Hydroxybenzoic Acids OCCURRENCE IN FRUITS AND VEGETABLES

A. Hydroxybenzoic Acids

HBAs have a general structure of the C6-C3 type derived directly from benzoic acid (Fig. 1), and variations in structure lie in the hydroxylations and methoxylations of the aromatic cycle. They are mainly present in fruits and vegetables in the form of O-glucosides, but glucose esters of p-hydroxybenzoic, vanillic, or syringic acids have also been reported, e.g., in garden cress (Table 1). In most of the important species of fruits and vegetables, because HBA conjugates are only found in trace concentrations, their identification is difficult. The presence of free HBA likely corresponds to degradation products from conjugates forms, during either extraction or subsequent hydrolysis. For example, salicylic, p-hydroxybenzoic, vanillic, gentisic, 3,4-dihydroxybenzoic, syringic, p-coumaric, and gallic acids were identified in the fruit of Diospyros lotus, whereas no information was reported about native forms [16].

Figure 1 Chemical structure of hydroxybenzoic acids and some derivatives identified in fruits and vegetables.

Table 1 Contents of Hydroxybenzoic Acids in Vegetables and Ripe Fruitsa

Three HBAs ( p-hydroxybenzoic, vanillic, and protocatechuic) are apparently universal in the angiosperms, and others (e.g., syringic, gallic, salicylic) are also frequently present in either complex structures, i.e., hydrolysable tannins, or as simple derivatives in combination with sugars or organic acids.

Gallic acid, hexahydroxydiphenic acid, and pentagalloylglucose (Fig. 1) are also constituents of hydrolyzable tannins. In addition, very low concentrations of gallic acid are found in fruits in the form of esters with quinic acid (theogallin) or glucose (glucogallin) and in the form of glucosides. Glucogallin has also been identified in persimmon and isolated only from astringent immature fruit, whereas free gallic acid was found in immature fruit of both astringent and nonastringent varieties [2]. Glucogallin was thus proposed as a good index for distinguishing between astringent and nonastringent varieties. Gallic acid is also found combined with naringenin in fruits of Acacia farnesiana or with (_)_epicatechin to form epicatechin 3-O-gallate, a constituent of unripe grapes [2] (Fig. 1). Ellagic acid, a dimer of gallic acid, is a component of ellagitannins, but it has also been reported in the free form and as arabinoside, acetylxyloside, or acetylarabinoside in raspberry and strawberry [17–19].

p-Hydroxybenzoic and vanillic acids are also present in numerous fruits and vegetables [1], and the native forms are frequently simple combinations with glucose (Table 1). Other derivatives have been detected in certain fruits [1,2]: the methyl ester of p-hydroxybenzoic acid in passion fruit, 3,4-dihydroxybenzoic aldehyde in banana, a phenylpropene benzoic acid derivative in fruits of Jamaican Piper species, and benzoyl esters and other derivatives in the fruits of Aniba riparia. Different new glycosides of HBA showing radical-scavenging activity [e.g., a new guaiacylglycerol-vanillic acid ether (Fig. 1)] have been identified in the fruits of Boreava orientalis [20].

Syringic acid or its glucoside has been reported in grape, plum, and some vegetables (Table 1), but its distribution appears to be very limited. It is very likely that p-hydroxybenzoic, vanillic, and syringic acids derive, at least partially, from the degradation of certain lignified zones of the fruit when these exist (stone, seed teguments, etc.).

Protocatechuic acid is found in a number of soft fruits and vegetables in the form of glucosides (Table 1), generally much less abundantly than those of p-hydroxybenzoic acid [1,2], except in onion peel, where it is prominent [21]. Salicylic and gentisic acids have been reported in very small quantities in the 6 Fleuriet and Macheix fruits of certain Solanaceae (tomato, eggplant, pepper), Cucurbitaceae (melon, cucumber) and other species (e.g., kiwi fruit, grapefruit, grape). Very low concentrations of p-hydroxybenzoic, protocatechuic, and t-cinnamic acids have been reported in different species of mushrooms (Agaricus and Lentinus species), along with traces of caffeic acid [22].