18.1 Fruits 835Table 18.27.Lignans in foodsa,b
Food Pino (I) Lari (II) Seco (III) Mat (IV) Sum
Linseed 33 30 2942 5 3011
Sesame 293 94 0. 7 5 393
Bread, wholemeal wheat 0. 30. 70 .2n.d. 1. 2
Bread, Rye, dark 1. 71. 20. 10. 13. 2
Broccoli 3. 29. 70 .4n.d. 13. 3
Garlic 2. 02. 90 .5n.d. 5. 4
Apricot 3. 11. 10 .3n.d. 4. 5
Strawberry 2. 11. 20. 05 n.d. 3. 3
Peach 1. 90. 80 .3n.d. 2. 9
aValues in mg/kg fresh weight.
bStructures see Formula 18.33, Name: pinoresinol (I), lariciresinol (II), secoisolariciresinol (III),
matairesinol (IV)
n.d.: not detected
18.1.2.5.6 Lignans
Lignans are polyphenols which belong to the
group of phytoestrogens (cf. 16.2.9). Four
compounds (I–IV in Formula 18.33) will be
presented here. Pinoresinol (I) is a dimer of
coniferyl alcohol. Lignans widely occur in low
concentrations in food (examples in Table 18.29),
linseed and sesame being especially rich.
(18.33)18.1.2.5.7 Flavonoid Biosynthesis
Flavonoid biosynthesis (cf. Formula 18.34) oc-
curs through the stepwise condensation of acti-
vated hydroxycinnamic acid (I) with three acti-
vated malonic acid molecules (II). The primary
condensation product, a chalcone (III), is in equi-
librium with a flavanone (IV) with the equilib-
rium shifted toward product IV. The condensation
directly yields a flavanone, hence chalcone is not
an obligatory intermediary product.
A 2,7-cyclization yields stilbenes (IIIa).
One pathway converts flavanones (IV) to flav-
ones (V) and, through another pathway, flavan-
ones are converted to flavanonols (VI). The latter
compounds are converted to flavandiols (VII),
flavanols (VIII) and flavonols (IX), as well as an-
thocyanidins (XII) via endiols (X) and enols (XI).18.1.2.5.8 Technological Importance
of Phenolic CompoundsThe taste of fruits is influenced by phenolic com-
pounds. The presence of tannins yields an astrin-
gent, harsh taste, similar to an unripe apple (or
an apple variety suitable only for processing). Ta-
ble quality apples are low in phenolic compounds.
Flavanones (naringin, neohesperidin) are the bit-
ter compounds of citrus fruits.
Phenolic compounds are substrates for polyphe-
nol oxidases. These enzymes hydroxylate
monophenols to o-diphenols and also oxidize
o-diphenols to o-quinones (cf. 2.3.3.2).
o-Quinones can enter into a number of other re-
actions, thus giving the undesired brown discol-
oration of fruits and fruit products. Protective
measures against discoloration include inactiva-
tion of enzymes by heat treatment, use of reduc-
tive agents such as SO 2 or ascorbic acid, or re-
moval of available oxygen.
Polyvalent phenols form colored complexes with
metal ions. For example, at pH>4, Fe^3 +forms