ROLE OF PHENOLIC ACIDS IN THE ORGANOLEPTIC AND NUTRITIONAL QUALITY OF FRUITS AND VEGETABLES

V. ROLE OF PHENOLIC ACIDS IN THE ORGANOLEPTIC AND NUTRITIONAL QUALITY OF FRUITS AND VEGETABLES

Phenolic acids contribute to the sensory and nutritional qualities of fruits, vegetables, and derived foods. Directly or indirectly, they play a role in color, astringency, bitterness, and aroma, and they also are of great interest to humans, because of their antioxidant capacity [2,4].

A. Phenolic Acids and Food Flavor

As reported, acylation of anthocyanins with p-coumaric and caffeic acids is common in fruits, and it is responsible for better color stability in fruit products [35]. For example, it has been shown that the difference of stability to light and heat of different Sambucus species results from the degree of anthocyanin acylation [96,97]. Diacylated anthocyanins are stabilized by the sandwich-type stacking caused by hydrophobic interaction between the anthocyanidin ring and the two aromatic acyl groups [98]. Intramolecular copigmentation involving p-coumaroylglucose units at three or four positions of delphinidin is responsible for the exceptionally deep blue color of Daniella sp. berries [37]. Furthermore, numerous flavonoids and HCA derivatives play a role in the intermolecular copigmentation by stabilizing the pigment in its colored form and being the cause of a bathochromic shift and of an increase in the absorbance in the visible band [98].

The color of plant organs may also be strongly modified by the appearance of brown compounds, which generally result from the enzymatic oxidation of phenolic compounds including caffeic esters [2,99]. These melanin-type pigments may appear naturally during maturation of certain fruits, but they generally occur after wounding and crushing of plant organs. The resultant discoloration affects both commercial quality and nutritional parameters. These aspects are discussed late in relation to the processing of fruits and vegetables.

The astringency of fruits results from the interaction of salivary proteins with tannins or other phenolics. Although chlorogenic itself has sometimes been reported to be astringent, HBAs play a major role as they participate in the formation of hydrolysable tannins [100]. For example, ellagitannins (ellagic acid esters of glucose) are responsible for the strong astringency of various fruits, e.g., pomegranate, persimmon, chestnuts, and fruits of Rosaceae. Furthermore, in rather rare cases, HBAs are also present in condensed tannins in the form of epicatechin-gallate [27].

The role of phenolic acids in the bitterness of fruit and fruit products is still a matter of discussion, but it was concluded that HCAs do not play any role in the taste of wines, even at high concentrations of caftaric acid and glutathionylcaftaric acid [27]. Verbascoside may contribute to bitterness in olives, but its concentration is always low in comparison to oleuropein concentration [2]. Phenylpropanoid sucrose esters with several acetyl groups are also responsible for the bitter taste of stone fruits of Prunus sp. [101].

The importance of phenolic acids in fruit aroma is low, though many simple aromatic phenols may be released by enzymatic or chemical reactions from glycosylated precursors during maturation or processing, e.g., in vanilla, passion fruit, mango, and apricot [102]. Such transformations are also at the origin of some aroma constituents in wines, ciders, and fruit juices, through the degradation of HCA conjugates. Vanillic acid participates, in addition to vanillin, in vanilla aroma, and cinnamaldehyde is the principal component of cinnamon flavor [103]. Ferulic acid is a potential precursor of off-flavors in stored citrus juice, and pasteurization increases both the release of free ferulic acid from bound forms and the formation of p-vinyl guaiacol [29].

B. Phenolic Acids as Antioxidants

Antioxidants play an important role in antioxidant defense mechanisms in biological systems, protecting lipids both in cells and in food products and having inhibitory effects on mutagenesis and carcinogenesis. Attention is now focused on natural antioxidants, since the use of synthetic antioxidants has been falling off because of their suspected action as cancer promotors [104]. Most natural antioxidants present a polyphenolic structure, and it is significant that most papers published since the early 1990s about the characterization of phenolic compounds, and especially phenolic acids, concern their antioxidant activity (see reviews 104–109; see also Ref. 110) and the different chapters of the present volume.

Along with numerous other phenolic compounds, hydroxycinnamates and gallic acid derivatives (methyl and lauryl esters, propylgallate) act as free radical acceptors and show strong antioxidant properties [4,6,50,105,108].

Many fruits and vegetables (e.g., grape, citrus, olive, black pepper, spices, soya, cereals) and the derived foods and beverages are a good source of phenolic antioxidants and constitute an important part of our daily diet [4,5,25,26,107–109,111–115]. A good correlation between phenolic content and antioxidant activity is often observed, as reported for monomeric and dimeric hydroxycinnamates of rye bran [75] and various caffeoyl quinic esters in peach puree [116], tart cherries [117], and prunes and prune juice [69,118]. Regular consumption of phenolic antioxidants may provide protection against diseases, including cancer and cardio-and cerebrovascular diseases [110], and it increases the serum antioxidant capacity in humans, as shown after consumption of strawberries, spinach, or red wine [119].

In addition to flavonoids, HCA derivatives protect food products from oxidation: for example, the remarkable stability of virgin olive oil is directly related to its phenolic antioxidants [104,120,121]. They are also widely used as food antioxidant additives to protect lipid structures [4,6] and are good candidates for successful employment as topical protective agents against ultraviolet (UV) radiation–induced skin damage [122]. Agricultural and industrial residues and by-products of plant origin (e.g., potato peel waste, grape seeds, olive, apple or cranberry pomace, citrus peels and seeds) are attractive and cheap sources of natural antioxidants [123–125]. An extensive review of these last aspects, including the influence of processing conditions, has been published [108], and it clearly shows that antioxidant capacity is often associated with the presence of phenolic acids (Table 4).

Table 4 Main Phenolic Acids in Crude Extracts from Agroindustrial Wastes

The relationships between chemical structures of HCA conjugates and their antioxidant and free radical scavenging activity have been reported [5,50,106,108,110,126]. Although this may vary with temperature and the nature of the test used, antioxidant activity was always higher for free caffeic acid than for its glucose or quinic esters (e.g., chlorogenic acid), whereas it was lower for ferulic acid [50,75]. For the less active HCAs, p-coumaric and ferulic acids, esterification to tartaric acid may enhance ability to inhibit low-density lipoprotein (LDL) oxidation [127]. The presence of hydroxyl groups in the ortho position increases antioxidant activity, as shown for caffeic and rosmarinic acids. Nevertheless, a limitation of the utilization of HCAs and their natural esters as lipid protectors