Food Biochemistry and Food Processing

21 Biochemistry of Fruits 491

become leaky to calcium ions and protons that are
highly compartmentalized. The membrane proteins
are also excluded from the gel phase into the li-
quid crystalline phase. Thus, during examinations of
membrane structure by freeze fracture electron mi-
croscopy, the gel phase domains can appear as
regions devoid of proteins (Paliyath and Thompson
1990).

PROTEINS

Fruits in general are not very rich sources of pro-
teins. During the early growth phase of fruits, the
chloroplasts and mitochondria are the major organ-
elles that contain structural proteins. The structural
proteins include the light-harvesting complexes in
chloroplast or the respiratory enzyme/protein com-
plexes in mitochondria. Ribulosebisphosphate car-
boxylase/oxygenase (rubisco) is the most abundant
enzyme in photosynthetic tissues. Fruits do not store
proteins as an energy source. Green fruits such
as bell peppers and tomato have a higher level of
chloroplast proteins.

ORGANICACIDS

Organic acids are major components of fruits. The
acidity of fruits arises from the organic acids that are
stored in the vacuole, and their composition can vary
depending on the type of fruit. In general, young
fruits contain more acids, which may decline during
maturation and ripening due to their conversion to
sugars (gluconeogenesis). Some fruit families are
characterized by the presence of certain organic
acids. For example, fruits of Oxalidaceae members
(e.g., Starfruit, Averrhoa carambola) contain oxalic
acid, and fruits of the citrus family, Rutaceae, are
rich in citric acid. Apples contain malic acid, and
grapes are characterized by the presence of tartaric
acid. In general, citric and malic acids are the major
organic acids of fruits. Grapes contain tartaric acid
as the major organic acid. During ripening, these
acids can enter the citric acid cycle and undergo fur-
ther metabolic conversions.
L-()-tartaric acid is the optically active form of
tartaric acid in grape berries. A peak in acid content
is observed before the initiation of ripening, and the
acid content declines on a fresh weight basis during
ripening. Tartaric acid can be biosynthesized from
carbohydrates and other organic acids. Radiolabeled

glucose, glycolate, and ascorbate were all converted

to tartarate in grape berries. Malate can be derived

from the citric acid cycle or through carbon dioxide

fixation of pyruvate by the malic enzyme (NADPH-

dependent malate dehydrogenase). Malic acid, as

the name implies, is also the major organic acid in

apples.

FRUIT RIPENING AND

SOFTENING

Fruit ripening is the physiological repercussion of

very complex and interrelated biochemical changes

that occur in the fruits. Ripening is the ultimate

stage of the development of the fruit, which entails

the development of ideal organoleptic characters

such as taste, color, and aroma that are important

features of attraction for the vectors (animals, birds,

etc.) responsible for the dispersal of the fruit, and

thus the seeds, in the ecosystem. Human beings have

developed an agronomic system of cultivation, har-

vest, and storage of fruits with ideal food qualities.

In most cases, the ripening process is very fast, and

the fruits undergo senescence, resulting in the loss

of desirable qualities. An understanding of the bio-

chemistry and molecular biology of the fruit ripening

process has resulted in development of biotechno-

logical strategies for the preservation of postharvest

shelf life and quality of fruits.

A key initiator of the ripening process is the gas-

eous plant hormone ethylene. In general, all plant

tissues produce a low, basal level of ethylene. Based

on the pattern of ethylene production and respon-

siveness to externally added ethylene, fruits are gen-

erally categorized into climacteric and nonclimac-

teric fruits. During ripening, the climacteric fruits

show a burst in ethylene production and respiration

(CO 2 production). Nonclimacteric fruits show a con-

siderably low level of ethylene production. In cli-

macteric fruits (apple, pear, banana, tomato, avocado,

etc.), ethylene production can reach levels of 30–500

ppm (parts per million, microliters/liter), whereas in

nonclimacteric fruits (orange, lemon, strawberry,

pineapple, etc.) ethylene levels usually are in the

range of 0.1–0.5 ppm. Ethylene can stimulate its

own biosynthesis in climacteric fruits (autocatalytic

ethylene production). As well, the respiratory car-

bon dioxide evolution increases in response to ethyl-

ene treatment (the respiratory climacteric). Climac-

teric fruits respond to external ethylene treatment by