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Figure 2 Representative fruit character impact flavor compounds.

also significant contributors to the ''foxy'' and ''candy-like'' notes, respectively, in Concord grape. This aroma character is in significant contrast to that of Vitis vinifera varieties used for production of table wines, in which ethyl heptanoate has an odor reminiscent of cognac (4). 4-Mercapto-4-methyl-2-pentanone provides Sauvignon grape character, as discussed earlier (5).

Two character impact compounds have been proposed for grapefruit flavor, the first being nootkatone, a sesquiterpene. The fresh juicy note of grapefruit juice is attributable to 1-p-menthene-8-thiol. This compound has a detection threshold of 10-1 ppt, among the lowest values reported for aroma chemicals (28). The (+)-R-isomer was found to have a lower aroma threshold in water than the racemic mixture, and it imparts a pleasant, fresh grapefruit juice character, as opposed to the extremely obnoxious sulfur note contributed by the (-)-S-epimer.

Among other citrus flavors, the basic flavor-impact compound of lemon is citral, a mixture of neral and geranial isomers, which together compose the aroma impression. The flavor character of lime results from a combination of a-terpineol and citral, even though limonene is the most abundant, but sensorily immaterial, volatile in lime and other citrus oils. In contrast to other citrus flavors, a single flavor impact compound is unknown for orange, and current belief is that orange flavor is the result of a complex combination of terpene, aldehyde, ester, and alcohol volatiles in specific proportions (29). The most odor-active compounds in fresh orange include the sesquiterpene aldehyde P-sinensal, octanal, and decanal (4). For Mandarin and tangerine flavors, methyl-N-methyl anthranilate and thymol are associated with character impact, with additional contributions from P-pinene and y-terpinene (29).

Lactones have characteristic aromas that contribute to peach, coconut, and dairy flavors and occur in a wide variety of foods. The y-lactones, specifically y-undecalactone and lesser for y-decalactone, possess intense peach-like odors (11). A doubly unsaturated 8-decalactone, 6-pentyl-2H-pyran-2-one, also has an intense peach character (4). As a point of distinction, the Q0-Ci2 8-lactones, particularly the ''creamy-coconut'' note of 8-decalactone, are flavor constituents of coconut as well as cheese and dairy products (11).

In addition to the character impact compound of raspberry, 4-(4-hydroxy-phenyl)-butan-2-one (raspberry ketone), alpha- and beta-ionone, geraniol, and linalool were concluded to be of importance in raspberry aroma (30). Its odor threshold was measured at 1-10 |g/kg. The ionones have chemical structure similarities and potencies comparable to P-damascenone (11).

The ''tropical'' category is one of the most important areas for new discoveries of key impact flavor compounds. Analyses of passion fruit and durian flavors have produced identifications of many potent sulfur aroma compounds (18). Among these is tropathiane, 2-methyl-4-propyl-1,3-oxathiane, which has an odor threshold of 3 ppb (15). For pineapple, 2-propenyl hexanoate (allyl caproate) exhibits a typical pineapple character (11); however, Furaneol, ethyl 3-methyl-thiopropionate, and ethyl-2-methylbutyrate are important supporting character impact compounds (31). The latter ester contributes the background ''apple'' note to pineapple flavor. Another character impact compound, allyl 3-cyclohexyl-

propionate, has not been discovered in nature, but it provides a sweet-fruity pineapple flavor note (11).

Blackcurrant flavor is very popular in Europe, and is associated with numerous health-related functional foods and with alcoholic drinks (cassis liqueur). The key aroma component in blackcurrant is 2-methoxy-4-methyl-4-butanethiol (18). Characterizing flavors for melons include (Z)-6-nonenal, which contributes a typical melon aroma impression, and (Z,Z)-3,6-nonadienol for watermelon rind aroma impact (22). 2,6-Dimethyl-5-hepten-1-al (Melonal™) has not been identified in melon, but it provides a melon-like note in compounded flavors (11). The flavor of muskmelons is more complex, with methyl 2-methylbutanoate, Z-3-hexenal, £-2-hexenal, and ethyl 2-methylpropanoate identified as the primary, non-characterizing odorants (32).

While an important constituent to both varieties, the flavor of sweet cherries (Prunus aviium) is less dominated by the character impact compound, benzaldehyde, than is the flavor profile of sour cherries (P. cerasus) (33).

C. Vegetable Flavors

Recent aroma research has been devoted to the identification of key flavor compounds in vegetables and is the subject of several contemporary reviews (31,34,35). Cucumbers, sweet corn, and tomatoes are botanically classified as fruits; however, for flavor considerations they are regarded as vegetables, because they are typically consumed with the savory portion of the meal. Overall, the knowledge base of character impact compounds for vegetables is much smaller than other flavor categories and warrants further investigation.

Identifying flavor impact compounds in vegetables depends considerably on how they are prepared (cutting, blending) and the form in which they are consumed (raw vs. cooked). For example, the character impact of fresh tomato is delineated by 2-iso-butylthiazole and (Z)-3-hexenal, with modifying effects from P-ionone and P-damascenone (31). Alternatively, thermally processed tomato paste has dimethyl sulfide as a major flavor contributor (31,34). Dimethyl sulfide is also a flavor impact compound for both canned cream corn and fresh corn; 2-acetyl-1-pyrroline provides a ''corn chip'' character. Other sulfur compounds such as hydrogen sulfide, methanethiol, and ethanethiol may contribute to the aroma of sweet corn due to their low odor thresholds (35). A summary of character impact compounds for vegetable flavors is outlined in Table 3.

The character-impact compound of green bell pepper, 2-isobutyl-3-methoxypyrazine, was the first example of a high-impact aroma compound because of its exceptionally low odor threshold of 2 parts-per-trillion (ppt) (15). It is also responsible for the ''vegetative'' aroma of Cabernet Sauvignon wine (23b). A similarly low odor threshold compound, geosmin, is the character impact

Table s Character Impact Flavor Compounds in Vegetables

Character impact compound(s)

CAS registry no.

Occurrence

Reference

Dimethyl sulfide

[75-1S-3]

Asparagus

0 0

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