Aroma, as is true for many quality attributes of fresh and processed
fruit and vegetable products, is affected by the culture of the plant
material prior to consumption. Culture, in this case, refers to the
cultivar selection and production and postharvest processes that affect
the physiology of the plant. In this workshop, we will focus on the
effects of cultural tools on the biology of aroma generation by
horticultural commodities. Importantly, the aroma of some fresh
horticultural products [e.g., strawberry (Fragaria ×ananassa Duch.),
tomato (Lycopersicon esculentum Mill.), and apple (Malus×domestica
Borkh.)] has received an increasing amount of attention from both
producers and consumers because of perceived deficiencies in the
sensory quality of commercially produced cultivars, so perhaps the
time is ripe for strengthening efforts to improve our understanding of
this important attribute.
We have loosely grouped commodities into three distinct classes,
based on the manner in which the product is typically consumed and
the means by which the bulk of the compounds with aromatic character
impact are generated. One class of plant products is usually consumed
after some form of thermal treatment that affects the synthesis of
aromatic compounds. Within this group one might place the edible
portions of, for example, sweetpotato [Ipomoea batatas (L.) Lam.],
potato (Solanum tuberosum L.), spinach (Spinacea oleracea L.), and
green beans (Phaseolus vulgaris L.). A second class is consumed
fresh, but some form of cellular disruption, and the mixing of cellular
constituents normally held apart by cellular compartmentation, are
responsible for the formation of characteristic aromas. Among this
group, one could include, for example, edible portions of tomato,
blueberry (Vaccinium sp.), spinach, and radish (Raphanus sativus L.).
In the third class, the production of characteristic aroma compounds is
spontaneous, relying primarily on the developmental stage of the
organ. Important in this class are fruits such as apples, bananas (Musa
sp.), strawberries, and peaches [Prunus persica (L.) Batsch.].
These three classifications are somewhat artificial in that they
integrate the habits of the consumer (some plant materials are consumed
either fresh or cooked) and the biology of the plant. Further, the
volatile profile will probably comprise compounds that are synthesized
via more than one of the three routes described. Cellular
disruption occurs, for instance, in all plant products as they are
processed or consumed, and aromas attending this disruption will
necessarily contribute to the mixture of compounds with olfactory
activity. Nevertheless, there are advantages to the classifications, as
they permit a focused presentation and discussion of biological systems
that affect final aroma and, by extension, the manner in which
plant processes can be altered by cultural techniques to affect the
product.
The first of the four papers in this workshop (Kays and Wang,
2000) approaches the subject of thermally induced aromas. As part of
the discussion, the authors describe the process of flavor perception by
humans, segregating and then integrating the influences of taste and
aroma on flavor. Flavor perception is the process linking plant biochemistry
with the physiology and psychology of the consumer and is
a particularly interesting and difficult area of inquiry. The authors
proceed to develop the concept of thermal induction of aromas, using
the sweetpotato as a primary example, and conclude with a discussion
of selection through plant breeding and preharvest factors as they
1002 HORTSCIENCE, VOL. 35(6), OCTOBER 2000
WORKSHOP
affect aroma. Among the interesting features of this work is the activity
of enzymes critical to flavor development during the heating process.
The second paper in the workshop (Baldwin et al., 2000) focuses
on the cell disruption-dependent synthesis of aroma in tomato. These
authors also treat the subject of aroma perception, with additional
perspective on the complexity and sensitivity of aroma detection by
humans and other herbivores. Among the concepts discussed is the
importance of the odor threshold of aromas as it pertains to perception,
as well as its use as a descriptive tool. Considerable attention is given
to the biochemistry of aroma generation following tissue disruption,
with a discussion of possible target enzymes for improving aroma.
The third and fourth papers describe the process of spontaneous
aroma generation for strawberry (Forney et al., 2000) and apple
(Fellman et al., 2000) fruit. The biosynthesis of esters figures prominently
in both crops. The crops differ, however, in that apple can be
said to have a specific character-impact compound such as hexyl
acetate or ethyl 2-methylbutyrate, whereas no such compound exists
for strawberry, the aroma of which is only apparent when a mixture of
esters and other volatiles are present. In both papers, the importance of
cultivar and postharvest handling procedures is discussed at some
length.
The task of preserving and improving the aroma of horticultural
products will probably be difficult. For instance, some treatments
currently used to prevent decay, or to preserve texture and color, can
compromise aroma quality. Elevated CO2, for example, is commonly
used to reduce decay in strawberry and other berry crops, but can
induce fermentation (Beaudry, 1993; Ke et al., 1991). Similarly,
reduced O2 levels, while extending the storage-life of apple and other
pome fruit, can reduce aroma and, at stress levels, cause off-flavors
(Brackmann et al., 1993). Perhaps more to the point, the task of
improving our understanding through scientific inquiry appears daunting.
The metabolic pathways responsible for the synthesis of aroma
compounds are diverse and often highly integrated with other portions
of both primary and secondary metabolism. Even so, we are poised on
the brink of new breakthroughs. For instance, the protein required for
the biosynthesis of esters has been characterized (Pérez et al., 1993;
Ueda and Ogata, 1977) and the gene controlling its synthesis recently
identified (Aharoni et al., 2000). As we develop a better understanding
of the genetic and biochemical factors that alter or control synthesis of
aroma, we should be able to develop better cultural tools to maintain
and/or improve flavor. We hope that the proceedings from this
workshop will inspire plant scientists with these possibilities and
instigate new, much-needed research.