2009_Book_FoodChemistry

18.1 Fruits 845
CO 2 production decreases. This provides an
explanation for the change in RQ during the
climacteric stage. CO 2 production increases
more rapidly than O 2 uptake, thus the RQ is
greater than 1. The shift from the citric acid cycle
to malate degradation in apples is also reflected
by the effect of citrate and malate on succinate
production. As ripening proceeds, production
of succinate from citrate drops to zero. An
increase in succinate content after addition of
malate in the initial stage of ripening is probably
a feedback reaction. In this case, a decrease
is also observed later on, suggesting a greater
change in metabolic patterns.
18.1.3.3 Changes in Individual Constituents
18.1.3.3.1 Carbohydrates
During ripening of fruits, significant changes occur
in the carbohydrate fraction. For example, between
picking and onset of decay in apples about
20% of the available carbohydrates have been utilized.
During the growth of apples on trees, the starch
content rises and then drops to a negligible level
by the time of harvest. This drop appears to be
related to the increase in climacteric respiration.
Contrary to starch, the sugar content rises. Other
sources in addition to starch should be available
for conversion to sugars. A decrease in hemicelluloses
suggests that they are a possible source.
Organic acids may also be an additional source of
sugars.
A marked decrease in starch in bananas parallels
an increase in the contents of glucose, fructose
and saccharose. Biosynthesis of the latter occurs
by two pathways:
1) UDPG + Fru-6-p → UDP + Sac-6 F -P
→ Sac + P in
2) UDPG + Fru → UDP + Sac (18.42)
The content of hemicelluloses drops from 9% to
1–2% (relative to fresh weight), hence they act as
a storage pool in carbohydrate metabolism. There
is also a drop in the sugar content in bananas during
the post-climacteric stage.
Differences in various fruits can be remarkable.
In oranges and grapefruits the acid content drops
during ripening while the sugar level rises. In
lemons, however, there is an increase in acids.
Decreases in arabinans, cellulose and other
polysaccharides are found in pears during
ripening. Cellulase enzyme activity has been
confirmed in tomatoes.
Remarkable changes occur in the pectin fractions
during ripening of many fruits (e. g., bananas,
citrus fruits, strawberries, mangoes, cantaloupes
and melons). The molecular weight of pectins
decreases and there is a decrease in the degree
of methylation. Insoluble protopectin is increasingly
transformed into soluble forms. Protopectin
is tightly associated with cellulose in the cell
wall matrix. Its galacturonic acid residues are
acetylated at OH-groups in positions 2 and 3 or
are bound to polysaccharides as lignin (R 1 =H,
CH 3 , polysaccharide: arabinan, galactan and possibly
cellulose; R 2 =H,CH 3 CO, polysaccharide,
lignin):
(18.43)
Soluble pectins bind polyphenols, quench their
astringent effect and, thus, contribute to the mild
taste of ripe fruits.
After prolonged storage there is a decrease in soluble
pectins in apples. This drop is associated
with a mealy, soft texture. Similar events occur
in pears, but much more rapidly and with more
extensive demethylation of pectin. Generally, the
degree of pectin esterification drops from 85%
to about 40% during ripening of pears, peaches
and avocados. This drop is due to a remarkable
increase in activities of polygalacturonases and
pectin esterases. The rise in free galacturonic acid
is negligible; therefore it appears that the release
of uronic acid is associated with its simultaneous
conversion through other reactions.