Food Chemistry

820 18 Fruits and Fruit Products

into the bitter tasting dilactone, limonin (II;
cf. Formula 18.5).
Bitter and nonbitter forms of the manyCucur-
bitaceaeare known. The bitter forms contain cu-
curbitacins (III) in fruits and seeds. For example,
Citrullus lanatus(watermelon) contains IIIE in
glycosidic form; whileCucumis sativus(cucum-
ber) contains IIIC andCucurbitaspp. (pumpkin)
contains IIIB, D, E and I (cf. Formula 18.6).

(18.6)

The common precursor in the biosynthesis of
limonoids and cucurbitacins is squalene-2,3-
oxide (IV). Based on some identified interme-
diary compounds, the biosynthetic pathway is
probably as postulated in Reaction 18.7.

18.1.2.3.4 Fruit Waxes

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 hydrocar-
bons, 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. Ap-

ples 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 (milli-

equivalents/100 g fresh weight)

Fruit Major acid Other acids

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