Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

BIOCHEMISTRY OF FRUITS 31
content is generally very high in ripe mangoes and can reach levels in excess of 90% of
the total soluble solids content. Fructose is the predominant sugar in mangoes. In contrast
to the bananas, the sucrose levels increase with the advancement of ripening in mangoes,
potentially due to gluconeogenesis from organic acids (Kumar and Selvaraj, 1990). As well,
the levels of pentose sugars increase during ripening, and could be related to an increase in
the activity of the pentose phosphate pathway.
3.3.1.3 Glycolysis
The conversion of starch to sugars and their subsequent metabolism occur in different compartments.
During the development of fruits, photosynthetically fixed carbon is utilized for
both respiration and biosynthesis. During this phase, the biosynthetic processes dominate.
As the fruit matures and begin to ripen, the pattern of sugar utilization changes. Ripening
is a highly energy-intensive process. And this is reflected in the burst in respiratory carbon
dioxide evolution during ripening. As mentioned earlier, the respiratory burst is characteristic
of some fruits that are designated as climacteric fruits. The postharvest shelf life of
fruits can depend on their intensity of respiration. Fruits such as mango and banana possess
high level of respiratory activity and are highly perishable. The application of controlled
atmosphere conditions having low oxygen levels and low temperature have thus become a
routine technology for the long-term preservation of fruits.
The sugars and sugar phosphates generated during the catabolism of starch are metabolized
through the glycolysis and citric acid cycle (Fig. 3.4). Sugar phosphates can also
be channeled through the pentose phosphate pathway, which is a major metabolic cycle
that provides reducing power for biosynthetic reactions in the form of NADPH, as well as
supplying carbon skeletons for the biosynthesis of several secondary plant products. The
organic acids stored in the vacuole are metabolized through the functional reversal of respiratory
pathway, which is termed as gluconeogenesis. Altogether, sugar metabolism is a
key biochemical characteristic of the fruits.
In the glycolytic steps of reactions (Fig. 3.4), glucose-6-phosphate is isomerized to
fructose-6-phosphate by the enzyme hexose phosphate isomerase. Glucose-6-phosphate is
derived from glucose-1-phosphate by the action of glucose phosphate mutase. Fructose-
6-phosphate is phosphorylated at the C1 position yielding fructose-1,6-bisphosphate. This
reaction is catalyzed by the enzyme phosphofructokinase in the presence of ATP. Fructose-
1,6-bisphosphate is further cleaved into two three-carbon intermediates, dihydroxyacetone
phosphate and glyceraldehyde-3-phosphate, catalyzed by the enzyme aldolase. These
two compounds are interconvertible through an isomerization reaction mediated by triose
phosphate isomerase. Glyceraldehyde-3-phosphate is subsequently phosphorylated at the
C1 position using orthophosphate, as well as oxidized using nicotinamide adenine dinucleotide
(NAD), to generate 1,3-diphosphoglycerate and nicotinamide adenine dinucleotide
plus hydrogen (NADH). In the next reaction, 1,3-diphosphoglycerate is dephosphorylated
by glycerate-3-phosphate kinase in the presence of ADP, along with the formation
of ATP. Glycerate-3-phosphate formed during this reaction is further isomerized to
2-phosphoglycerate in the presence of phosphoglycerate mutase. In the presence of the
enzyme enolase, 2-phosphoglycerate is converted to phosphoenol pyruvate (PEP). Dephosphorylation
of phosphoenolpyruvate in the presence of ADP by pyruvate kinase yields
pyruvate and ATP. Metabolic fate of pyruvate is highly regulated. Under normal conditions,
it is converted to acetyl coA, which then enters the citric acid cycle. Under anaerobic