Structure

Hydroxycinnamic acids are widely distributed in relatively high concentrations in most fruits and vegetables. The important acids are p-coumaric, caffeic, ferulic, and sinapic acids (Table 1). They rarely occur in free state, but rather as simple esters. The most common is quinic acid, especially 5-caffeoylquinic or 3-caffeoylquinic acid, which are commonly known as chlorogenic acid and neochlorogenic acid. Hydroxycinnamic acids often occur in fruits and vegetables as esters of malic acid, tartaric acid, hydroxycitric acid, tartronic acid, shikimic acid, galactaric acid, gluconic acid, and methoxyaldaric acid (5).

Flavonoids constitute one of the most distinctive groups of higher plant secondary metabolites. The term flavonoid embraces all those compounds whose structure is based on that of flavone (2-phenylchromone) (Fig. 1).

It can be seen that flavone consists of two benzene rings (A and B) joined together by a three-carbon link that is formed into a y-pyrone ring. The various classes of flavonoid compounds differ from each other only by the state of oxidation of this 3-C link. Various states of oxidation at the link produce a number of different compounds found in plants, such as flavan-3-ols (catechins), and 3-hydroxyfla-vones (flavonols, such as quercetin). Also included in the flavonoids are flavanones and anthocyanidins, chalcones, and isoflavones, as shown in Table 1. In the flavanones (naringenin), the double bond between C-2 and C-3 is reduced.

The individual compounds within each class are also distinguished by the number of hydroxyl, methoxyl, and other groups substituted in the two benzene rings (A and B in Fig. 1). Hydroxyl groups of the A ring are substituted at either both C-5 and C-7, or only at C-7, while those of the B ring are usually substituted by either one, two, or three hydroxyl or methoxyl groups. The hydroxylation pattern of the B ring thus resembles that found in the commonly occurring cinnamic acids and coumarins. Most flavonoid compounds except catechins occur in the plant as glycosides in which some of the phenolic hydroxyl groups (usually C-3) are combined with sugar residues such as galactose, arabinose, xylose, glucose, and rhamnose. More than 720 flavone and flavonol glycosides are known to occur in the plant kingdom (6).

Anthocyanins are the most important group of water-soluble plant flavonoid pigments visible to the human eye. With a few exceptions, they are universal plant colorants and largely responsible for the cyanic colors of flower petals and fruits. Most of the approximately 300 anthocyanins that have been characterized consist mainly of 17 known aglycones (7,8). Of these aglycones, six are important food colorants, due to their common occurrence (9). The sugars substituted on the aglycone are glucose, rhamnose, xylose,

Table 1. Common Phenolic Compounds Found in Fruits and Vegetables

Hydroxycinnamic acid derivatives

CH=CH—COOH

Cinnamic acids

R1 R2 = H, p-coumaric acid R1 = OH, R2 = H, Caffeic acid R1 = OMe, R2 = H, Ferulic acid R1 R2 = OMe, Sinapic acid

Catechins

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