2009_Book_FoodChemistry

820 18 Fruits and Fruit Products
into the bitter tasting dilactone, limonin (II;
cf. Formula 18.5).
Bitter and nonbitter forms of the many Cucurbitaceae
are known. The bitter forms contain cucurbitacins
(III) in fruits and seeds. For example,
Citrullus lanatus (watermelon) contains IIIE in
glycosidic form; while Cucumis sativus (cucumber)
contains IIIC and Cucurbita spp. (pumpkin)
contains IIIB, D, E and I (cf. Formula 18.6).
The fruit peel is often coated with a waxy layer.
In addition to the esters of higher fatty acids with
higher alcohols, these waxes contain hydrocarbons,
free fatty acids, free alcohols, ketones and
aldehydes. The ester fraction in apples and grapes
predominantly consists of alcohols of 24, 26 and
28 carbons, but their fatty acid patterns differ. Apples
contain mostly 18:1, 18:2, 16:0 and 18:0 fatty
acids, while grapes contain 20:0, 18:0, 22:0 and
24:0 fatty acids.
18.1.2.4 Organic Acids
L-Malic and citric acids are the major organic
acids of fruits (Table 18.13). Malic acid is
predominant in pomme and stone fruits, while
citric acid is most abundant in berries, citrus
and tropical fruits. (2R:3R)-Tartaric acid occurs
only in grapes. Many other acids, including the
acids in the citric acid cycle, occur only in low
amounts. Examples are cis-aconitic, succinic,
pyruvic, citramalic, fumaric, glyceric, glycolic,
glyoxylic, isocitric, lactic, oxalacetic, oxalic
and 2-oxoglutaric acids. In fruit juices, the
ratio of citric acid to isocitric acid (examples in
Table 18.14) serves as an indicator of dilution
with an aqueous solution of citric acid.
Table 18.13. Organic acids in various fruits (milliequivalents/100
g fresh weight)
Fruit Major acid Other acids
(18.6)
The common precursor in the biosynthesis of
limonoids and cucurbitacins is squalene-2,3-
oxide (IV). Based on some identified intermediary
compounds, the biosynthetic pathway is
probably as postulated in Reaction 18.7.
18.1.2.3.4 Fruit Waxes
Apple Malic 3–19 Quinic (in unripe
fruits)
Pear Malic 1–2 Citric
Apricot Malic 12 Citric 12,
quinic 2–3
Cherry Malic 5–9 Citric, quinic,
and shikimic
Peach Malic 4 Citric 4
Plum/prune Malic 4–6 Quinic (especially
in unripe fruits)
Strawberry Citric 10–18 Malic 1–3, quinic
0.1, succinic 0.1
Raspberry Citric 24 Malic 1
Currant, red Citric 21–28 Malic 2–4,
succinic, oxalic
Currant, black Citric 43 Malic 6
Gooseberry Citric 11–14 Malic 10–13,
shikimic 1–2
Grapes Tartaric 1.5–2 Malic 1.5–2
Orange Citric 15 Malic 3, quinic
Lemon Citric 73 Malic 4, quinic
Pineapple Citric 6–20 Malic 1.5–7
Banana Malic 4
Fig Citric 6 Malic, acetic
Guava Citric 10–20 Malic