2009_Book_FoodChemistry

822 18 Fruits and Fruit Products
Table 18.14. Ratio of citric acid (C) to isocitric acid (I)
in fruit juices
Fruit juice
C/I
Orange 80–130
Currant 60–140
Grapefruit 50–90
Raspberry 80–200
Lemon
≤250
Blackberry ca. 0.25
(18.8)
Human and animal metabolism oxidatively
degrade (2R:3R)-tartaric and meso-tartaric
acids into glyoxylic and hydroxypyruvic acids
respectively:
18.1.2.5 Phenolic Compounds
In plant foods, several hundred polyphenols have
been identified, which are classified according
to the number of phenol rings and their linkage,
as presented in Fig. 18.3. The flavonoids exhibit
a great multiplicity. According to Fig. 18.4, they
are divided into six subclasses. The polyphenols
are mostly present as glycosides, and partly also
as esters. They are antioxidatively active, their
activity depending on the number and position
of the OH-groups and on the pH (cf. 3.7.3.2.1).
As antioxidants, they are of interest from
(18.9)
(18.10)
(18.11)
Fig. 18.3. Chemical structures of polyphenols
Hydroxybenzoic acids (Hba), Hydroxycinammic
acids (Hca), Flavonoids (F), Chalcones (C), Stilbenes
(S, cf. 20.2.6.6), Lignans (L). R: H, OH or OCH 3