Radiation Chemistry and the Radiation Preservation of Food Irwin A. Taub Radiation Preservation of Food Division, Food Enaineerina Laboratory, U S Army Natick Research and Development Command, Natick, MA 01760 Irradiation can improve and preserve certain foods by reducing or eliminatin~deteriorative biological or physiological agents ( I ). It can deEtroy insects, senescence, retard mold growth, and eliminate disease-causing and spoilagecausine bacteria. As examoles: irradiatine" flour destrovs the infesting insects, including their eggs; irradiating potatoes or onions prevents the formation of sprouts; irradiating strawberries retards the spreading of fungi; irradiating poultry, using pasteurizing doses, reduces the levels of Salmonella (which cause salmonellosis) and of Pseudornonas and lactobacilli (which contribute to putrefaction); and irradiating pre-cooked meats, using s&rilizing doses, eliminates afi microorganisms (including C1. botuhnum, which causes botulism) and renders such foods indefinitely stable while stored a t ambient temperatures. Concomitant with these desirable effects, irradiation of food leads through a sequence of reactions to certain chemical changes in the food components. Some conversion of minor components takes place, some degradation of proteins and linids occurs. and some low molecular weieht volatile compounds are formed. Knowing the nature and levels uf the r:ldiolvsis oroducts in dlfferew t w d n irradiated under different conditions is essential for evaluating the wholesomeness of irradiated foods and, ultimately, for obtaining clearances from the Food and Drug Administration (FDA) for the unlimited marketing for public consumption of irradiated foods. Consequently, one must understand the radiation chemical reactions occurring in food components and put the effects observed into perspective in order to facilitate the acceptance and commercialization of radiation preserved foods. One needs to show that these reactions lead to key radiolysis products whose yields are low and can he predicted on the basis of food comoosition and irradiation conditions. Such generalizations enable health authorities in this country and abroad to clear those eenericallv related foods that are similarly irradiated. ~hese"genera1izationsalso serve as a guide to industry in developing and improving irradiated food products. Accordingly, common features in the radiation chemistry of food components will he described and our capability to predict product yields will he illustrated. As an aid in understanding these features and this predictability, certain considerations pertaining to irradiating food will he mentioned first. Data will then he presented that pertain to the radiolysis (in model systems and in meats) of nitrate ion, metmyoglobin, myosin, and tripalmitin to illustrate the approach being used. Bask Considerations The chemical and physical complexity of food introduces certain special features in the radiolysis. Several distinctly different components are present in several distinctly different phases. Depending on solubilities, many minor constituents are found either free or hound within one or more major components. Depending on the degree of homogenization or emulsification. there would he a wide ranee of interfacial contact area among otherwise immiscible components. These features have an influence on the formation and reaction of intermediates.
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To a first approximation, the energy deposited in a food will be apportioned according to the weight fraction of each component, leading to the formation of intermediate ions, excited molecules, and free radicals clustered in spurs in each major component. In the case of relatively solid foods, particularly foods with a high water content that are irradiated while frozen. most of these intermediates will remain nonu~~-~~~ niformly distrihuted. Some of the smaller and highly mohile intermediates could eventually become uniformly distributed. As a consequence of the rigidity of the medium and of the free radicals being associated with large protein and lipid molecules, the yields of radicals could be lower than in simple model systems and their subsequent reactions would not likelv be governed by simple laws ofkinetics. The products derived from these intermediates, therefore, will be influenced by the physical state of the food and the state. irradiation parameters (2). With regard to ~hvsical . . i n w n >?stems yield fener prad.lct- I I K I O 1 1 ~ syitem? 4 because thr rigid med~umpromutes rectrmhi~utlouoiprimary radicals. W ~ t hreeard to irradiatim onramrters: hieher dgws ... lead to a higher concentration ofprohucts hut not a change in the snectrum of oroducts. because the vields are linear: dose-rate does not cave a significant effeci, hecause the re: actions mainlv involve radicals in the s ~ u rand s because the "solute" concentrations are extremel; high; and whether oxygen is present also is important, because radical-oxygen reactions could take =olace. --Treatment of the radiation chemistry can he simplified. however, since each phase is effectively i n independent sys: tem. The irradiation of pre-cooked, frozen meats illustrates this independence. In effect, there are three main phases: an aqueous phase, constituting -65% of the weight and containing primarily soluble salts, free amino acids, soluble proteins, and specific micronutrients such as the vitamins; a hydrated protein phase, constituting -20% and containing minor compounds such as salts and bound vitamins; and a lipid phase (comnrised mainlv of trielvcerides derived from maior .. . tatty acids), constituting -1S0~andc~n~t;~inini: other far-solu l h nutrients inchdine. - vitaminn. ' l ' h ~fdlowine discussion of each phase or representative a m l s m e n t rrlntm to model sysknm and, where possible, t,, art.uai twds. ~~
to
Aqueous Phase: Radiolvtic Reduction of Nitrate Ion Nitrate ion is representative of a radiolytically sensitive salt in the aqueous phase. I t is added to certain meats as part of the curing solution, in part to stabilize the pink color of the cured meat. Its reduction to nitrite ion, which is directly involved in color formation (31, is of special interest. The mechanism and efficiency of its radiolvtic reduction under different conditions needed to be unde&od. Information on the nitrate radiolysis was obtained usingmodel svstems of nitrate, irradiating at r w m temprrature and at sub-treezing temperatures, and anslyling electrnchemically with nitrarr and nitrite ion-select~vrrkn.tndt.s. 'l'he model solutions contained t-hutanol to scavenge OH. radicals and were nureed with nitrneen to eliminate oxveen. ~ h major r effect on &ate is the reductiwi U, nitrite. Figure I shows the 106s of nitrate as function of closr in s~~lutions lo-? M in nitrate a t 20°C. The slope of this line amounts to -1.6
drop in G(N02-) when irradiating frozen systems and the need for high NOS- concentration to bring about reduction in the frozen system are consistent with the difficulty of scavenging e,- in the ice. The increase in G(N02-) in the frozen svstems as the temperature increases is consistent with G(e,-) increasing in ice with increasing temperature. Similar differences in effects between fluid and frozen systems have l w n UINrwd I W c,tIwr r t , n ~ ~ h ~ ,~7 ~ n d s TIw Iml).l,nrt.m dth,.,, r, sulf- r;,! irrndiarins mear i; thnt only a sm& fraction of nitrate in cured meat wodd be reduced a t -40°C. Such meat might contain 50 ppm nitrate, which is M. and be irradiated to 4 Mrad a t -40°C. annroximatelv Based on G(NO2-) s 0.0035 for this nitrate concentration a t this temperature, then no more than about 2% of the nitrate would be lost, unless radicals formed from soluble proteins also react with nitrate. Recent experiments on ham with added nitrate by Kamarei e t al. show only a small loss due to irradiation (8).
.~ .~
1
50
100
DOSE. KRADS
150
I
Figure 1. Loss of Nibate Ion as a Function of Gamma-Irradiation Dose in a Sodium Nitrate Solution at 20% Solution contained M NaN03 and 10C M (CH&COH, and was purged with N.,
Aqueous Phase: Radioiytic Reactions of MetMyoglobln Myoglobin, except for the kinetic influences of its ironnorohvrin center. is renresentative of the soluble nroteins in lhe ;quew: phaIXII, coordinate p d i m . For example, mctmwgld,in ~mel\lntconrnins tri\nlrnt ir.m nnll is brcwn: dt~eryrny~>gl ,.....,,
171 Tauh, I. A., Ksprielian, R. A..snd Hslliday,J. W. "Food Pr~sorvationby insdiation? IAEA.Vienna,1978,Vul. 1.371. 181 Karel, M.,Kamard, A. R..and Wierbicki, E., "Roc 26th European MootingofMeal Hosearch Wurkers."C~,lorndnSnrinza.. Seot. 19Pa.AmericanMeatSei.A-"..Chi-
. ..
.
.
c a p , Prpor E-15, in p m s . 19) Shieh,J..I., Whitburn,K., Hoffman,M, Land Taub. I. A., "Pm. I n t Symp on Recent Advancer in Povd and Technolugy:Tsipei, Jan. 1980,in PIOIE. (10) Shieh. J. .I.. Sellers. R. M., Hofimsn. M. %.. andTaub.I. A , in "RadiationBioloavand
Relevance to Chemi-clearance The concepts and data presented here should serve as a basis for "chemi-clearance," which is the use of chemical data by health authorities to 'clear irradiated foods for public consumotion. Part of this basis is the commonalitv and predictahilky of the chemistry. Commonality implies that similar reactions occur in homologous protein and lipid components; predictability implies that such reactions proceed independently of other components and in proportion to the ahundance of each component. Comparative ESR and analytical measurements made on pre-cooked ham, chicken, pork, and heef further confirm these imnlications. The ESR soectra a t -40°C for all the meats show the same pattern corresponding to contributions from protein and lipid radicals (2). The analyses of lipid products, as described in part above, show the similaritv of detectable comoounds and their dependence upon dose, total fat, fatty acidcomposition, and (to a first approximation) triglyceride composition (32). Several applications of chemi-clearance are now being pursued or soon will he p \ ~ r \ ~ wIntrrnationally, l. a meeting of the Joint Expert Committw un Food Irradiation (sponsored by WHO FA0 IAEAI will he wnvened in October 1980, to consider clearing all major classes of food irradiatedup to an average dose of 1Mrad. Domestically, the clearing of meats
1171 Canlron. W.. Xodiot. Rea. Rm., 3,305 119721. 1181 Sevilla, M. D.. D'Arey. .I. R.. and Morehouse, K. M., J Phys Chom., 83. 2893
(24) Hallid&J.
~.,~aa~ersen,~.~.,kekerson.C.L.,Rees,C.R.,andTaub,l.A.,IBEI:
Trans.Nur. Sci..NS-26,1771 11979). (25) Merritt,C. Jr..Angelini,P.,and Nawar, W. W., "Food Preservation by irradiation: IAEA, Vienna, 1978,Voi. 11.97. 126) Merritt,C., Jr.,A~gdini, P.. and Vajdi, M., in prepamtion. I271 Merrift. C., J?.,Appi.Sprcf. Re".. 3. 263 (19701. (281 Hallidav. J. W.and Taub.1. . A,.. in .oreoaratinn. . I291 Sevilla. M. D., privatz communication. I301 Taub,I.A.,Hallidsy.J.W., Walker,J.E..Anpelini.P..Vajdi. M..andMerritt.C.,Jr., '"Pm.26thEurupean Meeting 01MeaLRespsrch Workers: ColoradoSprinp,Sept. 19RO.Amer. Meat Sei. Assa.. Chieapo, I'rpe. E-18.in prerr. 1.31)Vsidi. M.. Nawar. W. W.. and Merritt. C. J. ..in ornoeration.
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February 1981
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