162 Produce Degradation: Reaction Pathways and their Prevention
Individual aroma-significant volatile components are generally present at low parts-
per-million (micrograms per gram), parts-per-billion (nanograms per gram), and
sometimes parts-per-trillion (picograms per gram) levels, making the detection and
identification of these compounds difficult. Nonetheless, due to modern advances in
analytical chemistry, especially application of instrumental-sensory techniques such
as gas chromatography-olfactometry (GCO) combined with GC-mass spectrometry
(MS), significant progress has been made toward our understanding of the flavor
chemistry of fruits and vegetables. This section provides a concise review of the
flavor chemistry of some selected fruits and vegetables, with emphasis placed on
discussion of character-impact aroma components. More extensive reviews and
compilations on fruit and vegetable flavor systems can be found elsewhere [55–60].
6.3.1 FRUIT FLAVORS
6.3.1.1 Pome Fruit
Among the various pome fruits, apples and pears are of greatest economic impor-
tance. Berger [55] covered the published literature on apple and pear flavor up to
about 1991. The following discussion will focus on apple flavor since it has under-
gone extensive study in recent years. Pear (Pyrus spp.) flavor has been the subject
of some recent studies [61–63].
6.3.1.1.1 Apple
It is nearly impossible to accurately describe the exact chemistry of apple flavor,
because each cultivar can have a unique flavor character. Furthermore, there also is
the factor of ripeness to consider. That is, some varieties, such as Granny Smith, are
intentionally eaten in their “green” or unripe state, while many other varieties are
preferably consumed in their ripe stage. A large number of volatile compounds have
been reported as aroma-impact components of apples. β-Damascenone has been
established by analysis of over 40 cultivars to be a predominant aroma component
of apple aroma [64,65]. Aldehydes, alcohols, and esters are other predominant
components. These compounds are formed in a manner that parallels respiration
(climacteric) to its maximum [66]. Secondary aroma compounds, such as hexanal
and (E)-2-hexenal, which provide green or unripe notes, are formed by lipoxygenase
action after disruption of the cells during cutting or chewing. 2-Methylbutanoic acid
and corresponding methyl to hexyl esters, 2-methylbutanol, and 2-methylbutyl ace-
tate have been reported as aroma-impact components of apples [67–69].
6.3.1.2 Stone Fruit
Stone fruits of commercial importance belong to the genus Prunus, subfamily
Prunoideae, and include apricots, cherries, peaches, and plums. A detailed discussion
of the recent literature on peach and nectarine flavor will be presented here. Studies
on apricot [70], cherry [71], and plum [70,72] flavors have been recently published.
6.3.1.2.1 Peach and Nectarine
Extensive investigation on peach and nectarine flavor has led to the identification of
over 100 volatile constituents [73]. Lactones, in particular γ- and δ-decalactones,