cause of the presence of other substances that can modify the reactions. The contributions of different polyphenols to the color of grape and apple juices depend on the amounts of certain polyphenols in the fruit.

In general, the significance of phenolic compounds in food can be related to ascorbic acid destruction, color formation with heavy metals, and antioxidant activity. When both active polyphenol oxidases and phenolic substrates are present in fruits and vegetables, tissue discoloration takes place. But this reaction does not begin until all of the ascorbic acid has been destroyed. The reason for this is that the oxidized polyphenols, in the course of oxidation, can reversibly transfer oxygen to ascorbic acid, being itself reduced to its original state.

Flavonoids affect anthocyanin color by copigmentation and by copolymerization. Polyphenols contribute most to browning when enzymatically oxidized to quinones, and the quinones polymerize to form relatively stable polymers. Phenolic compounds also give characteristic color reactions with a number of heavy metals, such as ferric salts and aluminum ions (33). For example, black spotting of cashew kernels is caused by iron interaction with catechin, and red-brown precipitates in wine are caused by the copper-procoanthocyanidin reaction (34). Since copper, iron, and other complexing ions catalyze oxidation of polyphenols, they can promote oxidative browning. Quercetin, chlorogenic acid, and gallic acid form highly colored complexes with cyanidin-3-glucoside in the presence of aluminum ions (35). The combination of anthocyanins with flavonoids of another types occurs in red wine during aging (36).

One of the discolorations caused by reactions of colorless plant polyphenols is the reddening or pinking that occurs when colorless anthocyanogens are converted to antho-cyanidins. Acidic media and heat are important factors in the conversion of proanthocyanidins to anthocyandins; these conditions often occur in foods, particularly canned fruits. Pink color in canned pears is associated with high procoanthocyanidin content and failure to cool the cans quickly after thermal processing (37). Pink color development in banana, cabbage, and broad beans is also related to polyphenols (38-40).

Browning and blackening in fruits and vegetables during processing and storage involves the production of many overlapping chromophores that have absorbances in the visible region. Reactions of polyphenols to given colored products are not necessarily oxidative, particularly those that involve reaction with the phloroglucinol portion of flavonoids. Phenol-aldehyde condensations are an example of this. Carbonyl-amine reactions can result in browning if the carbonyl is in a quinone molecule that can be observed in the Maillard sugar-amino acid type reaction.

Certain phenolic compounds are antioxidants that suppress activity of the enzymes, lipoxygenase (41) and /i-galactosidase (42). Lipoxygenase causes oxidation of carot-enoids in certain vegetables and thus lowers the vitamin A value of these foods. Among phenolic compounds, flavans exhibit a higher inhibitory effect on lipoxygenase than fla-vonols and phenolic acids (43). Catechin, chlorogenic acid, and quercetin glycosides suppress /i-galactosidase in apple and retard softening during cold storage (42).

Another important reaction of polyphenols in foods is the complexing with protein. It has long been known that the o-dihydroxy phenolic compounds and the carbonyl groups of protein interact by hydrogen bonding (44) and that quinone-protein reactions also participate in this interaction (45). This polyphenols-protein interaction affects the protein quality of certain foods. The presence of tannins has been shown to reduce the nutritive value of various grains such as sorghum and fava beans. Vegetable proteins that have been exposed to o-dihydroxyphenols in oxidizing condition would be expected to be less readily digested and to have less biologically available lysine and cystine (46). When polymerized tannins combine with the proteins of food, the complex is less likely to be absorbed. Condensed tannins also may pass unchanged through the digestive tract.

Phenolic compounds and proteins reduce apple juice quality by the formation of haze and sediment during storage. Phenolic compounds participate in formation of haze during processing of beer and wine. Gelatin treatments have been used to remove some of the tannins and thus minimize the problem (47). Oxidized polyphenols may also react with proteins; the mechanisms of these quinone-protein reactions are not completely understood.

Some beneficial effects of polyphenols in relation to color and flavor of food products are associated with tea and coffee. In the cultivation of the tea plant, conditions are designed to produce tea leaves that are rich in epicat-echin, epigallocatechin, and their gallate esters (46). A large number of phenolic compounds are produced during the roasting process of coffee, while some phenolics such as chlorogenic acid (which is known to contain as much as 4% by weight of coffee beans) is destroyed significantly (48).

Since bound phenolic residues are components of unlig-nified cell walls of many plants, some polyphenols may contribute to the fiber content of the human diet. Both primary and secondary cell walls of wheat endosperm, spinach, potato, and other vegetables contain polyphenols such as ferulic and coumaric residues that are linked to hemicelluloses or peptides (49).

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