Food Biochemistry and Food Processing

(Ben Green) #1

500 Part V: Fruits, Vegetables, and Cereals


acid cycle, through which it is completely oxidized
to carbon dioxide (Fig. 21.3). The conversion of
pyruvate to acetyl-CoA is mediated by the enzyme
complex pyruvate dehydrogenase, in an oxidative
step that involves the formation of NADH from
NAD. Acetyl-CoA is a key metabolite and starting
point for several biosynthetic reactions (fatty acids,
isoprenoids, phenylpropanoids, etc.).


Citric Acid Cycle


The citric acid cycle involves the biosynthesis of
several organic acids, many of which serve as pre-
cursors for the biosynthesis of several groups of
amino acids. In the first reaction, oxaloacetate com-
bines with acetyl-CoA, mediated by citrate synthase,
to form citrate (Fig. 21.4). In the next step, citrate is
converted to isocitrate by the action of aconitase.
The next two steps in the cycle involve oxidative
decarboxylation. The conversion of isocitrate to-
ketoglutarate involves the removal of a carbon diox-
ide molecule and reduction of NAD to NADH. This
step is catalyzed by isocitrate dehydrogenase.-
ketoglutarate is converted to succinyl-CoA by-
ketoglutarate dehydrogenase, with the removal of
another molecule of carbon dioxide and the conver-
sion of NAD to NADH. Succinate, the next product,
is formed from succinyl-CoA by the action of
succinyl-CoA synthetase, which involves the re-
moval of the CoA moiety and the conversion of ADP
to ATP. Through these steps, the complete oxidation
of the acetyl-CoA moiety has been achieved, with
the removal of two molecules of carbon dioxide.
Thus, succinate is a four-carbon organic acid. Suc-
cinate is further converted to fumarate and malate in
the presence of succinate dehydrogenase and fum-
arase, respectively. Malate is oxidized to oxaloac-
etate by the enzyme malate dehydrogenase, with the
conversion of NAD to NADH. Oxaloacetate, then
can combine with another molecule of acetyl-CoA
to repeat the cycle. The reducing power generated in
the form of NADH and FADH (flavin adenine dinu-
cleotide, reduced form; succinate dehydrogenation
step) is used for the biosynthesis of ATP through the
transport of electrons through the electron transport
chain in the mitochondria.


Gluconeogenesis


Several fruits store large amounts of organic acids in
their vacuoles, and these acids are converted back to


sugars during ripening, a process called gluconeoge-
nesis. Several irreversible steps in the glycolysis and
citric acid cycle are bypassed during gluconeogene-
sis. Malate and citrate are the major organic acids
present in fruits. In fruits such as grapes, where
there is a transition from a sour to a sweet stage dur-
ing ripening, organic acids content declines. Grape
contains predominantly tartaric acid along with mal-
ate, citrate, succinate, fumarate, and several organic
acid intermediates of metabolism. The content of
organic acids in berries can affect their suitability
for processing. High acid content coupled with low
sugar content can result in poor quality wines. Ext-
ernal warm growth conditions enhance the metabo-
lism of malic acid in grapes during ripening and
could result in a high tartarate/malate ratio, which is
considered ideal for vinification.
The metabolism of malate during ripening is
mediated by the malic enzyme, NADP-dependent
malate dehydrogenase. With the decline in malate
content, there is a concomitant increase in sugars,
suggesting a possible metabolic precursor product
relationship between these two events. Indeed, when
grape berries were fed with radiolabeled malate, the
radiolabel could be recovered in glucose. The me-
tabolism of malate involves its conversion to oxalo-
acetate mediated by malate dehydrogenase, the
decarboxylation of oxaloacetate to phosphoenol
pyruvate catalyzed by PEP-carboxykinase, and a
reversal of the glycolytic pathway, leading to sugar
formation (Ruffner et al. 1983). The gluconeogenic
pathway from malate may contribute only a small
percentage (5%) of the sugars, and decrease in ma-
late content could primarily result from reduced
synthesis and increased catabolism through the cit-
ric acid cycle. The inhibition of malate synthesis by
the inhibition of the glycolytic pathway could result
in increased sugar accumulation. Metabolism of
malate in apple fruits is catalyzed by the NADP-
malic enzyme, which converts malate to pyruvate. In
apples, malate appears to be primarily oxidized
through the citric acid cycle. In citrus fruits, organic
acids are important components; citric acid is the
major form of acid, followed by malic acid and sev-
eral less abundant acids such as acetate, pyruvate,
oxalate, glutarate, fumarate, and others. In oranges,
the acidity increases during maturation of the fruit
and declines during the ripening phase. Lemon fruits,
by contrast, increase their acid content through the
accumulation of citrate. The citrate levels in various
citrus fruits range from 75 to 88%, and malate levels
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