2009_Book_FoodChemistry

18.1 Fruits 829
Table 18.20. Occurrence of catechin and epicatechin in
foods a
Food (+)Catechin (-)Epicatechin
Apple 4–15 67–103
Apricot 50 61
Blackberry 7 181
Bilberry n.d. 11
Sweet cherry 22 95
Black currant 7 5
Red currant 12 n.d.
Kiwi n.d. 5
Mango 17 n.d.
Peach 23 n.d.
Pear 1–2 29–37
Plum 33 28
Strawberry 44 n.d.
commonly grown fruit varieties (Table 18.21) and
also in tropical fruits; approximately 70 have been
identified up to now.
(18.22)
The cation is to be regarded as a resonance hybrid
of the following oxonium and carbenium forms:
a Values in mg/kg fresh weight.
n.d.: not detected.
give dimers and higher oligomers, which are
called proanthocyanidins. As “condensed tanning
agents”, they contribute to the astringent taste
of fruits. The spectrum of oligomers depends on
the ratio of the rate of formation to the rate of
reduction of the cation. The proanthocyanidins
are soluble up to M r ∼7000, corresponding to
ca. 20 flavanol units. Plant tissues also contain
insoluble polymeric forms which frequently even
predominate and can be covalently bound to the
polysaccharide matrix of the cell. Procyanidins
(Formula 18.21, R = H) are the most common
group of proanthocyanidins; prodelphinidins (R
= OH) also occur.
The name proanthocyanidins, previously called
leucoanthocyanidins, implies that these are colorless
precursors of anthocyanidins. On heating
in acidic solution, the C C bond made during
formation is cleaved and terminal flavan units
are released from the oligomers as carbocations,
which are then oxidized to colored anthocyanidins
(cf. 18.1.2.5.3) by atmospheric oxygen
(Formula 18.21). Base-catalyzed cleavage via the
quinone methode is also possible.
18.1.2.5.3 Anthocyanidins
These red, blue or violet colored benzopyrylium
and flavylium salts (Formula 18.22) occur in the
form of glycosides, the anthocyanins, in most
(18.23)
The sugar residues at the 3 or 5 position are easily
cleaved in an acid catalyzed reaction with the
formation of the corresponding aglycones (anthocyanidins).
Table 18.22 provides data on the structure and
absorption maxima of the most important antho-
Table 18.21. Anthocyan content of fruit a,b
Type of fruit Dp Cy Pt Pg Pn Mv
Apple
Gala – 2.3 – – – –
Red Delicious – 12 – – 0.2 –
Blackberry – 244 – 0.7 – –
Bilberry 120 29 72 – 34 131
Cherry, sweet – 113 – 1.4 7.5 –
Black currant 333 133 7.3 1.9 1 –
Red currant 0.1 13 – – – –
Red grape 1.1 3.9 1.1 – 10 10
Peach – 4.8 – – – –
Plum – 19 – – – –
Raspberry – 90 – 1.9 – –
Strawberry – 1.2 – 20 – –
a Values in mg/kg fresh weight.
b
Dp, delphinidin; Cy, cyanidin; Pt, petunidin;
Pg, pelargonidin; Pn, peonidin; Mv, malvidin.
– not detected.