2009_Book_FoodChemistry

18.1 Fruits 835
Table 18.27. Lignans in foods a,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 0.3 0.7 0.2 n.d. 1.2
Bread, Rye, dark 1.7 1.2 0.1 0.1 3.2
Broccoli 3.2 9.7 0.4 n.d. 13.3
Garlic 2.0 2.9 0.5 n.d. 5.4
Apricot 3.1 1.1 0.3 n.d. 4.5
Strawberry 2.1 1.2 0.05 n.d. 3.3
Peach 1.9 0.8 0.3 n.d. 2.9
a Values in mg/kg fresh weight.
b Structures 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.
Flavonoid biosynthesis (cf. Formula 18.34) occurs
through the stepwise condensation of activated
hydroxycinnamic acid (I) with three activated
malonic acid molecules (II). The primary
condensation product, a chalcone (III), is in equilibrium
with a flavanone (IV) with the equilibrium
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 flavones
(V) and, through another pathway, flavanones
are converted to flavanonols (VI). The latter
compounds are converted to flavandiols (VII),
flavanols (VIII) and flavonols (IX), as well as anthocyanidins
(XII) via endiols (X) and enols (XI).
18.1.2.5.8 Technological Importance
of Phenolic Compounds
18.1.2.5.7 Flavonoid Biosynthesis
(18.33)
The taste of fruits is influenced by phenolic compounds.
The presence of tannins yields an astringent,
harsh taste, similar to an unripe apple (or
an apple variety suitable only for processing). Table
quality apples are low in phenolic compounds.
Flavanones (naringin, neohesperidin) are the bitter
compounds of citrus fruits.
Phenolic compounds are substrates for polyphenol
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 reactions,
thus giving the undesired brown discoloration
of fruits and fruit products. Protective
measures against discoloration include inactivation
of enzymes by heat treatment, use of reductive
agents such as SO 2 or ascorbic acid, or removal
of available oxygen.
Polyvalent phenols form colored complexes with
metal ions. For example, at pH > 4, Fe 3+ forms