Aroma contributors

Whereas the glucose ester 9 has been identified for the first time as a natural wine constituent, glycoconjugates of its reduced form, i.e. of the monoterpene diol 11, are known Riesling wine constituents (2). Under acidic conditions, diol 11 was partially converted into the bicyclic ether 12, the so-called dillether (2). In analogy to the formation of ether 12 from terpene diol 11, a likely formation of lactone 10 from acid 9A could be be expected (cf. Fig. 5). This so-called wine-lactone 10, first identified as an essential oil metabolite in the Koala (55), has recently been established by Guth (34) as a major aroma contributor in two white wine varieties. The 35,3a5,7aR-configured isomer of 10, which has been identified in wine, is reported to possess an unusual low flavor threshold of 0.01-0.04 pg/L of air and a sweet, coconut-like aroma (55). 

This book on the biogeneration of aromas comes at an opportune time. During the 1960 s and 70 s research in this area at the industrial and academic level was neglected in favor of classical analytical research aimed at unravelling the complexities of the chemical mixtures responsible for the aroma of food. In retrospect it appears that this was the proper way to approach this problem, since we had to identify the important aroma contributors before considering their biological origin. 

In E. foetidum a total of 13 aroma-active constituents with FD-factors > 9 were detected. These consisted of two n-aldehydes (nos. 2 and 6), seven 2-alkenals (nos. 3, 5, 7, 8, 9, 11, 28), (Z)-3-hexenal (no. 22) and three unknown compounds (nos. 29, 30 and 32)(Table V). All of these compounds were detected in C sativum with the exception of an unknown compound (no. 32) having a spicy, herbaceous note. ( )-2-Dodecenal was by far the predominant aroma component with an FD-factor of 2187. Unknown compound no. 30 had the second highest FD factor (=243). The musty, chlorine-like character of this compound may be important in the overall aroma of E. foetidum herb. Other aroma contributors include compounds with FD factors from 27 to 81, such as nos. 5, 6, 7, 11, 22, and 29. Despite having an FD factor of 27, (Z)-3-hexenal (no. 22) was probably derived via lipoxygenase action during sample preparation and may not be a characteristic aroma component of essential oil of E. foetidum herb. 

Unsaturated aldehydes have particularly irritating odors, and this is further increased by the presence of a triple bond. Like alcohols, some of the mid chain length unsaturated aldehydes such as trans-2-hexenal have very desirable green but more spicy odors. The di unsaturated aldehydes, e.g., 2,4-decadienal and 2,6-non-adienal, are important aroma contributors to meaty and fried aromas, and cucumber aroma, respectively. The aromatic aldehydes are powerful and display a wide spectrum of profiles, depending on their complexity. 

Here too we have a common contributor to the acid component of an ester, acetic acid in this case, coupled to various alcohol components to yield a variety of distinct biological responses, the aromas of different fraits. 

Like many odors, that for coffee has many individual contributors. Nonetheless, we do not, indeed cannot, identify the individual compounds that contribute to the aroma. All we get is the smell of coffee. With a single odorant, say jasmine, many of us can identify it correctly. But if an aroma contains two components, only about one person in six can identify both correctly. And only about one person in 25 can unravel a mixture containing three components. Nobody gets it right if the aroma contains four or more components. Our brain basically integrates all the inputs and gives us a single sensation, not multiple, individually identifiable sensations. 

DC064 Cronin, D. A., and P. Stanton. 2-methoxy-3-sec-butylpyrazine-an important contributor to carrot aroma. J Sci Food Agr 1976 27 145. 

The "impact compound that provides the primary stimulus for fruit character in the raspberry is the ketone, l-(p-hydroxphenyl)-3-butanone 11). Other important flavour contributors are cw-3-hexen-l-ol, a - and p - ionones, and a - irone (72, 13). In R. arcticus the characteristic aroma is considered to be from mesifiirane (70). It has, however, been reported that steam distillates of raspberries can be assessed for aroma content using a colorimetric procedure and 80% of aroma is accounted for by geraniol, nerol, linalool, a - terpineol and die ionones (13). 

For estimating the contribution of volatile compounds to bread aroma Rothe and coworkers (S) defined "aroma value" as the ratio of the concentration of some volatile compounds to the taste threshold value of the aroma. This concept was further developed by Weurman and coworkers (9) by introducing "odor value", in which aroma solutions were replaced by synthetic mixtures of volatile compounds in water. These mixtures showed the complexity of the volatile fractions of wheat bread, because none of them resembled the aroma of bread. Recently two variations of GC-sniffing were presented (10-11), in which the aroma extract is stepwise diluted with a solvent until no odor is perceived for each volatile compound separately in the GC effluent. The dilution factors obtained indicate the potency of a compound as a contributor to the total aroma. 

The most important flavour compound in raw onions is thiopropanal-S-ox-ide, the lachrymatory factor [145,146]. Other important flavour compounds are 3,4-dimethyl-2,5-dioxo-2,5-dihydrothiophene and alkyl alkane thiosulfonates such as propyl methanethiosulfonate and propyl propanethiosulfonate with a distinct odour of freshly cut onions [35, 36, 147]. Various thiosulfinates that have a sharp and pungent odour may also contribute to the flavour of onions. These compounds, however, are rapidly decomposed to a mixture of alkyl and alkenyl monosulfides, disulfides and trisulfides (Scheme 7.3) of which dipropyl disulfide, methyl ( )-propenyl disulfide, propyl ( )-propenyl disulfide, dipropyl trisulfide and methyl propyl trisulfide are the most important contributors to the aroma of raw and cooked onions (Table 7.5, Fig. 7.6) [148-150]. Recently, 3-mercapto-2-methylpentan-l-ol was identified in raw and cooked onions eliciting intense meat broth, sweaty, onion and leek-like odours [142, 151]. 

Charentais cantaloupe melon Cucumismelo L. var. cantalupensis Naud.) was characterised by abundant sweetness and a good aromatic flavour [68]. The aroma volatiles of Charentais-type cantaloupe melons, as with other cantaloupes, comprise a complex mixture of compounds including esters, saturated and unsaturated aldehydes and alcohols, as well as sulfur compounds [26, 65]. Among these compounds, volatile esters were quantitatively the most important and therefore represent key contributors to the aroma [68]. The linear saturated and unsaturated aldehydes seem to originate from the degradation of linolenic and linoleic acids [26, 32, 33, 67]. 

More recently, several aroma compounds were isolated from cupuacu pulp by vacuum distillation, solid-phase extraction, and simultaneous steam distil-lation-extarction and were analysed by GC, GC-MS, and GG-O [8]. The olfaction of the extracts obtained by solid-phase extraction indicated linalool, a-ter-pineol, 2-phenylethanol, myrcene, and limonene as contributors of the pleasant floral flavour. In this study, the esters ethyl 2-methylbutanoate, ethyl hexanoate, and butyl butanoate were involved in the typical fruity characteristics. 

A number of furans with thiol, sulphide or disulphide substitution have been reported as aroma volatiles, and these are particularly important in meat and coffee. In the early 1970s, it was shown that furans and thiophenes with a thiol group in the 3-position possess strong meat-like aromas and exceptionally low odour threshold values [50] however, it was over 15 years before such compounds were reported in meat itself In 1986,2-methyl-3-(methylthio)furan was identified in cooked beef and it was reported to have a low odour threshold value (0.05 pg/kg) and a meaty aroma at levels below 1 pg/kg [51]. Gasser and Grosch [52] identified 2-methyl-3-furanthiol and the corresponding disulphide, bis(2-methyl-3-furanyl) disulphide, as major contributors to the meaty aroma of cooked beef. The odour threshold value of this disulphide has been reported as 0.02 ng/kg, one of the lowest known threshold values [53]. Other thiols which may contribute to meaty aromas include mercaptoketones, such as 2-mercapto-pentan-3-one. 2-Furylmethanethiol (2-furfurylmercaptan) has also been found in meat, but is more likely to contribute to roasted rather than meaty aromas. Disulphides have also been found, either as symmetrical disulphides derived from two molecules of the same thiol or as mixed disulphides from two different thiols [54]. 

As for S-containing heterocycles, many N-containing heterocycles are also found in heat-treated foods as secondary flavours as a result of Maillard-type reactions between reducing sugars and amino acids. Pyrazines are N-heterocycles important contributors to the taste and aroma of roasted and toasted foods as well as vegetables and fermented foodstuffs. In cultures of Pseudomonas perolens ATCC 10757, amino acids such as valine, glycine and methionine were shown to... 

The significance of subthreshold concentrations as important contributors to aromas has been noted and discussed by others (5, 6,13). It is clear that much more must be learned before the relationship among chemical structure, concentration, and the sensory effect of these compounds can be understood. 

An investigation into the aroma of cooked small shrimp and krill revealed more than forty sulfur- and/or nitrogen-containing heterocyclic substances. Among these eight were newly identified as contributors to food flavor. The main ring structures encountered were pyrazine, trithiolane, dithiin and dithiazine. 

A heterocyclic sulfur-containing compound, 2-methyl-thiophene, was identified in boiled crayfish tail meat and pasteurized crabmeat. Thiazole and 3-methylthiopropanal were identified in the crayfish hepatopancreas. Heterocyclic sulfur-containing compounds play important roles in generating meaty aromas in a variety of meat products and are considered important volatile aroma components of marine crustaceans (12— 14). The 2-methylthiophene could be an important flavor cemponent in boiled crayfish tail meat. Both thiazole find 3-methylthiopropanal were important contributors to the desirable meaty aroma associated with crayfish hepatopancreas. The 3-methyl-thiopropanal, identified in boiled crayfish hepatopancreas, is derived from Strecker degradation of methionine (15), and has been considered to be an important cemponent in basic meat flavor (16). Pyridine was detected in the headspace of the hepatopancreas from freshly boiled crayfish. Pyridine and 2-ethylpyridine have been previously reported as components in the atmospheric distillate from a sample of crayfish hepatopancreas frozen for three months (2). 

Thiophenes and Furanthiols. Thiophenes are extremely important contributors to cooked meat flavor and are responsible for mild sulfurous aromas. The sulfur in thiophenes may be derived from amino acids (cysteine, cystine, methionine) or from thiamine. Over 36 thiophene derivatives have been found during various investigations of meat or meat constituents (32). 

Being volatile, the aldehydes formed in the Strecker degradation have often been thought to be important contributors to the aroma of foodstuffs and many patents have been granted which use the Strecker degradation to produce flavouring materials of various types, such as, maple, chocolate, coffee, tea, honey, mushroom, and bread.66... 

The aroma and flavour of ginger are determined by the composition of its steam-volatile oil, which is comprised mainly of sesquiterpene hydrocarbons, monoterpene hydrocarbons and oxygenated monoter-penes. The monoterpene constituents are believed to be the most important contributors to the aroma of ginger and they tend to be relatively more abundant in the natural oil of the fresh ( green ) rhizome than in the essential oil distilled from dried ginger. Oxygenated sesquiterpenes are relatively minor constituents of the volatile oil but appear to be significant contributors to its flavour properties. 

The aroma of mangosteen is contributed by 52 volatile compounds, 28 of which have been identified. In terms of quantity, the major compounds are (Z)-hex-3-en-l-ol (27%), octane (15%), hexyl acetate (8%) and a-copaene (7%). The main contributors to the mangosteen flavour are hexyl acetate, (Z)-hex-3-enyl-acetate (cis-hex-3-enyl-acetate) and (Z)-hex-3-en-l-ol (MacLeod and Pieris, 1982). The major groups of compounds found in mangosteen aril are alcohols, aldehydes and ketones, esters, hydrocarbons, terpenes, etc. The compounds present are given in Table 19.3. 

Wines aged under a flor film typically contain various compounds especially important among which are lactones. Thus, solerone (4-acetyl-y-butyrolactone) abounds mfino and Jura wines and has for years been deem one of the greatest contributors to their aroma profiles (Augustyn et al. 1971 Muller et al. 1973). However, subsequent studies have suggested that this lactone has no impact on wine aroma (Martin and Etievant 1991). 

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