found that chicken irradiated with a dose of 6 kGy contained approximately 27% less thiamin than non-irradiated cooked chicken which is comparable with the 26% reduction observed in this work at the 5 kGy dose level. It should also be noted that despite the fact there are reductions in the thiamin content of meats following irradiation, the vitamin is even more sensitive to heat than to irradiation. Whilst cooking generally tends to increase vitamin loss, in some instances, the combination of irradiation and cooking has been shown to actually reduce losses. When studying the e†ects of irradiation on the B vitamins in the legume red gram, Sreenivasan (1974) found that the vitamins, thiamin included, were retained better in samples irradiated and then cooked than in the corresponding non-irradiated controls. Irradiation resulted in a reduced cooking time for the red gram and as prolonged heating is known to destroy the B vitamins it was presumed that this reduction accounted for the better retention of the vitamins in the radiationÈ processed cooked samples. However, the Ðndings of Sreenivasan (1974) were not observed in this study as irradiation followed by cooking did not reduce thiamin loss. Nevertheless, as indicated by Diehl (1990), further studies of the combination e†ects of irradiation and heat would be advantageous. Whilst Hanis et al (1989) reported a threshold dose level of 0É5 kGy for chicken meat, the present study found a threshold e†ect up to 1É0 kGy. Fox et al (1989) did not observe any threshold e†ect for either chicken meat or pork. The greater thiamin losses reported by Hanis et al (1989) are difficult to explain although, since a spectrophotometric method was used, they may be due to interference from other food components present. The advantage of the HPLC methodology used in the present study, apart from speed, is its selectivity. As reported in the experimental section of this paper both the preparation of the thiachrome derivative and its subsequent injection into the HPLC system are critical due to the instability of the thiachrome and may be the root cause of any discrepancies. CONCLUSIONS Thiamin concentrations were reduced by irradiation but when the process was carried out at [20¡C the e†ect was minimised thus emphasising the importance of optimising irradiation conditions. The loss of thiamin following cooking was similar in both non-irradiated and irradiated samples, thereby illustrating the fact that other food processes can cause the same degree of W D Graham, the late M H Stevenson, E M Stewart 564 change to the nutrients. Although there was a decrease in the thiamin concentration of chicken meat following irradiation, it should be borne in mind that chicken is only one of a number of sources of thiamin in the human diet. Therefore, the reduction in total thiamin intake of an individual consuming irradiated chicken will be considerably less than 20%. REFERENCES BMA 1989 Infection Control, the British Medical Association Guide. Edward Arnold, London, UK, p 122. Coleby B, Ingram M, Shepherd H J, Thornley M J, Wilson G M 1961 Treatment of meats with ionizing radiation, VII. E†ect of low temperatures during irradiation. J Sci Food Agric 12 483È490. De Groot A P, van der Mijll Dekker L P, Slump P, Vos H J, Willems J J L 1972 Composition and Nutritive V alue of Radiation-Pasteurized Chicken (Report No R3787). Central Inst for Nutr and Food Res, The Netherlands. Diehl J F 1990 Nutritional adequacy of irradiated foods. In : Safety of Irradiated Foods. Marcel Dekker, New York, USA, pp 195È208. FDA 1990 Irradiation in the Production, Processing and Handling of Food ; Final Rule 21CFR Part 179 (55, No. 85). FDA, USA. Finglas P M, Faulks R M 1984 The HPLC analysis of thiamin and riboÑavin in potatoes, Food Chem 15 37È44. Fox J B, Thayer D W, Jenkins R K, Phillips J G, Ackerman S A, Beecher G R, Holden J M, Morrows F D, Quirbach D M 1989 E†ect of gamma irradiation on the B vitamins of pork chops and chicken breasts. Int J Radiat Biol 55 (4) 689È703. Fox J B, Lakritz L, Hampson J, Richardson R, Ward K, Thayer D W 1995 Gamma irradiation e†ects on thiamin and riboÑavin in beef, lamb, pork and turkey. J Food Sci 60 (3) 596È598,603. Hanis T, Jelen P, Klir P, Mnukova J, Perez B, Pesek M 1989 Poultry meat irradiationÈe†ect of temperature on chemical changes and inactivation of microorganisms. J Food Protec 52 (1) 26È29. Joint Expert Committee on the Wholesomeness of Irradiated Food (JECFI) 1981 Report of the W orking Group on Irra- diation of Food (WHO Technical Report Series no 659). WHO, Geneva, Switzerland. Josephson E S, Thomas M H, Calhoun W K 1978 Nutritional aspects of food irradiation : An overview. J Food Proc Pres 2 299È313. Kampelmacher E H 1983 Irradiation for control of Salmonella and other pathogens in poultry and fresh meats. Food T echnol 37 117È119, 169. Kilcast 1994 E†ect of irradiation on vitamins. Food Chem 49 157È164. Klinger I, Lapidot M 1993 Irradiation of Poultry Meat and Its Products (IAEA-TECDOC-688). IAEA, Vienna, Austria. Mulder R W A W 1982 The use of low temperature and radiation to destroy enterobacteriaceae and salmonellae in broiler carcasses. J Food T echnol 17 461È466. Mulder R W A W 1984 Ionizing energy treatment of poultry. Food T echnol Aus 36 418È420. Roberts D 1990 Sources of infection. Food L ancet 336 859È 861. Stevenson M H 1994 Nutritional and other implications of irradiating meat. Proc Nutr Soc 53 317È325. Sreenivasan A 1974 Compositional and quality changes in some irradiated foods. In : Improvement of Food Quality by Irradiation (Panel Proceedings Series). IAEA, Vienna, Austria, pp 129È155. Thayer D W, Shieh J J, Jenkins R K, Phillips J G, Wierbicki E, Ackerman S A 1989 E†ect of gamma ray irradiation and frying on the thiamine content of bacon. J Food Qual 13 115È134. Thayer D W, Fox J B Jr, Lakritz L 1991 E†ects of ionizing radiation on vitamins. In : Food Irradiation, ed Thorpe S. Applied Science, London, UK, pp 285È325. Thomas M H, Calloway D H 1957 Nutritive value of irradiated turkey. II. Vitamin losses after irradiation and cooking. J Am Dietet Assoc 33 1030È1033. Urbain W M 1983 Radurization and radicidation : meat and poultry. In : Preservation of Food by Ionizing Radiation (Vol 3), eds Josephson E S & Peterson M S. CRC Press Inc, Boca Raton, FL, USA, pp 1È11. Williams C, Yen J, Fenton F 1958 E†ects of radiation and cooking on the quality of baby beef liver. Food Res 23 473È 491. Wilson G M 1959 The treatment of meats with ionising radiation. II. Observations on the destruction of thiamine. J Sci Food Agric 10 295È300.