Hydroxycinnamic Acids OCCURRENCE IN FRUITS AND VEGETABLES

B. Hydroxycinnamic Acids

Among fruit and vegetable phenolics, HCA derivatives play an important role that is due to their abundance and diversity. They all derive from cinnamic acid and are essentially present as combined forms of four basic molecules: coumaric, caffeic, ferulic, and sinapic acids (Fig. 2). Two main types of soluble derivatives have been identified (Fig. 2): (1) those involving an ester bond between the carboxylic function of phenolic acid and one of the alcoholic groups of an organic compound (e.g., quinic acid, glucose), for example, chlorogenic acid, which has been identified in numerous fruits and vegetables; and (2) those that involve a bond with one of the phenolic groups of the molecule, e.g., p-coumaric acid O-glucoside in tomato fruit. The diversity of HCA conjugates thus results from the nature of the bonds and that of the molecule(s) involved. In addition, for each of these compounds, the presence of a double bond in the lateral chain leads to the possible existence of two isomeric forms: cis (Z) and trans (E). Although native compounds are mainly of the trans form, isomerization occurs during extraction, purification, and processing under the effect of light or other chemical and physical factors.

1. Hydroxycinnamic Acids in Fruits

Quinic esters of HCA have been reported for a long time in fruits. The first were chlorogenic acid (5-O-caffeoylquinic acid*) and p-coumaroylquinic acid in apple [23]. Chlorogenic acid was subsequently found in many other fruits (Table 2), often accompanied by other caffeoylquinic isomers such as neochlorogenic acid (3-O-caffeoylquinic acid) and cryptochlorogenic acid (4-O-caffeoylquinic acid) isochlorogenic acid (a mixture of several di-O-caffeoylquinic acids) in coffee beans or coffee pulp, apple, avocado, pineapple, cherry, peach, eggplant [1,2], and in loquat fruit [24]. Red berries are particularly rich in caffeoylquinic esters, which confer on them, along with anthocyanins, high antioxidant activity [25,26]. The presence of tri-or tetra-O-caffeoylquinic acids in some fruits or leaves seems to be rather uncommon [4].

Figure 2 Chemical structure of hydroxycinnamic acids and some common derivatives identified in fruits and vegetables.

Table 2 Contents of Hydroxycinnamic Derivatives in Ripe Fruits

Quinic derivatives of other HCAs have also been identified in numerous fruits, e.g., several isomers of p-coumaroylquinic acid in apple and 5-Oferuloylquinic acid in tomato [2]. Although quinic derivatives are generally abundant in fruits, some contain none at all, e.g., grape, cranberry, and strawberry (Table 2). Mixed quinic di-esters of caffeic and ferulic acids are also present in robusta coffee beans [4].

Tartaric esters are limited to certain fruits of Vitis species and to some vegetables of the Asteraceae family (Tables 2 and 3). HPLC separations during the 1980s fully confirmed previous data by showing that the only combined form of caffeic acid in grape was in fact caffeoyltartaric acid (=caftaric acid) (Fig. 2). In addition, p-coumaroyl and feruloyl-tartaric acids (respectively named coutaric and fertaric acids) were found in varying proportions according to species and physiological stages [27]. Caffeoylshikimic esters (Fig. 2) are not widespread in plants, but they are very abundant in date fruit, where they participate in enzymic browning [2].

HCA derivatives with other hydroxyacids have rarely been identified in fruits, although p-coumaroylmalic acid is present in pear skin [28] and 2V-O-pcoumaroyl-, 2V-O-feruloylgalactaric acids, 2V-O-p-coumaroyl-, 2V-O-feruloyl-, and 2V,4V-O-diferuloylglucaric acids in the peel of citrus fruits [29].

Since the identification of 1-O-p-coumaroylglucose (Fig. 2) and caffeic acid 3-O-glucoside in potato berry, numerous derivatives of HCA with simple sugars have been identified in various fruits [2] (Table 2), and cinnamoylglucose itself has been reported in blood orange [30]. Glucose esters and glucosides may be present simultaneously, for example, in tomato fruit, where p-coumaric and ferulic acids are present both as glucosides and as glucose esters (Fig. 2), whereas caffeic acid is only represented by caffeoylglucose. Glucose esters of sinapic acid have also been reported in tomato and in Boreava orientalis, where it is present along with a glucosinolate salt [31,32]. Different new phenylpropanoid derivatives with simple sugars have been shown in the fresh fruit of Piscrama quassioides [33]. Verbascoside (Fig. 3) is an example of a rather more complex chemical combination that was identified in olives and in the fruit of different members of the Oleaceae family, along with several other caffeoyl glycosides [2].

Although HCA derivatives with sugars and hydroxyacids are simultaneously present in numerous fruits [e.g., apple, tomato, cherry (Table 2)], several exceptions should be reported. Glucose derivatives of HCA are not present or are present only as traces in pear and in grape, whereas HCAs are only present in the form of conjugates with sugars in strawberry and cranberry [1,2].

The presence of hydroxycinnamoyl amides in fruits and vegetables has rarely been reported. Feruloyputrescine (Fig. 2) occurs in grapefruit and orange juice [29] but has not been found in tangerine or lemon. The p-coumaroyl or caffeoyl amides of hydro-or dihydroxyphenylalanine have been reported in 10 Fleuriet and Macheix cocoa [34]. Two new phenolic amides were isolated from the fruit of white pepper (Piper nigrum L.), N-trans-feruloyltyramine (Fig. 2) and N-transferuloylpiperidine, together with some other derivatives of piperidine and phenolics [2].

Acylation of anthocyanins with certain phenolic acids has been known for a long time [35]. Grape has been studied extensively, and it was shown that pcoumaric acid plays a major role in the acylation of malvidin (Fig. 3) and of all the other anthocyanins present, whereas caffeic acid combines only with malvidin 3-glucoside, a condition common in fruits and vegetables [35]. In eggplant, delphinidin is acylated with coumaric and caffeic acids; delphinidin 3-(p coumaroylrutinoside)-5-glucoside is a major pigment in purple-skinned varieties. In the fruit of Solanum guineese (garden huckleberry), petunidin 3-(p-coumaroyl-rutinoside)-5-glucoside forms at least 70% of anthocyanins and is accompanied by very small quantities of several other acylated derivatives [36]. An extreme case concerns the blue berries of Dianella species, which contain delphinidin tetraglucosides bearing p-coumaroyl groups on two, three, or four of the sugars [37] (Fig. 3).

Flavonoid glycosides other than anthocyanins can also be acylated with HCA, but they have only rarely been reported in fruits, e.g., in the form of kaempferol p-coumaroylglycosides in Tribulus terrestris, 7-O-p-coumaroylglycoside-naringenin in Mabea caudata, or rhamnetin-3-p-coumaroylrhamninoside in Rhamnus petiolaris [38]. p-Coumaric and ferulic acids are also present in combination with betanidin monoglucoside (Fig. 3) in fruits of Basella rubra [39].

HCA may also be covalenty attached to aliphatic components of cutin and suberin. The amount of covalently bound phenolic compounds (m-, p-coumaric acids and flavonoids) in tomato fruit cutin increased during fruit development and accounted for as much as 6% of cutin membranes. Protoplasts isolated from immature tomato fruit secrete a wall that has been shown to contain suberin, in which phenolic compounds formed 25% of total monomers [3].

2. Hydroxycinnamic Acids in Vegetables and Cereals

Most of HCA conjugates previously described in fruits are also present in vegetables [1,4], but concentrations may be very different according to the botanical origin and the nature of the plant organ. An extensive review of the different HCA conjugates encountered in most vegetables consumed was published in 1999 [4], and here we only summarize some peculiar points.

Caffeoylquinic esters have been reported in most vegetables (Table 3): cabbages, endive, artichoke, potatoes, carrot, etc. In addition to the classical dicaffeoylquinic acids, diferuloylquinic acids are present in carrot root [40]. Several points must be underlined in Brassica vegetables: (1) 3-O-caffeoyl-Phenolic Acids in Fruits and Vegetables 11 quinic acid is always isomers and a similar condition is found for p-coumaroyl and feruloyl quinic esters; (2) feruloyl and sinapoyl glucose esters are important in kale and red cabbage; (3) mixed feruloyl-sinapoyl esters of gentibiose are present in broccoli [41]; (4) malic esters are present in radish tuber and leaf, whereas quinic and glucose esters are present only as traces [1].

Chlorogenic acid is also detected in fennel teas prepared by infusion or decoction [42] and in the leaves of Corchurus olitorius used as a vegetable for soup [43]. Artichoke capitula is characterized by significant amounts of chlorogenic acid and various dicaffeoylquinic esters, especially 1,3-dicaffeoylquinic acid, known as cynarin [44]. In addition to the previous compounds, 3,5-dicaffeoyl-4-succinylquinic acid is present in garland [45] and several caffeoyl-methylquinic acids with a strong antioxidant activity were characterized in bamboo shoots [46].

Along with quinic esters, caffeoyl and dicaffeoyltartaric acids are prominent in the leaves of some of the Asteraceae [1], e.g., lettuce, endive, and chicory. Although they are rarely present in fruits, malic esters of HCA are more frequently found in vegetables, e.g., in the leaves and pods of faba bean and in lettuce or spinach leaves (Table 3). Nevertheless, in the latter case, the prominent HCA conjugate is p-coumaroyl-meso-tartaric acid [1,47]. Tartronic acid occurs as p-coumaroyl, feruloyl, and caffeoyl-tartronic esters in the leaves of mung bean (Vigna radiata) [48]. Rosmarinic acid, a caffeic ester of 3,4-dihydroxyphenyllactic acid (Fig. 3), is found at a high level in extracts of various culinary and medicinal herbs (up to 1 g/kg fresh weight in thyme), where it shows remarkable antioxidant activity [49–51].

As previously shown in the case of fruits, sugar esters of HCA are also present in numerous vegetables, especially p-coumaroyl, caffeoyl, and sinapoyl glucose esters in Brassiceae, spinach leaves, and rhubarb stalk (Table 3). Root and/or derived cell cultures of red beet are rich in different HCA esters, e.g., several feruloylglucose conjugates, a feruloylsucrose monoester, a ferulicaspartic acid amide, and a feruloylglycerol glucuronide [52,53]. Furthermore, red beet also contains low concentrations of two conjugates of HCA with betacyanins (the major coloring substances of red beet): lampranthin I (p-coumaroylbetanin) and lampranthin II (feruloylbetanin) [53]. In addition to the case of tomato fruit previously reported (Table 2), HCA glucosides have been identified in faba beans (leaves and pods) and are present as traces in carrot [1].

Although the presence of chlorogenic acid itself has rarely been reported in barley grains [54], HCAs, and ferulic acid in particular, are generally found as insoluble forms in various glucidic fractions of the cell wall. These compounds have not been reported in fleshy fruits but exist in Graminaeae and some other plants from which they are easily liberated by chemical or enzymatic hydrolysis. Several reviews on the subject were published in 1999 [6,55], and only a brief summary is given here. Ferulic and p-coumaric acids are bound through an ester linkage to the arabinoxylans or xyloglucans of Gramineae (wheat, maize, barley, rice, etc.), leaves, straw, and grain (bran and aleurone layer). A part of ferulic acid also exists as dehydrodimers (Fig. 2) (e.g., in grasses, cereals, Chinese water chestnut, sugar beet, carrot), which cross-link and strengthen the wall [56–58], and a small amount of ferulic acid is also found in the cell walls of the thick cuticle of fleshy scales of onions [21]. In some dicotyledons (e.g., sugar beet, spinach, beans) ferulic and p-coumaric acid are also bound to the galactose or arabinose residues of pectins [4,59].

Apolar esters of sterols and stanols with ferulic or p-coumaric acids have been reported in corn bran and other cereals [60]. Furthermore, oats contain numerous caffeic and ferulic esters of glycerol, long-chain alkanols, and xhydroxyacids, in addition to avenanthramides (esters of anthranilic acid with either p-coumaric, caffeic, or ferulic acids) [61].

Suberized potato includes long-chain fatty acids and phenolic derivatives [62]. Furthermore, in addition to the HCA esters of p-coumaric acid, a significant number of ligninlike monolignol structures exist within suberin [63].