Flavor and Volatile Metabolism in Produce 157
product. Therefore, when we speak of aroma substances, we generally refer to only
those volatile compounds that have perceivable odors at relatively low concentra-
tions. Sometimes a food contains only one or just a few volatile compounds, so
called character-impact components, which are responsible for its characteristic
aroma; however, in many cases a large number of compounds make a seemingly
equal contribution to the aroma.
In the following discussions, the flavor chemistry of fruits and vegetables will
be presented, along with current knowledge in the area of metabolism and posthar-
vest effects on aroma composition.
6.2 FLAVOR COMPONENTS OF FRUITS AND
VEGETABLES6.2.1 NONVOLATILE TASTE COMPOUNDS
Sweetness, sourness, saltiness, bitterness, and astringency are some key flavor
attributes of fruits and vegetables [1]. Sweetness in fruits and vegetables is usually
due to the presence of various sugars, such as glucose, fructose, and sucrose. Total
sugars are usually expressed as percentage of soluble solids, which may be directly
related to sweetness (e.g., citrus). Sourness (or acidity) is usually measured as
percentage of titratable acidity, which is most often expressed in terms of the major
organic acid(s) found in a particular fruit or vegetable (e.g., citric acid in orange).
Compounds responsible for bitter and astringent flavors are often more difficult to
determine since many compound classes may be involved (e.g., phenolic compounds,
tannins, flavonoids, and alkaloids). During ripening, the sweetness of some fruits
may increase due to starch to sugar conversions (e.g., banana and apple), while at
the same time acidity, bitterness, and/or astringency generally decreases [2]. In fact,
percentage of soluble solids (°Brix) and °Brix-to-acid ratio are often used as indices
of ripeness and flavor quality.
6.2.2 VOLATILE AROMA COMPOUNDS
Numerous compound classes occur naturally (endogenously) in the volatile profiles
of fruits and vegetables and, depending on species or variety, one or more classes
may predominate. Among the thousands of volatile compounds identified to date
only a relative few actually contribute to the distinctive aromas of fruits and vege-
tables. The great diversity of chemical structures indicates the involvement of numer-
ous chemical reactions in their formation. In fruits and vegetables there are numerous
common reactions that produce similar or overlapping aroma profiles. The following
section is devoted to the discussion of the biogenesis of important classes of volatile
constituents of produce.
6.2.3 AROMA BIOGENESIS
The biosynthesis of volatile compounds in plants is a dynamic process. The activities
of the various enzymes and pathways change at different stages of maturity and