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 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 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.