Biochemistry of Fruits - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 25

exposure to cold temperature could be counteracted by increasing the proportion of fatty
acyl chains having a higher degree of unsaturation and therefore a lower melting point.
Thus, the membrane will tend to remain fluid even at a lower temperature. An increase in
gel-phase lipid domains can result in the loss of compartmentalization. The differences in
the mobility properties of phospholipid acyl chains can cause packing imperfections at the
interface between gel and liquid crystalline phases, and these regions can become leaky to
calcium ions and protons that are highly compartmentalized. The membrane proteins are
also excluded from the gel phase into the liquid crystalline phase. Thus, during examinations
of membrane structure by freeze fracture electron microscopy, the gel-phase domains can
appear as regions devoid of proteins (Paliyath and Thompson, 1990).

3.2.3 Proteins

Fruits, in general, are not very rich sources of proteins. During the early growth phase of
fruits, the chloroplasts and mitochondria are the major organelles that contain structural pro-
teins. The structural proteins include the light-harvesting complexes in chloroplast or the res-
piratory enzyme/protein complexes in mitochondria. Ribulose-bis-phosphate carboxylase/
oxygenase (Rubisco) is the most abundant enzyme in photosynthetic tissues. Fruits do not
store proteins as an energy source. The green fruits such as bell peppers and tomato have a
higher level of chloroplast proteins.

3.2.4 Organic acids

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 that 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 (ex. 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 character-
ized 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 further 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 (nicotinamide adenine dinucleotide
phosphate (NADPH)-dependent malate dehydrogenase). Malic acid, as the name implies,
is also the major organic acid in apples.

3.3 Fruit ripening and softening

Fruit ripening is the physiological repercussion of a very complex and interrelated bio-
chemical changes that occur in the fruits. Ripening is the ultimate stage of the development