488 Part V: Fruits, Vegetables, and Cereals
BIOCHEMICAL COMPOSITION
OF FRUITS
Fruits contain a large percentage of water, which can
often exceed 95% by fresh weight. During ripening,
activation of several metabolic pathways often leads
to drastic changes in the biochemical composition
of fruits. Fruits such as banana store starch during
development, and hydrolyze the starch to sugars
during ripening, which also results in fruit softening.
Most fruits are capable of photosynthesis, store
starch, and convert starches to sugars during ripen-
ing. Fruits such as apple, tomato, and grape have a
high percentage of organic acids, which decreases
during ripening. Fruits contain large amounts of fi-
brous materials such as cellulose and pectin. The
degradation of these polymers into smaller water-
soluble units during ripening leads to fruit softening,
as exemplified by the breakdown of pectin in tomato
and cellulose in avocado. Secondary plant products
are major compositional ingredients in fruits. An-
thocyanins are the major color components in
grapes, blueberries, apples, and plums; carotenoids,
specifically lycopene and carotene, are the major
components that impart color in tomatoes. Aroma is
derived from several types of compounds that in-
clude monoterpenes (as in lime, orange), ester vol-
atiles (ethyl, methyl butyrate in apple, isoamyl ace-
tate in banana), simple organic acids such as citric
and malic acids (citrus fruits, apple), and small-
chain aldehydes such as hexenal and hexanal (cu-
cumber). Fruits are also rich in vitamin C. Lipid
content is quite low in fruits, the exceptions being
avocado and olives, in which triacylglycerols (oils)
form the major storage components. The amounts of
proteins are usually low in most fruits.
CARBOHYDRATES, STORAGE, ANDSTRUCTURAL
COMPONENTS
As the name implies, carbohydrates are organic
compounds containing carbon, hydrogen, and oxy-
gen. Basically, all carbohydrates are derived by the
photosynthetic reduction of CO 2 , and the hexoses
(glucose, fructose) and pentoses (ribose, ribulose)
that are intermediates in the pathway are further
converted to several sugar monomers. Polymeriza-
tion of several sugar derivatives leads to various
storage (starch) and structural components (cellu-
lose, pectin).
During photosynthesis, the glucose formed is
converted to starch and stored as starch granules.
Glucose and its isomer fructose, along with phos-
phorylated forms (glucose-6-phosphate, glucose-1,6-
diphosphate, fructose-6-phosphate, and fructose-
1,6-diphosphate) can be considered to be the major
metabolic hexose pool components that provide a
carbon skeleton for the synthesis of carbohydrate
polymers. Starch is the major storage carbohydrate
in fruits. There are two molecular forms of starch,
amylose and amylopectin, and both components are
present in the starch grain. Starch is synthesized from
glucose phosphate by the activities of a number of
enzymes designated as ADP-glucose pyrophos-
phorylase, starch synthase, and a starch branching
enzyme. ADP-glucose pyrophosphorylase catalyzes
the reaction between glucose-1-phosphate and ATP
that generates ADP-glucose and pyrophosphate.
ADP-glucose is used by starch synthase to add glu-
cose molecules to amylose or amylopectin chains,
thus increasing their degree of polymerization. By
contrast to cellulose, which is made up of glucose
units in-1,4-glycosidic linkages, the starch mole-
cule contains glucose linked by-1,4-glycosidic
linkages. The starch branching enzyme introduces
glucose molecules through-1,6 linkages to a linear
amylose molecule. These added glucose branch
points serve as sites for further elongation by starch
synthase, thus resulting in a branched starch mole-
cule, also known as amylopectin.
The cell wall is a complex structure composed of
cellulose and pectin, derived from hexoses such as
glucose, galactose, rhamnose, and mannose, and pen-
toses such as xylose and arabinose, as well as some
of their derivatives such as glucuronic and galactur-
onic acids. A model proposed by Keegstra et al.
(1973) describes the cell wall as a polymeric struc-
ture constituted by cellulose microfibrils and hemi-
cellulose embedded in the apoplastic matrix in asso-
ciation with pectic components and proteins. In
combination, these components provide the struc-
tural rigidity that is characteristic of the plant cell.
Most of the pectin is localized in the middle lamella.
Cellulose is biosynthesized by the action of -1,4-
glucan synthase enzyme complexes that are local-
ized on the plasma membrane. The enzyme uses
uridine diphosphate glucose (UDP-glucose) as a
substrate, and by adding UDP-glucose units to small
cellulose units, it extends the length and polymeriza-
tion of the cellulose chain. In addition to cellulose,