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pyrazine, was determined to make a minor contribution to the aroma of fresh popcorn, because its odor intensity was considerably lower (51). An intense (0.06 ppt threshold) roasty, popcornlike odorant was recently identified in a ribose/ cysteine model system as 5-acetyl-2,3-dihydro-1,4-thiazine (52).

The flavor formed by the cooking of fragrant rice (e.g., Basmati) is described as popcornlike, hence it is not surprising that 2-acetyl-1-pyrroline is the character impact volatile (51). In masa corn tortillas, 2-aminoacetophenone provided the character impact resulting from the lime treatment of corn (53); in corn chips, its contribution is modified by 2-acetyl-1-pyrroline and unsaturated

Figure 4 Representative thermally generated character impact flavor compounds.

aldehydes (54). Representative chemical structures for thermally generated flavor impact compounds are shown in Figure 4.

Two compounds that create the characteristic odor notes in the pleasant aroma of wheat bread crust have been identified as the popcornlike 2-acetyl-1-pyrroline and 2-acetyltetrahydropyridine (51). The aroma of the bread crumb portion is principally due to lipid-derived unsaturated aldehydes such as (E )-2-nonenal and (E,E)-2,4-decadienal, which create stale aromas at high levels. The malty notes that predominate in yeast and sourdough breads are attributed to 2-and 3-methylbutanal and Furaneol (51,55).

Potato chip aroma is associated with 2-ethyl-3,5-dimethylpyrazine and 2,3-diethyl-5-methylpyrazine, whereas 2-vinylpyrazine and (Z)-2-propenyl-3,5-dimethylpyrazine provide an intense roasted potato smell (56).

Chocolate represents a highly complex flavor system for which no single character impact has been identified. Vanillin and Furaneol contribute to the sweet, caramel background character of milk chocolate (57). 5-Methyl-2-phenyl-2-hexenal provides a ''deep bitter, cocoa'' note, and is the aldol reaction product from phenylacetaldehyde and 3-methylbutanal, two Strecker aldehydes formed in chocolate (58). 2-Methoxy-5-methylpyrazine and isoamyl phenylacetate have ''chocolate, cocoa, nutty'' and ''cocoa-like'' notes, respectively, and both are used in synthetic chocolate flavors (59). Systematic studies of key odorants in milk chocolate were performed using aroma extract dilution analysis; however, character impact compounds unique to chocolate flavor were not reported (57,60).

E. Nut Flavors

Pyrazines are the major compound classes in peanuts, formed through the thermally induced Maillard reaction (with the exception of methoxy pyrazines) (61). Two pyrazines that represent peanut flavor character are 2,5-dimethylpyrazine (nutty) and 2-methoxy-5-methylpyrazine (roasted nutty) (Table 5).

Benzaldehyde has long been known as the character impact of oil of bitter almond. It possesses an intense almond-like flavor in the context of savory applications; in sweet systems, it becomes cherrylike. 5-Methyl-2-thiophene-carboxal-dehyde also provides almond flavor character and occurs naturally in roasted peanuts (59).

The character impact compound of hazelnuts, (E )-5-methyl-2-hepten-4-one (filbertone), undergoes isomerization during the roasting process (62). Of the four possible geometric and enantiomeric isomers formed, all exhibit the typical

Table 5 Character Impact Flavor Compounds in Nuts

Character impact compound(s)

CAS registry no.

Occurrence

Reference

Benzaldehyde

[100-51-6]

Almond

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