18.1 Fruits 825Table 18.19.Occurrence of hydroxybenzoic acids in
fruita
Type of fruit 4-Hydroxy- Protoca- Gallic
benzoic techuic acid
acid acid
Blackberry 10–16 68–189 8–67
Black currant 0–6 10–52 30–62
Raspberry 15–27 25–37 19–38
Red currant 10–23 3–08
Strawberry 10–36 11–44
White currant 5–19 3–38
aAfter hydrolysis; values in mg/kg fresh weight.
The hydroxybenzoic acids that are found in
various fruits and occur mostly as esters in-
clude: salicylic acid (2-hydroxybenzoic acid),
4-hydroxybenzoic acid, gentisic acid (2,4-di-
hydroxybenzoic acid), protocatechuic acid
(3,4-dihydroxybenzoic acid), gallic acid (3,4,5-
trihydroxybenzoic acid), vanillic acid (3-
methoxy-4-hydroxybenzoic acid) and ellagic
acid (IX, Formula 18.13), the dilactone of
hexahydroxydiphenic acid (Table 18.19). Ta-
ble 18.19 shows the most important sources
of 4-hydroxybenzoic acid, protocatechuic acid
and gallic acid. Strawberries (0.2–0.5), rasp-
berries (1.2) and blackberries (1.9–2.0) contain
higher concentrations of free and bound ellagic
acid (g/kg).
(18.13)Apart from the proanthocyanidins (cf.
18.1.2.5.2), esters of gallic acid and hexa-
hydroxydiphenic acid form one of the two main
classes of plant tanning agents, the “hydrolyzable
tanning agents” or tannins. In addition to simple
esters with different hydroxy components,
such asβ-D-glucogallin (X in Formula 18.14),
theogallin (XI) and the flavan-3-olgallates XII
and XIII, found, e. g., in tea leaves, complex
polyesters withD-glucose are known.
They have molecular weights of Mr500–3000,
are generally readily soluble, and contribute their
astringent properties to the taste of foods of plant
origin.
Apart from gallic acid, most of the tanning agents
of this type contain as acyl residues intermolecu-
lar gallic acid esters (depsides XIV), their ethers
(depsidones, XV), and hexahydroxydiphenic
acid (XVI) formed by oxidative coupling of
two gallic acids. Some of the polyphenols de-
rived fromβ-pentagalloyl-D-glucose are shown(18.14)