Irradiated meat volatiles

Most of the other products found in irradiated meat volatiles except those containing sulfur or aromatic rings may also be accounted for by mechanisms associated with alkyl free radical formation in the fat. Oxygenated compounds are far less abundant than hydrocarbons, but appreciable amounts of a homologous series of n-aliphatic alcohols up to hexanol are found. Of these, only ethanol is detected in the unirradiated controls. Since the water content of meat averages nearly 60%, the formation of alcohols may be thought to occur by reaction of the alkyl free radical with water. Such a mechanism is supported by the fact that only traces of alcohols are found in irradiated dry butterfat and were undetected in irradiated triglycerides or methyl esters of fatty acids. 

Table III. Miscellaneous Compounds in Irradiated Meat Volatiles...

The primary objective was to conduct a continuing investigation and to assemble facts relating to the analytical composition of volatile compounds produced in irradiated meat and of a number of model systems,... 

The procedures for irradiation, collection, and analysis of the volatile compounds have all been described (1, 2, 3, 5, 7, 9, 11). A cryogenically programmed gas chromatograph coupled to a rapid scanning mass spectrometer provided for the analysis of the volatile components isolated from several irradiated meats and lipid substances (6). These studies have now been extended to include other component substances in order to acquire still further understanding of the source of the irradiation-induced volatile compounds from among the various meat constituents. 

It is now well established (6,10) that the hydrocarbons, except possibly those having three or four carbon atoms, found in the irradiated meats can come only from the lipid. This hypothesis has been verified in earlier studies, when the volatiles from irradiated methyl oleate (10) were found to contain appreciable quantities of alkanes and alkenes, and now in more detail from studies of both triglycerides and fatty acid esters. 

In meats, of course, there are components which arise from the protein which cannot be present in the products from pure fat. Table III shows some of the sulfur compounds and aromatic compounds which are also found in irradiated meats. Many of these can be postulated as arising from direct bond cleavage of amino acid moieties. Benzene and toluene may come from phenylalanine and phenol and p-cresol from tyrosine. Recent studies have been directed to considering the origin of some of the compounds from proteinaceous substances. Some of the sulfides, disulfides, and mercaptans can derive directly from cysteine or methionine, but those containing more than two carbon atoms in a chain require more than a superficial explanation. In order to evaluate the contribution of the volatiles from the protein as well as the lipid constituents of meat, volatile components produced in various protein substances have also been analyzed. 

It thus seemed that the origin of the various components in meat volatiles could best be established by analyzing irradiation-induced compounds in meat protein and meat fat separately. Accordingly, a 500-gram sample of meat, the same size of sample normally used in irradiation studies of whole meat, was separated into a protein, a lipid, and a lipoprotein fraction by means of a methanol-chloroform extraction of the fat. The dry, air-free, fractions were then irradiated separately with 6 megarads of gamma radiation in the manner used for whole meat. The analytical results (Table V) show clearly that mainly sulfur compounds and aromatic hydrocarbons are formed in the protein fraction, whereas mainly aliphatic hydrocarbons are formed from the lipid. The lipoprotein fraction produced, as expected, both aliphatic hydrocarbons and sulfur compounds. Only the lipoprotein fraction had a characteristic irradiation odor. 

Irradiation of the lipid (fet soluble) phase of a meat extract does not produce the characteristic off-odor while irradiation of the aqueous (water soluble) portion of die meat extract results in a typical irradiation odor (20). 3) Irradiation of sulfur-containing amino acids or polypeptides produced a similar off-odor as the irradiation odor (21). 4) The amount of VSCs increased with radiation dose while volatiles from lipids were not always correlated with radiation dose (19). Several earlier researchers suggested that hydrogen sulfide (H2S) and methanethiol (MT) were important for the development of the off-odor (12, 20, 22). Patterson and Stevenson (23), using GC-olfactory analysis, showed that dimethyl trisulfide (DMTS) was the most potent off-odor compound in irradiated raw chicken meats followed by cis-3- and trans-6-nonenals, oct-l-en-3-one and bis(methylthio-)methane. Aim and his colleagues have published extensively on irradiation-induced volatile compounds in raw meats (11). They have identified MT, dimethyl sulfide (DMS), dimethyl disulfide (DMDS) and DMTS in different types of irradiated raw meats using GC-FID and GC-MS. 

K. M. Morehouse and I. A. Taub, Relationship of volatile hydrocarbons to lipids in irradiated meats, in Proceedings of International Conference on Future Nuclear Systems, American Nuclear Society, LaGrange, 1999, Paper No. 144. 

Du et al. (50-52) reported no differences in TBARS between irradiated and nonirradiated chicken breast fillets and chicken and turkey rolls. As dietary CLA increased, however, the TBARS values of chicken rolls decreased. This could be caused by the decreased unsaturated fatty acid content in meat after dietary CLA treatment (53). We have observed similar results in vacuum-packaged irradiated meats in that ready-to-eat (RTE) turkey rolls from birds fed CLA treatment had lower TBARS than did those fed the control diet. The main reason for the improved oxidative stability could be due to the decreased proportion of unsaturated fatty acids in meat caused by the dietary CLA (54). Irradiation had a significant influence on numerous volatiles, mainly sulfur compounds, aldehydes, and alkanes. Dimethyl sul-... 

Increase of shelf life under refrigeration and control of pathogenic nonsporeforming bacteria in fresh meat and poultry can be achieved by a 1-3 kGy dose. Doses for irradiation are selected under the consideration of threshold dose levels for sensory changes (off-odor), which depends on the type of animal meat (Table 7) [45]. Off-odor is due to the generation of volatile compounds from lipids and nitrogenous compounds formed by the reaction of these constituents with the reactive species produced by the radiolysis of water. 

One of the primary reactions of ionizing radiation with saturated fatty acids is decarboxylation and alkane formation (2). Dimers tend to be produced by reaction of ionizing radiation with unsaturated fatty acids (2). When meats are irradiated C -C 7 n-alkanes, C2-C17 n-alkenes, and C4-Cg iso-alkanes are the predominant products from the lipid fraction (10), Irradiation of the lipoprotein fraction of meat results in the formation of the following volatile compounds Ci-C 7 n-alkanes, C2-C1J n-alkenes, dimethyl sulfide, benzene, and toluene (10). 

Table I summarizes the various meats, meat constituents, and other related substances which have been analyzed, including substances reported on previously (6) as well as those for which new data are given. The substances chosen are intended to provide a cross-section of the type of inherently related material from which volatile irradiation odor and flavor compounds might be expected to form. Thus, in addition to several whole meats, the volatile irradiation products from a number of protein and lipid substances have been analyzed. Among the lipid substances included are typical whole fats and separate moieties such as triglycerides, fatty acid esters, and cholesterol, as an example of a steroid. Among the proteinaceous substances included are a protein, a polypeptide, and some individual amino acids. Finally, beef itself has been separated into a protein, a lipid, and a lipoprotein fraction, and these have been separated, irradiated, and analyzed.

Analysis of the volatiles from irradiated ground beef, pork, mutton, lamb, and veal showed that the compounds formed are essentially the same in all the meats. All of the samples were irradiated at a dose of 6 megarads, and the volatiles produced show the presence of more than 80 compounds,... 

It has been known that various antioxidants reduce VSCs in raw meats. Nam et al. 40) showed that addition of antioxidants such as tocopherols, gallic acid and sesamol reduced the production of some VSCs in raw pork homogenates and patties. Addition of ascobic acid at 0.1% (wt/wt) or sesamol + a-tocopherol each at 0.01% level to ground beef before irradiation effectively reduced lipid oxidation and VSCs 41). Patterson and Stevenson (25) found that dietary supplementation of a-tocopherol and ascorbic acid to hens reduced the yield of total volatiles. Dietary vitamin E added to turkey diets reduced production of MT, DMS, CS2 DMDS and some hydrocarbons and aldehydes of raw turkey meat 42). However, Lee et al. 43) found antioxidant combinations (sesamol+a-tocopherol and gallate+a-tocopherol) had very little effect on the development of off-odor and the formation of VSCs due to irradiation in raw turkey meat. 

Vegetable oil (none) or antioxidants in vegetable oil were added as ingredients in the raw meat emulsions for bologna manufacture. The antioxidants tested were 500 ppm sodium erythorbate (eiythorbate), 200 ppm sodium nitrite (nitrite), and 0.075% rosemary extract (rosemary). The emulsions were cased, cooked, and irradiated at 3 kGy. Volatile compounds measured die next day, and expressol as square root of peak ar count. The numbers were means of four replicates. Adopted from ref (26). 

The VSCs in irradiated RTE meats have not been quantified due to the complexity of meats, and the instability, volatility and reactivity of VSCs. So far, VSCs are routinely reported as peak area counts. However, area counts do not always correlate to actual amounts of die compounds. Attempts should be made to accurately quantify the amount of VSCs in irradiated foods, so that the impact of individual VSCs can be determined. 

The overall yield of radiolysis products is relatively low and their distribution will depend on the fatty acid composition of the triglycerides. In meats irradiated in the absence of oxygen, low levels of the free fatty acid, the associated propanedioldiester, hydrogen, and products derived from the triglyceride radical are to be expected. Much lower yields of volatile hydrocarbons are produced that provide insight into other scission processes and reaction pathways [27, 30, 31],... 

Hydrocarbons were identified as marker substances in the 1960s and early 1970s. Pioneer work was done by Kavalam and Nawar (1969) and by Champagne and Nawar (1969) who intensively investigated volatile components. Nawar (1986) reported the occurrence and quantitative distribution of hydrocarbons in irradiated poultry and meat. Some examples of concentrations of hydrocarbons are given in Table 7.4. A strong correlation between dose and hydrocarbon concentration can be observed. This effect can be used to estimate the applied dose. 

A characteristic chromatogram from chicken meat irradiated with a 5 kGy dose at a dose rate of 0.9 kGy h is shown in Fig. 7.1. The chromatogram indicates that as well as the characteristic hydrocarbons (1-tetradecene, pentadecane, 1,7-hexadecadiene, 1-heptadecene and 1,8-heptadecene) many non-characteristic volatile compounds have been isolated and detected, which demonstrates the necessity of blank experiments. 

Du, M., Ahn, D.U., Nam, K.C., and Sell, J.L. (2001) Volatile Profiles and Lipid Oxidation of Irradiated Cooked Chicken Meat Patties from Laying Hens Fed Diets Containing Conjugated Linoleic Acid, Poult. Sci. 80, 1749-1756. 

Rababah T, Hettiarachchy NS, Horax R, Cho MJ, Davis B, Dickson J (2006) Thiobarbituric acid reactive substances and volatile compounds in chicken breast meat infused with plant extracts and subjected to electron beam irradiation. Poult Sci 85 1107-1113... 

Du, M, Ahn, DU, Nam, KC and Sell, JL (2001) Volatile profiles and hpid oxidation of irradiated cooked chicken meat from laying hens fed diets containing conjugated hnoleic acid. Poultry Sd., 80, 235—241. 

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