Flavor irradiation

Irradiation. Although no irradiation systems for pasteurization have been approved by the U.S. Food and Dmg Administration, milk can be pasteurized or sterilized by P tays produced by an electron accelerator or y-rays produced by cobalt-60. Bacteria and enzymes in milk are more resistant to irradiation than higher life forms. For pasteurization, 5000—7500 Gy (500,000—750,000 tad) are requited, and for inactivating enzymes at least 20,000 Gy (2,000,000 rad). Much lower radiation, about 70 Gy (7000 tad), causes an off-flavor. A combination of heat treatment and irradiation may prove to be the most acceptable approach. 

Nishimura and coworkers57-59 studied the y-radiolysis of aqueous solutions of sulfoxide amino acids. Sulfoxide amino acids are the precursors of the flavors of onions (S-propyl-L-cysteine sulfoxide, S-methyl-L-cysteine sulfoxide and S-(l-propenyl)-L-cysteine sulfoxide) and garlic (S-allyl-L-cysteine sulfoxide). In studies on sprout inhibition of onion by /-irradiation it was found that the characteristic flavor of onions became milder. In the y-radiolysis of an aqueous solution of S-propyl-L-cysteine sulfoxide (PCSO)57,58 they identified as the main products alanine, cysteic acid, dipropyl disulfide and dipropyl sulfide. In the radiolysis of S-allyl-L-cysteine sulfoxide (ACSO) they found that the main products are S-allyl-L-cysteine, cysteic acid, cystine, allyl alcohol, propyl allyl sulfide and diallyl sulfide. The mechanisms of formation of the products were partly elucidated by the study of the radiolysis in the presence of N20 and Br- as eaq - and OH radicals scavengers, respectively. 

The diazines pyridazine, pyrimidine, pyrazine, and their benzo derivatives cinnoline, phthalazine, quinazoline, quinoxaline, and phenazine once again played a central role in many investigations. Progress was made on the syntheses and reactions of these heterocycles, and their use as intermediates toward broader goals. Some studies relied on solid-phase, microwave irradiation, or metal-assisted synthetic approaches, while others focused attention more on the X-ray, computational, spectroscopic, and natural product and other biological aspects of these heterocycles. Reports with a common flavor have been grouped together whenever possible. 

Robles and Bochet showed that nitrotoluene derivatives 106 (Scheme 50) can be used as a photoremovable protecting group for aldehydes. Irradiation of 106 released aldehydes in good yields, and the authors demonstrated that aldehydes such as phenylacetalaldehyde and citronellal, which are important for flavor and fragrances, are released efficiently from 106. 

Smoked meats, particularly processed pork products, show little loss of flavor and aroma after treatment. Since the storage properties of these products are usually adequate without radiation there is little point in discussing this area of endeavor. The same comment applies to cooked meats, whether they be cooked before, during, or after irradiation. 

The packaged life of cole slaw at 40°F. is extended from 6 or 7 days to 25 days after moderate radiation treatment (B15). Another delicatessen item is smoked foods, which have an irradiated type of flavor prior to treatment (Sec. IVC2). Their flavor will, if anything, be enhanced. 

The industrial flavor producers offer a very broad selection of natural and synthetic flavors,mainly in the form of liquid concentrates.The majority of flavor constituents in such concentrates exhibit considerable sensitivity to air,light irradiation and elevated temperature. These flavor concentrates are moreover oily,greasy rather lipophilic materials,which are difficult to work with. The natural plant extracts also have microbiological contaminations that need to be removed. 

The molecular encapsulation of flavors with cyclo-dextrins was found to improve the resistence of light sensitive flavor constituents against daylight and ultraviolet irradiation. The photodecomposition of adsorbed and complexed flavors was tested both in the solid state and in aqueous solutions. The results of the light stability tests are demonstrated in the example of complexed and adsorbed citral,beta-ionone and cinnamaldehyde formulations /Table III./. 

Table III. Stability of complexed and adsorbed flavors to UV/365nm/ irradiation in the solid state at room temperature...

Methional, formed by the degradation of the amino acid methionine, has been reported (Patton 1954 Velander and Patton 1955) to be the principal contributor to the activated flavor. Samuelsson (1962) reported, in studies of dio- and tripeptides containing methionine, that irradiation did not result in any hydrolysis of the peptides, and the... 

Day, E. A., Forss, D. A. and Patton, S. 1957. Flavor and odor defects of gamma-irradiated skim milk. I. Preliminary observations and the role of volatile carbonyl compounds. J. Dairy Sci. 40, 922-931. 

Khatri, L. L. 1966. Flavor chemistry of irradiated milk fat. Diss. Abst. 26, 6638-6639. 

Knowledge of the volatile components of irradiated and nonir-radiated beef is reviewed. Concurrent and nonconcurrent irradiation procedures produce the same compounds but in different relative quantities. Storage of irradiated beef decreases irradiation flavor and the quantity of volatile constituents. Methional, 1-nonanal, and phenylacetaldehyde are of primary importance in beef irradiation off-flavor produced under the conditions described. 

A problem associated with beef sterilized by irradiation at approximately room temperature is the production of an unpleasant flavor and aroma. This paper summarizes knowledge of the volatile components of enzyme-inactivated irradiated and nonirradiated beef, reviews the effects of concurrent and nonconcurrent irradiation procedures and of storage on these components, and presents evidence that methional (3-methylmercaptopropion-aldehyde), 1-nonanal, and phenylacetaldehyde are of primary importance to irradiation off-odor in beef thus processed. 

Any basic study of the chemistry of irradiation flavor is complicated by the fact that volatile components of nonirradiated beef must be known. Otherwise, those components produced by irradiation (and thus may be responsible for off-flavor) and those which are normally present in nonirradiated beef cannot be determined. 

The contribution to irradiation off-flavor of individual components was unknown, though the sulfur- and nitrogen-containing substances were suspected to be significant because of their inherent strong unpleasant odors. Merritt (9) suggested that dimethyl sulfide, 1-hexene, and n-hexane were important components of irradiation odor and pointed out that the quantity of these compounds produced increased directly with radiation dose. 

Figure 1. Temperature-programmed separation of irradiation flavor isolates on a 20% Carbowax 20M column...

Merritt, as a result of elegant analytical work on raw beef, has suggested (8,10,12) that the series of n-alkanes and 1-alkenes produced during irradiation are responsible for irradiation flavor. In our work no evidence has been found which supports this suggestion. The reason for this contradiction may be the different conditions used during irradiation. Merritt worked with raw beef which was irradiated in vacuum or an inert atmosphere. Our beef, on the other hand, had been partially cooked during enzyme-inactivation and then irradiated in the presence of air. It may also be that 1-nonanal, methional, and phenylacetaldehyde are not the only substances which when mixed in correct proportions give rise to typical irradiation odor. 

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.

A number of investigators (2, 5, 15, 21, 34, 41, 42, 48, 82, 53) have tried to isolate and to characterize the chemical compound or compounds which give rise to irradiation flavor in meat or to correlate irradiation flavor scores with the production of specific compounds or types of compounds during the irradiation of meat or meat fractions (3,4,32,44> 49,50). These investigations have indicated some probable and some improbable sources of irradiation flavor and the order of magnitude of the concentration of the compounds responsible for irradiation flavor. Wick et al. (53) have offered impressive chemical and organoleptic data connecting the 20 2 1 ratio of methional, 1-nonanal, and phenylacetaldehyde found in irradiated beef at the parts per million level with typical irradiation odor. 

Merritt (42) has carefully studied the yields of hydrocarbons on irradiation of meat and meat components and proposed mechanisms for their formation during irradiation. Despite this recent progress, the chemical characterization of irradiation flavor in meats is far from complete. Little is known about the radiation-induced chemical processes giving rise to the compounds proposed as important to irradiation flavor or the identity of the chemical precursors of these compounds. However, irradiation flavor in beef appears to be associated largely with the protein constituents in meat (21). 

These four compounds were also selected because they were commercially available in pure form and reasonably soluble in unbuffered solutions at room temperature. In addition, the radiation chemistry of glycine, glycylglycine, and methionine has been studied at room temperature (4, 5, 6, 12, 36, 49, 50, 51). Also, sulfur-containing amino acids have been suggested by chemical-irradiation flavor correlation studies (4, 21, 38, 44f 49, 50, 53) as being related to irradiation flavor. 

The effects of seven processing variables which, on the basis of previous literature reports (7, 27) and our chemical studies on model systems, might affect the irradiation flavor intensity or consumer acceptance of irradiated steaks were evaluated in statistically designed and analyzed experiments. Steaks were then prepared using the processing conditions shown to be optimum by these experiments and evaluated for consumer acceptance and storage stability. 

IV. Smaller but possibly significant effects of warming rate and packaging environment are shown in Figure 6, complicated by interaction with the irradiation temperature. The effects of both packaging and warming rate variables were largest at — 196°C. The lowest irradiation flavor intensity... 

Table IV. Mean Irradiation Flavor Intensity Scores of Enzyme-Inactivated Beefsteaks after Irradiation at 6.0 Megarads in Initial Process Variable Screening Experiment...

The cooling rate showed no appreciable effect on flavor scores and was eliminated as a variable in subsequent tests. Since there was some indication of a tendency toward texture deterioration on rapid cooling, a standard method of cooling to 4°C. followed by vapor phase cooling (essentially slow) to irradiation temperature was followed in subsequent studies designed to measure the effects of the other variables more accurately. 

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