Flavor of Meat, Meat Products and Seafoods 006

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3 The flavour of beefG. MACLEOD

3.1 IntroductionThe flavour of beef h asbeen investigated more extensively than any o thermeat flavour, probably because of its greater consumer popularity, andhence its commerical significance in the creation of successful simu latedmeat flavourings. Literature reports over the past 30 years show that theflavour of beef is highly complex. In its simplest format, it consists oftaste-active compounds, flavour enhancers and aroma components.

3.2 Taste-activecompoundsWith regard to taste (MacLeod and Seyyedain-Ardebili, 1981; MacLeod,1986; Kuninaka, 1981; Haefeli and Glaser, 1990),sweetnesshasbeen asso-ciated with glucose, fructose, ribose and several L-amino acids such asglycine,a lanine, serine, threo nine, lysine, cysteine, m ethionine, asparagine,glutamine,proline andhydroxyproline. Sourness stems from aspartic acid,glutamic acid, histidine and asparagine, together with succinic, lactic,inosinic, ortho-phosphoric and pyrrolidone carboxylic acids. Saltiness islargelydue to the presence ofinorganic saltsand the sodium saltsofgluta-mate and aspartate; while bitterness may be derived from hypoxanthinetogether withanserine, carnosine and other peptides, and also the L-am inoacids histidine, arginine, lysine, methionine, valine, leucine, isoleucine,phenylalanine, tryptophan, tyrosine, asparagine and glutamine.The umami taste has acharacteristic sav oury qua lity and issuppliedbyglutamic acid, monosodium glutamate (MSG), 5'-inosine monophosphate(IMP), 5'-guanosine monophosphate (GMP) and certain peptides.Although genera lly speaking, glutam ate is the m ost impo rtant contributo r,its presence at a lower concentration in beef than in pork or chicken, forexample, gives rise to a lower perceived umami taste intensity in beef(Kato and Nishimura, 1989; Kawamura, 1990). Similarly, the effect ofconditioning these threespecieshas shown a significant increase in inten-sity of the savoury, brothy taste in pork and chicken after aging, yet nosignificant difference in conditioned beef (Nishimura et al., 1988). Thisdisparity was paralleled by the observation that the increased concentra-tion of free amino acids and of oligopeptides on aging was significantly


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smaller in beef than inpork or chicken (Nishimuraet al.,1988). However,sub sequ ent rates of changes in concentration of these no nvolati le compo-nents and of others on continued heating, and the associated aromamanifestations, could well alter the overall comparative conclusions onflavour improvement , in general, on aging different meat species.3.3 Flavour enhancersA m ore im po rtant sensory contribution tha n the taste characteristics ofglutamic acid, MSG, IM P and, to a lesser extent, GMP in beef, is theirflavourenhancing property. It hasbeen proposed thatour sensory recep-tors for flavour enhancement are independent and stericallydifferent fromthe trad itiona l basic taste receptors (K un ina ka , 1981; K aw am ur a, 1990).The 5'-ribonucleotides have strong flavour potentiating effects individu-ally, but more importantly, they exhibit a potent synergistic effect whenpresent, as in meat, in conjunction with glutamic acid or MSG (Kawa-mu ra ,1990). It appears that, in some mammals, this synergy is due to aninduced increased strength of bind ing of glu tam ate to the receptor proteinsite, w hereas in other m amm als, an enhanced am ount of glutamate is actu-ally bound (Kawamura, 1990).The 5'-nucleotides arereportedto enhancemeaty, brothy, MSG-like, mouthfilling, dry and astringent qualities; theysuppress sulphurous and HVP-like notes, while sweet, sour, oily/fatty,starchy and burnt qual i t ies remain unchanged (Kuninaka,1981).Thermal decomposition of both classes of flavour enhancers may occur,witha resultant lossofactivity.For example,at1210Cand a pH of4.5-6.5(e.g. during canning), an initial loss of the phosphate group from IMP andGM P, co nverting the nucleotide into the corresponding nucleoside, is fol-lowed by slow hydrolysis releasing the base - either hypoxanthine (fromIMP)orguanine (from GMP) (Shaouland Sporns,1987).The first trigger-ing reaction of the phosphate loss is depressed in the presence of certaindivalentm etals, e.g. calcium ions(Kuchibaet al.,1990). U nd er sim ilar he at-ing conditions (100C/pH 4-6),g lutam ic acidand MSG are converted intopyrrolidon e carbox ylic acid (PC A) (Gayte-Sorbier etal.,1985).The reversereaction isalso possible,but isfavouredbyextrem epHvaluesof11(Airaudoetal. 1987).No t only is there no flavour enhancem entactivity fromany of the decom position produ cts (K un inak a, 1981; G ayte-Sorb ier,1985),but a distinctoff-flavour is associated with PCA at certain concentrations.

3.4 Aroma componentsAroma components are generated inbeef from nonvolatile precursors oncooking. Primary reactions occurring are (1) lipid oxidation/degradation;(2) thermal degradation and inter-reactions of proteins, peptides, amino


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acids,sugarsand ribonucleotides;and (3)therm al degradat iono fthiamine.But reaction products become reactants, and the end result is a complexand intertwining network of reactions. In consequence, th e most recentedition of the now classic TNO-CIVO publicationVolatile Compounds inFood lists 880 vo latile components repo rted from cooked beef (Maarseand Visscher, 1989). To place this figure in a better perspective, a roughbreakdown of the chemical classes represented in shown in Table 3.1(Maarse and Visscher, 1989).With such a large number ofpotential contributors to the sensory per-ception ofcooked beef arom a,the critical questionis ,'W hat is the relativesensory significance ofthese volatiles?'The answeris not totally clear, butthree conclusions can be drawn. First, many are relatively unimportant.Second,theterm'm eatiness'can beclea nly dissected sensorially into ab ou tten different odour qualit ies (Gait and MacLeod, 1983), in which case,many of the identified volatiles are acting as'aroma modifiers' contribut-ingbu ttery, caramel, roast, b urn t, sulphuro us, green,fragrant, oily/fatty andnut ty qualities. Structure-activity correlations, or at least associations, doexistin the l i teratureform a nyofthese o dou r qua lities. Th ird, some arom acomponentsdocontributeatruly specific 'meaty ' odour,and are therefore

Class of com pound N um ber of components reportedAliphaticHydrocarbonsAlcoholsAldehydesKetones

Carboxylic acidsEstersEthersAminesAlicyclicHydrocarbonsAlcoholsKetonesHeterocyclicLactones

Furans and derivativesThiophenes and derivativesPyrroles and derivativesPyridines and derivativesPyrazines and derivativesOxazol(in)esThiazol(in)esOther sulphur heterocyclicsBenzenoidsSu lphu r com pounds (no t heterocyclic)Miscellaneous

1037055492456720443183844402021541329138072

7From Maarse and Visscher (1989).

Table 3.1 Chemical classes of aroma componentsreported fromcooked beef


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character impact compounds. Severalpotent, key and trace meaty com-pounds are present in natural cooked beef aromas and many remain to beidentified.Clearly,th e ul t im ate chemicalandsensory reso lution of the beefflavour complex relies primarily on denning these particular meaty/beefycom pounds and the reactions which generate them.A search of the l i terature for ind ividua l chem ical com pounds describedas meaty (e.g. MacLeod, 1986; Werkhoff et al., 1989, 1990, 1996; Guntertet al., 1990) shows that, of the 880cooked beef arom a com ponents iden-tified to date (Maarse and Visscher, 1989; Werkhoff et al., 1989, 1990,1996), only25 have been reported to possess a meaty odour . These arepresented in Figure 3.1. The meaty quality of several of these has beenunder a t tack by some workers, probably because it is a difficult qual i tyto define with precision. Nevertheless, theyare includedfor completeness.Also, many compounds are meaty only at certain concentrations, usuallyvery low concentrations which are often unsp ecified. In the discussionbelow, which considers how various cooked beef aroma components areformed during cooking,the m eaty compounds ofFigure 3.1 are numbered1-25. All other compounds are quoted wi thouta numericalcode label.3.4.1 f f e c t of heat on sugars and/or amino acidsTo the seasoned flavour chemist, evenacursory glanceatTable3.1showstha t ahigh proportion of the tota l numberofvo latilesidentified isderivedfrom reactions which result from the effect of heat on sugars and or aminoacids. Strecker degradations and Maillard reactions are critical contribu-tors, both chemically and sensorially. Furthermore, a host of secondaryreactions can occur involving the products of the above reactions (e.g.H 2S, N H3, thiols and simple carbonyl compounds), thereby increasingquite considerably the variety of compounds which may be formed.Strecker degradation is depicted in Figure 3.2 (MacLeod and Ames,1988),and several Strecker aldehydesare well-known cooked beef arom acomponents, e .g. acetaldehyde (from a lanine) , methylpropanal (fromvaline), 2-methylbutanal (from isoleucine),3-m ethybutanal (from leucine),phenylacetaldehyde (from phenylalanine) and methional, which readilydecomposes into me thane thiol, dim ethy l sulphide, dim ethyl disulphide andpropenal (from methionine) .

The Maillard reaction between compounds containing a free aminogro up (e.g. am ino acids, am ines,peptides,proteins, am m onia)andcarbonylcompounds (e.g. aldehydes, ketones, reducing sugars) is an open-endedcomplex reaction consisting of a series of inter-reactions and decomposi-tions and resulting in numberous volati le products. The first stage in thereaction of ana-aminoacidwithan aldose or ketose is the fo rm ation of anA ma d o r iorH eyns com pou nd, respectively. Bothareno n-volati lebut theyare heat labile and they decompose thermally. The second gross stage


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Figure 3.1 Compounds identified from cooked beef aromas (Maarse andVischer, 1989;Wodour (bArctander, 1969; 'Baltes, 1979; aBrinkman et aL, 1972; hEvers et aL, 1976; "Furet aL, 1980; Macleod,1986; MacLeod andAmes, 1986; xMussinan et aL, 1976; cNishimura1974; eRoedel and Kruse, 1980; rSelf et aL, 1963; fShibamoto, 1980; kTressl and Silwar, 19Van der Linde et aL, 1979; vVernin,.1979, >1982; "Werkhoff et aL, 1

burnsi.

roast beefc

Jroast beefcmeaty,onion,bouillonb

Jmeaty 1-5ppb),

oniona

1meaty,onmetallic,

ghcooked meat

iXImeaty ippb)

beef broth,hjroast meat

ro16

meatys15

meatyslftfboiled beef dil.)onion, sulphurous cone.)13 proast meat

23meaty,nu

onion922

smoky,fatty,meatyy 9.V.Xmeaty,roasted,nutty, green veg. 9.V.Wmeaty,cocoa


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Figure 3.2 The Strecker degradation ofa-amino acids and the subsequent formationMacLeod, G. and Ames,J.M. (1988) CRC Crit. Revs Food Sd. Nutr., 27,219-40

Schiff basea -aminoacid

a,p-dicarbonylcompound

alkylpyrazine

dihydropyrazine

R1R1, R H,alkyl


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involves rearrangement and subsequent decomposit ion of the Amadoriand Heyns compounds, asexem plifiedinFigure 3.3(MacLeod andAmes,1988). Thus,2-furaldehyde forms from pentoses, and5-hydroxym ethyl-2-furaldehyde from hexoses. Additionally, a number of dicarbonyl andhydroxycarbonyl fragmentation products can be formed, e.g. glyoxal,glycolaldehyde, glyceraldehyde, pyruvaldehyde, hydroxyacetone, dihy-droxyacetone, diacetyl, acetoin and hydroxydiacetyl.These are all highlyreactive compounds, and therefore the final global stage of the Maillardreaction can beconsidered as thedecomposition andfurther inter-reactionof furanoidsand aliphatic carbonyl com pounds, formed as exemplifiedinFigu re 3.3, w ith othe r reactive species p resent in the system , e.g.H2S,N H3,amines and aldehydes. Yayalanhas recentlyproposed that, since the cur-rently accepted mechanismsof 1,2-and 2,3-enolizationsof the open chainformof the A m ado ri compounds, and their subsequent dehydration (as justdescribed), do not adequately account for many products observed inm odel systems and in foods, alternativ e m echanisms should be considered,e.g. direct dehydrations from cyclic forms of the A m adori com pounds(Yayalan, 1990). By whichever mechanism, the final stage reactions areextrem ely large in nu m be r and varied in na ture and canno t be gene ralized.Examples are asfollow s.Furanoids often arise, as shown in Figure 3.3, via the 1,2-enolizationpathway. One exception is 2-acetylfuran wh ich probably is derivedfrom a 1-deoxyhexosone intermediate (Tressl et aL, 1979). Some otherfuranoids, e.g. 4-hydroxy-5-methyl-(2//)furan-3-one and 2,5-dimethyl-4-hydroxy-(2//)furan-3-one, also arise by this 2,3-enolization route. 2-Furaldehyde is an important precursor of other furanoids,and indeed ofother heterocyclic compounds too, such as thiophenes and pyrroles. Thisis because the oxygen of the furan ring, in the presence ofH2S or N H3,m ay be s ub stituted by sulph ur or nitrogen, form ing the correspondingthiophenes or pyrrole derivatives. The formation of such compounds isshown in Figure 3.4 (MacLeod and Ames, 1988).The most likely pa thway for the formation of alkylpyrazines is byself-condensation of the a,p-aminoketones form ed during Strecker degra-dation, as show n in Figure 3.2. W hile alkylpy razines are freq ue ntlyoccurring volatiles in manyheated foods, the bicyclic pyrazines, namelythe alkyl-(5//)6,7-dihydrocyclopenta[6]pyrazines and the pyrrolo[l,2-0]pyrazines, are unique to grilled and roasted beef aromas (Maarse andVisscher, 1989). The former arise from reaction of an alky lhyd roxy -cyclopentenone (e.g. cyclotene) with ana,p-dicarbonyl andNH3as shownin Figure 3.5 (Flament et aL, 1976; MacLeod and Seyyedain-Ardebili,1981), while the latter derive from condensation of ana,p-aminoketone(from Strecker degradation) with a hydroxy a,p-dicarbonyl compound(formed from reducing sugars) as shown in Figure 3.6 (Flament et aL,1977; MacLeod and Seyyedain-Ardebili,1981).


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Figure 3.3 Decomposition of Amadori and Heyns compounds. (Reprinted with permissionRevs Food Sd. Nutr., 27 , 219-400. Copyright CRC Press,I

a 1-deoxyosoneCOMPOUND Y


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Figure 3.4 Formation of some related furan, thiophene and pyrrole derivatives from Mail-lard intermediates. (Reprinted with permission from MacLeod, G. and Ames, J.M. (1988)CRC Crit. Revs Food ScL Nutr. 27,219-400. Copyright CRC Press, Inc., Boca Raton, FL.)

Maillardtype reactionsofcysteine a nd/or cysteine h ave been extensivelystudied, and a recent review summarizes the various cysteine-specificMaillard products (Tressl et al., 1989). Reaction products include carbonylcompounds, amines, benzenoids, acids, lactones, thiols, sulphides, fura-noids, thioph enoids , thianes, thiolanes, pyrroles, pyridines, pyra zines,thiazoles, thiazolines and thiazolidines. The type of products formed, andtheir concentrations, are strongly influencedin model systemsby the sol-vent. For exam ple, reaction of cysteine/dihydroxyacetone in water favoursthe form ation of sulphu r products, especially m ercaptopropa none andsome thiophe nes, w hereas in triacylglycerol or g lycerine systems, deh yd ra-tion reactions are fa vou red produ cing prefe rentially various pyrazines andthiazoles (O kum ura et al., 1990).


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Figure 3.5 Formation of alkyl-(5//)6,7-dihydrocyclopenta[]pyrazines by Maillard reaction.(Re printed w ith permission from MacLeod,G. and Seyyedain-Ardebili, M .(1981)CRC Crit.Revs Food ScL Nutr. 14 309^37. Copyright CRC Press, Inc., Boca Raton, FL.)

The long listof chemical classes derivingfrom cysteine/cystine systems,and indeed the identification of several representatives within each class,serves to stress the considerable impact of cysteine (in particular) as aStrecker/M aillard reactant. T his significanceofcysteineisattributed to thevery high reactivity of its initial Strecker degradation products, namelymercaptoacetaldehyde, acetaldehyde, H2S and N H3, all of which undergonumerous further reactions. Sheldon et al. (1986) recently showed that


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Figure 3.6 Formation of pyrrolo[l,2-fl]pyrazines by Maillard reaction. (Reprinted withpermission fromMacLeod, G. and Seyyedain-Ardebili, M.(1981) CRC Crit. Revs Food ScLNutr. 14,309-437. Copyright CRC Press, Inc., Boca Raton, FL.)

some M aillard reaction betw een cysteine and glucose occurs even at roomtem pera ture in the da rk. A t high tem perature s ap prox ima ting roasting con-ditions (i.e. Shigematsu conditions), the products formed reflect the factthat amino acid pyrolysis (rather than Strecker degradation) is an impor-tant reaction (d e Rijke et aL,1981),and cysteine ispr imari ly transformedinto six produ cts, nam elym ercaptoacetaldehy de,a cetaldehyde,cysteamine,ethane-l ,2-dithiol ,H2Sand N H3(Tressl et aL,1983).These highly reactivecomp ounds trigger a num ber ofaldoland other co ndensation reactions w ithsugardegradation productsandw itheach other.Forexam ple,the reactionofethane-l ,2-dithiol/acetaldehy de/H2S gives rise to 2-m ethy l-l,3-dithiolan e(13) and3 -m eth yl-l,2,4- trithiane (17) (Tressl et aL,1983).


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Further examples of Figure 3.1 compounds arising from Maillard typereactionsare thiazole (19) from cysteine/pyruvaldehyd e (Ka toet al, 1973),2,4,5-trimethyloxazole (24) from cysteine/butanedione (Ho and Hartman,1982), thiophene-2-carboxaldehyde (12) (Scanlan et al, 1973), 3-methyl-1,2,4-trithiane (17) and thialdine (18) (deRijkeetal 1981) from cysteine/glucose; 3-methyl-l,2,4-trithiane (17) and 2,4-dimethyl-5-ethylthiazole(21) from cysteine/cystine-ribose (M ulde rs, 1973), 2-m eth ylfu ran-3-th iol(7),2-m ethyl-3-(m ethylthio)furan (8) and2,4-dim ethyl-5-ethylthiazole (21)from cysteine/ribose (Whitfield et al., 1988; Farmer etal 1989; Mottramand Salter, 1989; Farmer and Mottram, 1990; Mottram and Leseigneur,1990), methional (2) and 2-methyl-3-(methylthio)furan (8) from methion-ine/reducing sugar (Tressl etal 1989) and the 2- and 3-methylcyclopent-anones (3, 4) from cylcotene/H2S/NH 3 reaction (Nishimuraetal 1980).Frequently the effective sulphur reactant isH2S as shown, for exampleinFigure 3.4, explainingthe generation ofsome thiophenes. Thiazol(in)esderive from th e reaction of an a,(3-dicarbonyl compound, an aldehyde,H 2S and N H3 asshown inFigure 3.7(MacLeod and Ames, 1988; Takkenet al, 1976). In the absence of H2S, parallel reactions yield oxazol(in)es.On a similar theme, inter-reactions involving acetaldehyde, H2S andN H 3explaintheforma tiono fseveral other cyclic sulp hu r com pounds iden-tified in cooked beef aromas, e.g. 3,5-dimethyl-l,2,4-trithiolane (14),trithioacetaldehyde (15) and thialdine (18). These reactions are shown inFigure 3.8 (MacLeod and Seyyedain-Ardebili, 1981; Takken et al, 1976).The thiadiazine is decomposed on storage into thialdine (Kawai et al,1985). Interestingly, th e isomeric trithiolanes (14) are major products(63% of total volatiles) from heated glutathione, whereas H2S/NH 3 inter-action products such as the dithiazine and thiadiazine predominate (70%of total volatiles) from heated cysteine (Zhang et al, 1988). This isexplained by the fact that a very much milder degradation occurs withglutathione and itreleases H2S more readily thanN H3(Zhang et al, 1988).It is generally established that glutathione is the main H2S precursor inmeat during the early stages of cooking, but cysteine takes over thismajor role on prolonged heating. The formation of another reportedlyimportant beef aroma component is shown in Figure 3.8, i.e. -(methylthio)ethanethiol(1) which derives from reaction of acetaldehydewith methanethiol and H2S.The reactants required for the reactions just described are liberated onheating either aqueous cysteine (Shu et al, 1985b) or cystine (Shu et al,1985a) alone. Shu et al (1985b) have reported the thermal degradationproducts ofcysteine inwater at 160C/30 min at different pHvalues. Theyidentified 26volatile products ofwhichfivehave been described asm eaty ,i.e. 2-methyl-l,3-dithiolane (13), 3-methyl-l,2-dithiolane, 3,5-dimethyl-1,2,4-trithiolane (14), 3-methyl-l,2,4-trithiane (17), thiazole (19) andl,2,3-trithiacyclohept-5-ene. The most vigorous reaction occurred at the


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Figure 3.7 Proposed formation pathway for thiazoles from Mail lard reactions. (Reprinted wCRC Crit. Revs Food ScL Nutr. 27, 219-400. Copyright CRC P


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Figure 3.8 Some reactions of aldehydes with hydrogen sulphide/ammonia/methanethiol:a = atmospheric pressure; b = closed vessel + excess hyd rogen sulphide. (Reprinted withpermission from MacLeod, G. and Seyyedain-Ardebili, M .(1981) CRC Crit.Revs Food ScL

Nutr. 14 309-437. Copyright CRC Press, Inc., Boca Raton, FL.)

isoelectric point (IEP) of 5.1, which is also th e most relevant pH as faras beef is concerned. The volatile mixture was relatively simple andcontained 55% acetone. This pH also favoured the formation of theisomeric 3,5-dimethyl-l,2,4-trithiolanes (14). At pH 2.2, the main volatileprod ucts w ere cyclic sulp hu r com pounds containing 5-, 6- and 7-mem beredrings, togethe r w ith thiophenes. Them ajor component (34% of the isolate)was the novel m eaty compou nd l,2,3-trithiacyclohept-5-ene. Only a milddegradation occurred at pH 7.1 and, apart from butanone, only cycliccom pounds were identified. M ajor products w ere the 3,5-dimethyl-l,2,4-


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trithiolanes (14), 3-methyl-l,2,4-trithiane (17) and 2-propanoylthiophene(Shu et al.,19855).From aqueous cystine treated similarly,42compounds were identified;they w ere m ainly thiazoles, aliphatic sulphides, thiolanes and thianes (Shuetal. 1985a). Significantly more thiazoles were present than in the corre-sponding cysteine system (Shu et aL, 1985b). Several compounds reportedto be meaty were generated at the pH of meat (pH 5.5). Figure 3.1compounds produced were 3-methylcyclopentanone (4), 2-methyl-l,3-dithiolane (13), 3,5-dimethyl-l,2,4-trithiolane (14), 3-methyl-l,2,4-trithiane(17), trithioacetaldehyde (15) and thiazole (19) (ShuetaL,1985a). On thew hole, cyclic sulph ur com pou nds w ere generated m ore read ily at pH 2.3,which is the converse ofw h a t w as reported for the cysteine system wherethe thiolanes formed preferentially at the IEP rather than below it (Shuetal., 1985b). It was suggested that this was related to the thermal stabilityof the disulphide bond of cystine at different pH values.Often an integral part of the degradations just described, but not neces-sarilyso , is aseriesofreactions ofsimplelowm olecular w eight compounds,inparticular aliphatic aldehydes, alkane-2,3-diones, methanethiol,H2Sa ndNH3.A cetaldehyde isinvolved inm anyof them and its relevant reactionsare schematically represented in Figure 3.9 (Katz, 1981; Mussinan etal.1976; Takken et al.,1976), e xp lainingtheformationo f the reportedly meaty -(methylthio)ethanethiol (1), 3,5-d imethy l-l,2,4-trithiolane (14), tr ithio -acetaldehyde (15), thialdine (18), 2,4-dimethylthiazole (20), 2,4-dimethyl-5-ethylthiazole (21), 2,4,5-trimethyl-3-thiazoline (23), 2,4,5-trimethyl-oxazole (24) and 2,4,5-trimethyl-3-oxazoline (25).

3.4.2 Reactions of hydroxyfuranonesTwo im po rtant cooked beef arom a precursors are 4-hydroxy-5-methyl-(2//)furan-3-one (HMFone) and 2,5-dimethyl-4-hydroxy-(2//)furan-3-one(HDFone). Both have been isolated from beef (Tonsbeek et al. 1968,1969) but not from any other meats. Natural precursors of HMFone inbeef are ribose-5-phosphate and PCA or taurine (to a lesser extent) orboth, and the fura none formsonheating at 100C/2.5 h in a dilute aqueousmedium of pH 5.5 (Tonsbeek et al., 1969). PCA itself is readily formedfrom ammonia and glutamic acid or glutamine on heating (Tonsbeeket al., 1969). The furanone can also be derived from 5'-ribonucleotides viaribose-5-phosphate obtained, for example, by heating at 6O0C (Macyet al. 1970) or during autolysis in muscle (Jones, 1969). HMFone is alsogenerated from Maillard reaction of pentoses, e.g. ribose. Precursors ofthe related HDFone are hexoses, e.g. glucose or fructose under Maillardconditions. HDFone is a relatively unstable compound. Its opt imumstability is at pH 4 and its decomposition increases rapidly with tem per-ature (Hirvi et al., 1980). Shu et al. thermally degraded HDFone in water


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Figure 3.9 Schematic reactions of acetaldehyde-generat ing compounds from Figure 3.1.

at 160C/30 min at pH 2.2, 5.1 and 7.1 (Shu et al., 1985c). D eg rad ationoccurred m ore read ily at the lower pH values. They identified 20 volatileproducts, mostofw hich we re reactive sim ple aliphatic carbonylsanddicar-bonyls; the remainder of the volatile reaction product mixture consistedof alkyl-substituted (2/f)furan-3-ones, which were favou redat higher pHvalues. Most of these were shown to derive from pentane-2,3-dione asintermed iate, indicating that theHDFone ring opened initially on h eating .The significance of these two hy drox yfura none s in cooked beef arom ais in their reaction with hydrogen sulphide. For HMFone/H2S, the reac-tionissumm arizedinFigure 3.10 (MacLeod and Seyyedain-Ardebili, 1981;van den Ouweland andPeer,1975; van den Ouwelandet al., 1978). Again,ring opening is indicated and an initial partial substitution of the ringoxygen with sulphur occurs. The reaction product mixture possesses anoverall odour of roasted meat and the majority of the reaction productsare mercaptofuranoids and mercaptothiophenoids, most of whichpossessmeaty odours (van den Ouweland and Peer, 1975; van den Ouweland


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Figure 3.10 Form ation from ribonucleotides of 4-hy drox y-5-m ethyl-(2//)furan-3-on e, and itsreaction with hydrogen sulphide. (Reprinted with permission from MacLeod, G. andSeyyedain-Ardebil i , M.(1981) CRC Crit. Revs Food ScL Nutr., 14 309-437.Copyright CRCPress, Inc., Boca Raton, FL.)

et al., 1978). Only one of these (asterisked inFigure 3.10) has so far beenidentified from na tur al cooked beef arom as, i.e. 5-m ethyl-4-m ercapto-tetrahydrofuran-3-one (11) (Ching, 1979). It is l ikely, however, that theremainderare p resent, since several of them possessed GC retention tim esand odour port assessments which were very similar to those of trace

RlBONUCLEOTlOE

4-hydroxy5methyI-3 2H)furanone

ribose 5-phosphate

4-hydroxy-5-methyl-3(2H)thiophenone

sweetmeat-like meatymaggi-like meatysavoury nutty roasted meat

green,meatymaggi-like roasted meat rubbery rubberymeaty roasted meat

green meatyherbaceous fatty onion-likegasoline meaty sweetroasted meat

green green cabbage acetylenic cis &transmeaty

meat-like mushroom butter-like


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components detected from a natural beef broth aroma isolate (Ching,1979). Bodrero et al.(1981), using surface response m ethodology to stud ythe contribution of various volati les to cooked beef aroma, showed thatthe HMFone/HS2S reaction product mixture gave the highest predictedscore of their entire study.HDFone reacts with H2S in a similar manner(van den Ouweland and Peer, 1975; van den Ouweland et al., 1978)producing at least tw o meaty compounds, i.e. 2,5-dimethylfuran-3-thioland 2,5-dimethyl-4-hydroxy -(2//) thiophen-3-one, neither ofwhich has yetbeen identified from beef. The chances are that other compounds identi-fied from this reaction are meaty also, but their individual sensoryproperties havenot been reported.Shu et al.have reactedHDFonewithcystineat 160C/30min inwateratpH 2.4 (Shu e tal. 1985d). The volatile pro du cts changed on storage for twoweeks at 40C and were therefore analysed after holding for two weeks.When compared with the products generated from HDFone (Shu e t al.,1985c) and from cystine (Shu et al., 1985a) treated similarly, th e ma jo rdifference was the presence of relatively large concentrations of hexane-2,4-dione (16% ) and of three thiopheno nes (22.5%). One of these was the2,5-dimethyl-4-hydroxy-(2//) thiophen-3-one mentioned above and previ-ously reported from HDFone/H2S reaction, but the o ther two were novel,namely 2,5-dim ethyl-2-hydrox y-(2//)thiophen-3-one (possessing a roastedonion odou r) and 2,5-dimethyl-2,4-dihydroxy-(2//)thiophen-3-one possess-ing a mea ty ,pot roast aroma and taste. Y et again, this meaty compoundhas eluded identification in natu ral beef. Op timum conditions for theformation of m eaty com pound s (including these thioph enones and the iso-m eric 3,5-dim ethyl-l ,2,4-tri thiolanes (14)) were 160C/aqueous medium;75% water /pH 4.7 (Shu and Ho, 1989). More recently, all three thiophe-nones have been reported from cysteine/glucose reaction (Tressl e t al.1989).In a study involving HD Fone/cysteine, Shu et al. (1986) showed thatthe twonovel thioph enone s m entioned above w ere only trace com ponents.Instead, two novel thiophenes were characterized, namely3-me thyl-2-(2-oxopropyl) thiophene and 2-methyl-3-propanoylthiophene. Their odourprope rties w ere not described. M eaty com pounds form ed prefe rentiallyatpH 2.2 and 5.1ra ther thanat pH 7.1, when various secondary reactionsgenerated a host of different compounds (Shu and Ho, 1988).3.4.3 Therm al degradation ofthiamineThetherm al degradat ionofthiamine produces some im portant com pounds(Werkhoff etal. 1989,1990;Gu ntert et al.,1990; van der Lind e et al.,1979;H a r tma n etal. 1984; Reineccius and Liardon, 1985; Dwivediand Arnold,1973). van der Linde et al. (1979) reported five primary products, themain component being 4-methyl-5-(2-hydroxyethyl)thiazole, which is


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responsible for the formation of several thiazoleson further degradation.The other sulph ur-containing prim ary product is 5-hydroxy-3-m ercapto-pentan-2-one, a key intermediate compound giving rise to a number ofaliphatic sulphu r com pounds, furans and thiophenes (vand er Linde, 1979).Two of these were also present in the previously discussed reaction prop-erties of HMFone/H2S (van den Ouweland and Peer, 1975; van denOuweland et al., 1978), namely 2-methyltetrahydrofuran-3-one, and themeaty 2-methyl-4,5-dihydrofuran-3-thiol. Hartman et al. (1984) degradedthiamine at 135C/30 min in water (pH 2.3) and also in propane-l,2-diol.Few decomposition products were formedfrom thediol system but severalcarbonyls, furanoids, thiophenoids, thiazoles and aliphatic sulphur com-pounds were isolated from the aqueous reaction. In addition, anovel com-pound was reported, i.e. 3-methyl-4-oxo-l,2-dithiane, also present (and inenhanced concentration) from a model system of cystine/ascorbic acid/thi-amine (Har tman et al., 1984). Reineccius and Liardon (1985) studied thevolatileproducts from thiamineat lower temperatures (4O0C,6O0C, 9O0C)anda t pH5,7and 9respectively.A t pH 5 and 7, them eaty2-m ethylfuran-3-thiol (7) and bis(2-m ethy l-3-fury l) disulph ide (9), togeth er w ith variousthiophenes were the major products, but at pH 9, the meaty compounds(7) and (9) were not significant and the thiophenes predominated. Apartfrom compounds (7) and (9), other compounds which are reportedlymeaty and which have been identifiedfrom therm ally degraded thiam ineare 3-mercaptopentan-2-one, 2-methyl-4,5-dihydrofuran-3-thiol,2-methyl-tetrahydrofuran-3-thiol,4 ,5-dimethylthiazole and 2,5-dimethylfuran-3-thiol(van der Linde et al.,1979; H artm an et al., 1984).In their recent paper reporting the formation of selected sulphur-containing compounds from various meat model systems, Guntert et al.(1990) have proposed very comprehensive reaction schemes for the ther-mal degradation of thiamine. An extract of these schemes, in so far as itrelatesto the formationo fsome com pounds rep orted to bemeaty,isshownin Figure 3.11 (Guntert et al., 1990). In the following, meaty compoundsfrom thiamineare coded (A)-(U);allother compounds are uncoded. Oneof th e prim ary de gradation products, i.e. 4-m ethyl-5-(2-hydroxy ethyl)thiazole, as mentioned above, is responsible for the subsequent generationof several thiazoles, such as 4,5-dimethylthiazole (B) and thiazole itself(19). Othe r prim ary deg radation produ cts are4-amino-5-(am inomethyl)-2-methylpyrimidine (a), formic acid and the key intermediates H2S and 5-hydroxy-3-mercaptopentan-2-one (c).The latter canform seven a dditionalintermediates, only one of which, i.e. 3,5-dimercaptopentan-2-one (b), isshown in Figure 3.11. Further reactions of blead to the meaty compounds3-acetyl-l,2-dithiolane (A), 2-methyltetrahydrothiophene-2-thiol (C),2-methyl-4,5-dihydrothiophene-3-thiol (D ) 2-methylthiophene-3-thiol (F)andb is(2-methyl-3-thienyl)disulphide (G) as show n, w hereas intermediatec isresponsible for the formation of2-methyl-4,5-dihydrofuran-3-thiol (E),


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Figure3.11 Formation of some meaty compounds from th e thermal degradation of thiamifollows: 3-acetyl-l,2-dithiolane (A) = meaty, onion, shiitake, liver (Guntert e t aL, 1990)poultry (Pit tet and Hruza, 1974; Vernin, 1979); 2-methyltetrahydrothiophene-2-thiol (C(Gunter t et aL, 1990); 2-methyl-4,5-dihydrothiophene-3-thiol (D) = meaty(van den Ouwe= roast meat (van den Ouweland and Peer, 1975; 2-methylthiophene-3-thiol (F) = roast thienyl)disulphide (G) = sulphurous, metal l ic, rubbery, s l ightly m

thiamin


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2-methylfuran-3-thiol (7) and bis(2-m ethyl-3-fury l)disulphide (9). Twofur ther meaty compounds, which are not shown in Figure 3.11, were alsoidentified (Guntert et al., 1990). These were mercaptopropanone andtetrahydrothiophene-2-thiol . To date, only compounds 7, 9 and 19 havebeen identified from beef; mercaptopropanone has been reported fromcanned pork (Maarse andVisscher, 1989), and it isinterestingto note thatpork has asign ificantly higher thiamine content than beef.As a sequel to this stud y just described, W erkho ff et al. (1989, 1990)have recently published their excellent work on the identification of inter-esting volatile sulphur compounds giving meaty notes to a model meatsystem co nsistingofcystine/thiamine/glutam ate/ascorb ic acid/wa ter heatedat 120C/0.5 h at an initial pH of 5.0. They positively characterized 70sulphur components, of which 19 possessed individua l odo urs describedas meaty. These are presented in Figure 3.12. The majority are new tothe flavour literature. Four have already been identified from naturalcooked beef, nam ely2-methylfuran-3-thiol (7) (Gasser and Grosch,1988),bis(2-methyl-3-furyl)disulphide (9) (Gasser and Grosch, 1988), 2-methyl-3-[2-methyl-3-thienyl)dithio]furan (10) (Werkhoff etal 1989, 1990) and -(methylthio)ethanethiol(1)(Maarse andVisscher, 1989). Apart from th elatter, the others are all recent identities from beef, the most recent being2-methyl-3-[(2-methyl-3-thienyl)dithio]furan (10), identified by retro-spective use of informa tion gained from the above model systeminwhichit was the main component. The disulphides 9, 10, G, H and J of Figure3.12 are derived from oxidation of the corresponding thiols. Even airoxidation of the monomers results in dimerization without effort (Werk-hoff et al 1989, 1990). l-[2-Methyl-3-thienyl)thio]ethanethiol (1) andl-[(2-methyl-3-furyl)thio]ethanethiol (U) aretotallynewcompounds. Theyhave flavour threshold values in water of


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Figure 3.12 Meaty compounds from a model mea t system of cystine/thiamine/glutam


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Figure 3.13 Decomposition of alkoxy radicals (RO') derived from monohydroperoxides anis saturated; example B is RO' where R has one double bond; example C is RO* where Ra diene. (Reprinted with permission from MacLeod, G. and Ames, J.M. (1988) CRC Crit. RInc., Boca Raton, FL.)


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Figure3.14 Schematic representationofsome possible reaction pathways in thethermal oxfromMacLeod, G. and Ames, J.M. (1988) CRC Crit. Revs Food ScL Nutr., 27,219-400.

SATURATED FATTYACID


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Other cooked beef aroma components created by lipid degradation areseveral benzenoids, e.g. benzaldehyde, benzole acid, alkylbenzenes andnaphthalene.Lipid oxidation starts in raw beef and continues during cooking. Evenin lean muscle,the intramuscular lipidsare asourceof avery large numberof volatiles, many of which are present in relatively high concentrations(Bailey and Einig, 1989; Buckholz, 1989). In fact, they create quite anuisance effect, analytically speaking, because they dominate gaschrom-atograms of aroma isolates from lean beef and hinder the detection oftrace components.The role of lipids inbeef flavour has been considerably clarified by thework of Mottram and his colleagues. They showed that the addition ofadipose tissue to lean beef does not givea proportional increase in lipid-derived volatiles, indicating that the intramuscular lipids are the majorsource of volatile components (Mottram etal 1982). It has also beenshown that species-specific flavour precursors are present in lean beef,al though the addition of fat induces fat/lean interactions of some kindwhich enhance speciesdifferences. Intramuscular lipids consist ofmarblingfat (mainly triacylglycerols)and structuralormembrane lipids, which arelargely phospholipids, and which contain a relatively higher content ofunsaturated fatty acids. Selected removal of the inter- and intramuculartriacylglycerols from lean beef causes no significant chemical or sensoryaroma differences, but removal of both triacylglycerolsand phospholipidsgenerates marked chemical and sensory differences (Mottram andEdwards,1983).The aromaisless meatyandmore roasted,and itcontainsless lipid oxidation products but higher concentrations of certain hetero-cyclic compounds, including some alkylpyrazines (Mottram and Edwards,1983). This implies that, in beef, lipids (or their degradation products)m ay inhibitthe formationofsome heterocycles specifically generated fromMaillard reactions (Mottram and Edwards, 1983).This hypothesis was tested using model systems, e.g. of glycine/riboseand of cystein/ribose, both in the presence and absence of lecithin(Whitfield et al 1988; Farmer et al 1989; Mottram and Salter, 1989;Farmer and Mottram, 1990; Mottram, 1987; Salter etal 1988).The sameoveral leffectwasobservedinboth cases (i.e.alower concentration ofsomeheterocyclic compounds generated by theaminoacid-ribose reaction. Forsome compoundsin the cysteine system, e.g. 2-methylfuran-3-thiol(7) andth iophene-2- thiol , the decrease was as much as 65% or more (Mottramand Salter,1989).This isexplained bycompetition by lecithin degradationproducts for H2Sand NH3derived from cysteine,and thiswasproved tooccurby the additional presencein the reaction product mixtureofsomedifferent heterocycles known to arise from such reactions. The possibleinteraction of different lipids was also tested.For example, the additionof beef triacylglycerol had no effect on the aroma of the cysteine/ribose


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reaction produ ct m ixture , but the addition of beef phospho lipid caused asignificant increase ofmeaty aroma,asignificantly decreased concentrationof the normal Maillard reaction heterocycles and a significantly increasedconcentration of new heterocycles specific to lipid-Maillard interactions(Mott ram and Salter, 1989). Examples of such products are 2-pentyl-pyridine (from deca-2,4-dienal/NH3 interaction),2-pentyl-,2-hexyl- and 2-hex-1-enyl-thiophenes (from alka-2,4-dienals/H2S or from 2-alkylfurans/H 2S), heptane-1-thiol and octane-1-thiol (from alkan-l-ols/H2S) and4,5-dimethyl-2-pentylthiazole (from hexanal/diacetyl/H2S/NH 3). The for-mation of heterocyclic compounds with long-chain alkyl substituents hasalso been confirmed in model systems of deca-2,4-dienal and dec-2-enalwith H2S(vanden Ouweland et#/., 1989), ofdeca -2,4-dienal w ith cysteineor glutathione (Zhang and Ho, 1989) and from acetol/NH3/pentanal orhexanal reaction (Chiu et al.,1990).Therefore, inbee f, some lipid isnecessaryfor afullmeaty aroma, but theintramuscular tissue phospholipids (which constitute only about 1% ofthe m uscle com position) aresufficient, and the triacylglycerols are no t essen-tial (Mottram andEdwards,1983).Interactions betw een w ater-soluble andphospholipid-derived components occur andcouldbe important.3.4.5 Selected aroma components of high sensory significanceGrosch and his co-workers have recently developed ascreening procedu reforhighlightingthemost im portant volatile compounds inaroma isolates. Inthis technique, the volatiles of an extract are 'arranged' in order of theirflavoursignificance according to their 'flavourdilution factors' (FD factors).The FD factor of any compound is proportional to its 'arom av alue' wh ichisdefinedas the ratio of the concentration of the flavour compound to itsodour threshold. When the technique wasapplied to cooked beef, Gasserand Grosch(1988)identified35comp ounds possessingFDfactors 4.Theseare listed, together with the odour qualities apportioned to each, in Table3.2. Seventeen aroma components (of which 15w ere identified)had rela-tivelyhighFD factors^64),thereby contributing w ith high aroma valuestothe flavour of the cooked beef. Only two compounds were described asmeaty,i.e.2-methylfuran-3-thiol (7) andbis(2-methyl-3-furyl)disulphide (9).Both possessed thehighestFDfac tor measured (512), highligh ting the ir con-siderable odour potency. The odour thresholds of those tw o compoundswere determined as 0.0025-0.01ngI-1 (air) and 0.0007-0.0028 ng H (air)respectively (Gasser and Grosch, 1990a,b). Gasser and Grosch have alsoreported a virtua lly identical study on chicken volatiles (Gasser and Grosch,199Oa) and on commerical meat flavourings (Gasser and Grosch, 199Ob).Them ajor differences between beef andchicken we re that th esulphur com-pounds bis(2-methyl-3-furyl)disulphide (9) (mea ty) and m ethional (2)(cooked potato) predominated in beef whereas volatiles from oxidation of


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of theavailable gaseous oxygenforperoxidation reactions, thus protectingthe thiolagainst oxidation to the disulphide. This hypothesis links with thepreviouslymentioned findings ofW hitfield et a l. (1988) whoshowed averysignificantdecrease inconcentrationof2-methylfuran-3-thiol (7)from acys-teine/ribose model system when lecithin was added. They suggested thatcarbonylcompounds (from thelecithin) were preferentially capturing reac-tants suchasH2S.Sincethecombined levelsof 2-methylfuran-3-thiol (7) anditsdisulphide were much lowerinchicken thaninbeef, this difference couldwell be due to the reactions proposed by Whitfield et al,Furthermore, thehigher FD factor for methional inbeef, compared with chicken volatiles,tends to indicate a partial inhibition of the Strecker degradation ofmethio-nineon heating chicken (Gasser and Grosch, 199Oa).

A familyoffuranswith sulphur substituents in the3-position- asexem-plified bycompounds 7-10 ofFigure3.1 and others also discussed inthischapter,but so farunidentifiedinbeef- arelikelyto be ofextreme impor-tance in cooked beef aromas. Many have been synthesized, patented anddescribed sensorially (Werkhoffetal 1989,1990;Everset al. 1975; Tressland Silwar, 1981; Gasser and Grosch, 1988).These four compounds haveonly recently been reported from cooked beef however (Werkhoff e t al.1989, 1990; MacLeod andAmes, 1986; Gasser andGrosch, 1988).van denOuweland et al.have proposed that an essential structural requirementfo rmeaty aroma is a 5- or6-membered ring, whichismore or less planarand substituted with an enol, thiol and a methyl group adjacent to thethiol, as exemplified by the compounds shown in Scheme 3.1 (van denOuweland , 1989).

A comprehensive investigation into structure/activity correlation inmeaty compounds has been reported by Dimoglo et al. (1988). Theyconcluded that the generalized molecular fragment shown in Scheme 3.2accounts for meaty odour (see also van Wassenaar et al. 1995; Wanget al. 1996; Hau etal. 1997).

Scheme 3.1

green pea roast meat burnt,roast meat green,herbaceous

meaty, brothy meaty,Phenolic

burnt,greenfatty burnt


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Scheme 3.2In Scheme 3.2,X = O, S and 3 =agroup cop lanarw ith th ea-carbon andthe methyl group carbon (e.g.C=O) or an atom different from O, e.g. S.For furan and thiophene derivatives, the methyl group on C2 plays animportant role inm eaty odour; addi t ional ly,tw ofuran rings and/or two or

more su lphur a toms favour meatiness. They also showed that allm e a tycom pounds contain the general s t ructuralfragment XH2w here X = O, N,S and H2= two hydrogen atoms belonging,as a rule, to a m ethyl group.The methyl group must rota te freely. For X = O, N and inter-atom icdistance (X-H1) of0.262-0.277nm, compounds are described as 'meaty,meat sauce-like, meat soup odour'; for X=SwithX-H1 distance of0.278-0.306nm, compounds possess a'roast meat' odour (Scheme 3.3).T he extremely low odour thresholds for compounds 9 and 7 havealreadybeen mentioned. Related structures are likelyto be potent odor-ants too. It follows therefore that only minute traces of these types ofcompounds needbepresent forthemto bea roma effective, creating enor-mous analyt icaldifficulties for their detection.

3.5 ConclusionsBecause of these difficulties, many researchers have investigated relevantm odel systems. Such studies hav e led to the characterization of im portantvolati les, mostly sulphur compounds, the presence of which can then beinvestigated retrospectively in natural cooked beef aromas. The extremesuccess of this philosophy is now evident, particularly so in the recentli terature. Several com pounds exhibiting m eaty/b eefy odou rs have beenfully characterized, both chemically and sensorially, and formation

Scheme 3.3

2-methylfuran-3-thiolmeatsauce,soup) 2-methylthiophene-3-thiol roast meat)

* denotes X H2 fragments


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mechanisms from defined precursors have been proposed. A few of thesecompounds have also been isolated from beef itself, with strong indica-tions that others are inherently present in trace quantities.There is nodoubt that the successes of the last decade have evolved from suchapproaches.ReferencesAiraudo, C.B., Gayte-Sorbier, A. and Armand, P. (1987). Stabilityofglutamine and pyro-glutamic acid under model system conditions. J. FoodSd. 52,1750-1752.Arctander, S. (1969). Perfume and Flavor Chemicals, published by the author, N ew Jersey.Bailey, M.E. and Einig, R.G. (1989). Reaction flavors of meat. In Thermal Generation of

Aromas eds. T.H. Pariiment,RJ. McGorrin andC.-T. Ho. American Chemical Society,Washington, DC, pp.421-432.Baltes ,W. (1979). Rostaromen. Deutsch. Lebensm. Riwdsch. 72, 2-7.Bodrero,K.O.,Pearson, A.M.andMagee, W.T.(1981).Evaluationof thecontributionof fla-vorvolatilesto the aromaofbeef bysurface response methodology. J.Food ScL,46 ,26-31.Br i nkman , H.W., Copier, H., de Leuw, JJ.M. and Tjan, S.B. (1972). Componentscontributing to beef flavor. J. Agric. Food Chem. 20,177-181.Buckholz , L.L. Jr. (1989). Maillard technology as applied to meat and savory flavors. InThermal GenerationofAromas eds. T.H. Pariiment,RJ. McGorrin andC.-T. Ho. Amer-icanChemical Society, Washington,DC, pp. 406-420.Ching, J.C.-Y. (1979). Volatile Flavor Compounds from Beef and Beef Constituents. PhDThesis, UniversityofMissouri.Chiu, E.-M., Kuo,M.-C,Bruechert, LJ. and Ho, C.-T. (1990) Substitutionofpyrazinesbyaldehydes in model systems.J. Agric. Food Chem. 38,58-61.de Ri jke , D.,van Dort, J.M. and Boelens, H. (1981).Shigematsu variationof the Maillardreaction. In Flavour 81 ,ed. P. Schreier. de Gruyter, Berlin,pp.417-431.Dimoglo,A.S.,Gorbachev,M.Y.,Bersuker, LB., Greni, A.L, Vysotskaya, L.E., Stepanova,O.V.andLukash, E.Y. (1988).Structuraland electronic origin of meat odour of organicheteroatomic compounds. Nahrung 32,461-473.

Dwivedi , B.K. and Arnold, R.G. (1973). Chemistry of thiamine degradation in foodproducts and model systems./. Agric. FoodChem. 21,54-60.Evers,WJ., Heinsohn, H.H. Jr., Mayers,BJ.andSanderson, A. (1976). Furans substitutedat the three position with sulfur, in Phenolic, Sulfur and Nitrogen Compounds in FoodFlavors,eds.G.Charalambousand I.Katz. American Chemical Society, Washington,DC,pp. 184-193.Farmer,JJ.and Mottram, D.S. (1990). Recent studieson the formationofmeat-likearomacompounds. InFlavour Scienceand Technology,eds.Y.Bessiere andA.F. Thomas. Wiley,Chichester,pp.113-116.Farmer, LJ., Mottram, D.S. and Whitf ield, F.B. (1989). Volatile compounds produced inMaillard reactions involving cysteine, ribose and phospholipid. J. ScL Food Agric. 49,347-368.Flament, L,Kohler,M. and Aschiero, R. (1976). Sur 1'arome de viande de boeuf grille: II.Dihydro-6,7-5H-cyclopenta[Z?]pyrazines, identificationet mode de formation. HeIv. Chim.Acta 59 2308-2313.Flament, L, Sonnay, P. and Ohloff , G. (1977). Sur 1 arome de boeuf grille: III. Pyrrolo-[l,2-0]pryazines, identificationet synthese. HeIv. Chim.Acta 60,1872-1883.Furia, T.W. and Bellanca, N . (1975). Fenaroli s Handbook of Flavor Ingredients,2nd edn,Vol.1. CRC Press, Ohio.Gai t ,A.M.andMacLeod, G.(1983).The applicationoffactor analysistocooked beef aromadescriptors./. Food ScL,48,1354-1355.Gasser, U. and Grosch, W . (1988). Identificationof volatile flavour compounds with higharoma values from cooked beef. Z. Lebensm. Unters. Forsch.,186, 489-494.


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Kubot a , K .,Kobayashi, A. and Yamanishi, T. (1980). Some sulfur-containing compounds incookedodorconcentrate fromboiled Antarctic krills.J. Agric. Food Chem. 44,2677-2682.Kuchiba , M ., Kaizaki,S., Matoba, T. and Hasegawa, K . (1990). Depressing effect of saltson thermal degradation of inosine5'-monophosphateand guanosine5'-monophosphateinaqueous solution.J. Agric. Food Chem. 38,593-598.Kuninaka , A. (1981). Taste and flavour enhancers. In Flavor Research: Recent Advanceseds. R.Teranishi and R.A. Flath. Marcel Dekker, NewYork, pp.305-353.Maarse,H. andVisscher, C.A. (1989). Volatile Compounds inFood- QualitativeandQuan-titative Data.TNO-CIVO,Zeist,The Netherlands.MacLeod, G. (1986). The scientific and technological basis of meat flavours. In Develop-ments in Food Flavours eds. G.G. Birch and M.G. Lindley. Elsevier Applied Science,London, pp.191-223.MacLeod, G. and Ames, J.M. (1986). 2-Methyl-3-(methylthio)furan: a meaty characterimpact compound identified from cooked beef.Chem. Ind. 175-177.MacLeod, G. and Ames, J.M.(1988).Soy flavor and its improvement.CRC Crit.Revs FoodScL Nutr. 27,219-400.

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