Food Chemistry

822 18 Fruits and Fruit Products

(18.8)

Human and animal metabolism oxidatively
degrade (2R:3R)-tartaric and meso-tartaric
acids into glyoxylic and hydroxypyruvic acids
respectively:

(18.9)

(18.10)

(18.11)

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

18.1.2.5 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

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