Flavor and Volatile Metabolism in Produce 159
6.2.3.2 Lactones
The γ- and δ-lactones impart intense fruity, peachy, or coconut-like aromas and are
important aroma constituents of several fruits. Apricot aroma can be largely attrib-
uted to γ-octa- and γ-decalactones [12,13]. β-Oxidation, reduction, chain elongation,
and hydroxylation are involved in the biosynthesis of lactones [14]. The biosynthesis
of lactones is sometimes chiroselective, leading to formation of predominantly the
R enantiomer in some fruit, such as apricots [15], while in other fruit such as
pineapples both the R and S enantiomers may be present [14], indicating that various
biosynthetic pathways may be involved in the formation of lactones.
6.2.3.3 Terpenoids
Terpenes are ubiquitous components of fruits and vegetables, although they may not
necessarily contribute to aroma. For example, in some fruits such as strawberries
and tomatoes terpenes make essentially no aroma impact. On the other hand, terpenes
make a substantial contribution to citrus, carrot, and blueberry aromas.
The arrangement of the groups of five carbon atoms found within terpenes is
related to the structure of the molecule isoprene [16]. The key step in the biosynthesis
of most monoterpenes is the cyclization of an allylic pyrophosphate catalyzed by
enzymes collectively known as cyclases [17,18]. Most naturally occurring mono-
terpenes are oxygenated (e.g., camphor, menthol, citral, and carvone). Oxygenated
monoterpenes are of particular importance because they impart characteristic aroma
notes to essential oils, such as citrus. Oxygenated monoterpenes may be derived
from the monoterpene hydrocarbons via oxygenation involving multifunction oxi-
dases and molecular oxygen [19].
6.2.3.4 Norisoprenoids
Norisoprenoids comprise of a large group of C 13 -carotenoid-derived volatile com-
pounds that make a substantial contribution to the aromas of many fruits and
vegetables [20,21]. For the most part, the biochemical mechanisms involved in
carotenoid catabolism are still not well understood. The formation of norisoprenoids
is believed to initiate via enzymatic cleavage (dioxygenase system) of a C 40 -caro-
tenoid (e.g., β-carotene) chain at the 9,10 double bond to form C 13 fragments, which
then undergo enzymatic and acid-catalyzed conversions to yield both bound forms
(glycosides) of C 13 -norisoprenoids and volatile aglycones, such as the well-known
β-damascenone and β-ionone [20]. β-Damascenone has an extremely low threshold
of 2 pg/g in water [22]. Therefore, this compound probably makes a significant
contribution to the aroma of any fruit or vegetable in which it has been found. Many
C 13 -norisoprenoids, including β-damascenone and β-ionone, dominate the volatile
profile of some fruit, such as starfruit [23].
Although norisoprenoids represent the major carotene-derived aroma com-
pounds, cleavage of the carotenoid occurs not only at the 9,10 position; there are a
number of other products formed with a different number of carbon atoms, such as
C 10 -norterpenoids, that arise from cleavage of the 7,8 double bond [21,21,24].