[Advances in Food Research] Advances in Food Research Volume 5 Volume 5 || Flavonoid Compounds in Foods

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<ul><li><p>Flavonoid Compounds in Foods </p><p>BY E. C. BATE-SMITH </p><p>Low Temperature Research Station, University of Cambridge, and Department of Scientific and Industrial Research, Cambridge, England </p><p>CONTENTS Page </p><p>. . . . . . . . . . . . . . . . . . . . . . 267 </p><p>1. Properties Depending upon Their General Phenolic Character.. . . . . . . 268 a. Destruction of Ascorbic Acid.. . . . . . . . . . . . . . . . . . . b. Reactions with Metals.. . . . . . . . . . . . . . . . . . . . . . . . . . . c. Antioxidant Action. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . </p><p>a. Color . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . </p><p>c. Pharmacological Action. . . . . . . . . . . . . . . . . . . . . . . 275 </p><p>. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277 111. The Genetic Situation.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278 </p><p>1. Genetic Situation in Fruits and Vegetables.. . . . . . . . . . . . . . . . . . . . . . . 280 a. Apples. . . . . . . . . . . . . . . . . . . . . . . . . . . . b. Peaches.. . . . . . . . . . . . . . . . . . . . . . . . . . c. Grapes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . d. Potatoes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282 e. Onions.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283 </p><p>IV. Systematic Distributiori . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283 1. Anthocyanins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283 </p><p>a. Zsoflavones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286 2. Anthoxanthins . . . . . . . . . . . . . . . . . . . . . . . . . </p><p>3. Catechins and Leuco-anthocyanins. . . . . . . . . . . . . </p><p>2. Tea. . . . . . . </p><p>c. Fermentation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293 </p><p>. . . . . . . . . . . . . . . . . . . . . . . . . . </p><p>261 </p></li><li><p>262 E. C. BATE-SMITH </p><p>I . INTRODUCTION </p><p>The task of reviewing the subject of flavonoid compounds in foods has been made very much easier by the appearance recently of several reviews dealing with particular aspects of the subject. One of these, Theories of the Biogenesis of Flavonoid Compounds (Geissman and Hinreiner, 1952), is especially valuable in providing a complete survey of these compounds, in tabular form, and a discussion of their chemical and biological relationships. The definition of flavonoid compounds em- ployed by these authors is as follows: The flavonoid compounds are characterised by their possession of a Cs-C8-Ce carbon skeleton con- sisting of two aromatic rings linked by an aliphatic three-carbon chain. Chiefly on the basis of the oxidation state of this aliphatic fragment the very large number of compounds included in the flavonoid classification is subdivided into such well-known types as anthocyanins, flavones, chalcones, etc. </p><p>Flavone, the type substance of the whole class, which occurs as a dust on the leaves and stems of certain species of Primula, has the structure (I). The other naturally occurring compounds of this class have a number of phenolic hydroxyl or methoxyl groups substituted in each of the two aromatic rings, usually in certain characteristic patterns (Table I) e.g., apigenin (11). Flavones substituted with hydroxyl specifically in the </p><p>6 Co </p><p>I </p><p>111 IV </p><p>0 </p><p>V VI </p></li><li><p>FLAVONOID COMPOUNDS IN FOODS 263 </p><p>3 position are known as flavonols, e.g., quercetin (111). In the flavanones, e.g., naringenin (IV), and flavanonols, e.g., taxifolin (V), the double bond between carbon atoms 2 and 3 is reduced; together with the isoflavones (VI), these four types are sometimes collectively known as anthoxanthins. </p><p>The catechins, and probably the leuco-anthocyanins (whose structure is not completely known) are derived from a reduced flavone, i.e., a flavan molecule; catechins, e.g., catechin (VII), being flavan-3-01s. Anthocyanins, in the colored anionic form in which they usually occur, are flavylium salts (e.g., cyanidin, VIII). The anthocyanins may, how- ever, occur in a colorless form, known as a pseudo base, possibly (IX), </p><p>+ </p><p>VII VIII </p><p>and the leuco-anthocyanins probably have some structure such as (X), related both to this pseudo-base form of the anthocyanins and to the catechins. </p><p>IX X </p><p>Closely related to the flavones and anthocyanins are the coumarins (XI) and chlorogenic acid (XII), which may be regarded as derivatives of cinnamic acid. O;:,; Ho(j CHOH </p><p>CH-COOCk CHOH I I c&lt; CH, CH2 </p><p>\ / \ CH </p><p>\Ci/OH)COOH </p><p>XI XI1 </p><p>These, although not, properly speaking, flavonoid in structure, have hydroxylation patterns similar to that of the B ring in flavones (Table I) and must therefore be kept in view in any survey of flavonoid compounds. </p></li><li><p>Cinnamic acids </p><p>Coumarm </p><p>Anthocyanidin: </p><p>Catechins </p><p>Flavonols </p><p>CH=CH. COOH p-Coumaric </p><p>TABLE I The Principal Naturally Occurring Flavonoid, and Some Related, Compounds </p><p>H O G </p><p>CH=CH.COOH Caffeic </p><p>Pelargonidin Cyanidin </p><p>I Catechins </p><p>Kaempfeml R = H Fisetin I R- OH Quercetin </p><p>C H , O G </p><p>CH=CH .COOH Ferulic </p><p>Scopoletin </p><p>Peonidin </p><p>Isorhamnetin </p><p>CH= CH . COOH Sinapic </p><p>R = H Fraxetin R = CH, Isofraxidin </p><p>+ OR HO ooH </p><p>\ &amp;*OH OR' OH CH </p><p>R = R' = H Delphinidin R'-H, R-CHI Petunidin R' = R = CHI h5alvidin </p><p>Gallocatechins </p><p>K = H Robinetin R - OH hlvricetin </p></li><li><p>Flavones </p><p>Flavanonols </p><p>Flavanones </p><p>Chalcones </p><p>Isoflavones </p><p>Apigenin </p><p>H O P O o H ,CHOH </p><p>OH co Katsurenin </p><p>(Aromadendrin) </p><p>R - H Liquiretigenin R= OH Xaringenin </p><p>HO' ' ? H a o H U c 6 C H </p><p>Dahlia chalcone </p><p>R = R' - H Daidzin R - OH, R' - H Genistein R = H , R'-CHa Formononetin R - OH, R E CHI Biochanin-A </p><p>Luteolin I Chrysoeriol I Tricin </p><p>I Taxifolin I Ampelopsin </p><p>I R = H Butin Homoeriodictyol R = OH Ericdictyol I </p><p>Butein I </p><p>Orobin </p></li><li><p>266 E. C. BATE-SMITH </p><p>The only other types of flavonoid substances which have to be con. sidered are benzalcoumaranones (aurones, cf., Bate-Smith and Geissman, 1951), e.g., sulphuretin (XIII, R = H) and aureusidin (XIII, R = OH), chalcones, e.g., butein (XIV), and dihydrochalcones, e.g., phloretin (XV). The relationships between these types are admirably illustrated by Geissman and Hinreiner (1952). Table I1 is a somewhat modified </p><p> TABLE^ I1 Equivalent Level of Oxidation of 3-Carbon Fragments </p><p>Catechins -CHz.CHOH.CHOH- Leuco-antho- Structure uncertain </p><p>Anthocyanins -CHz.CO.CO cyanins </p><p>or -CHOH.CHOH.CO- </p><p>Dihydrochalcones -CO.CHn.CH2 Chalcones Flavtmones } --CO.CHz.CHOH . Flavones -CO.CHz.CO- </p><p>Benaalcoumaranones -C0.C0.CH2- Flavanonols -CO.CHOH.CHOH- Flavonols -CO.CO.CHOH- </p><p>version of their chart. It shows in a purely formal way the level of oxida- tion of each of the three carbon atoms of the central ring or chain. It is interesting to note that in the jlavanone series no higher state of oxidation of the Ca fragment than that of the flavonols is possible; and similarly no higher state of oxidation in the JEavan series is possible than that of the anthocyanins. </p><p>XIII XIV </p><p>From the examples given in Table I it will be noted that variation within a type is due to hydroxylation and methoxylation of the two benzene nuclei. As pointed out above, an important point to note is that the pattern of hydroxylation and methoxylation is common to a great many types. In some families of plants, and especially in the Rutaceae and Malvaceae, still more highly hydroxylated and methoxylated flavonoids occur. Some of these are dealt with in connection with citrus fruits (p. 289). </p></li><li><p>FLAVONOID COMPOUNDS IN FOODS 267 </p><p>1. Glycosidation </p><p>With the exception of the catechins and possibly the leuco-antho- cyanins, the flavonoid compounds occur in the plant as glycosides in which certain of the phenolic hydroxyl groups are combined with sugar residues. The sugar-free molecules shown in Table I are termed aglycones. If an extract of plant material is chromatographed on paper, numerous spots will be found which give the typical reactions for flavonoid compounds. Usually these are clearly visible by their fluorescence in ultraviolet light, and many change both their visible and fluorescent colours on fuming with ammonia. If the extract is hydrolyzed with mineral acid, a chromatogram of the extract now shows only a few spots-often only one-reacting as described. The numerous spots on the original chromatogram are, in fact, numerous glycosyl forms of just a few parent substances. The variety of glycosyl forms arises both from the number and variety of sugars that can combine with any one phenolic hydroxyl group and from the numerous hydroxyl groups capable of glycosylation present on the flavonoid mole- cule. Sugars which commonly occur in glycosyl combination with fla- vonoid substances include galactose, arabinose, xylose, and, especially, glucose and rhamnose. These can, and often do, occur not only attached as single sugar residues to particular hydroxyl groups, but as di- or tri- saccharides, and several positions on the same molecule may be so glycosylated. Thus it is quite possible for three or four different glycosides of each parent phenolic compound to be clearly visible on a chromato- gram. I n such circumstances the particularization of any one of these as a constituent of the foodstuff in question (unless it far outweighs all other flavonoid constituents in quantity or is known to have some property of especial importance in regard to the behavior of the food) would be more misleading than instructive. Furthermore, the practice of giving specific glycosides-or even specific aglycones-names derived, as has usually been the case, from the botanical species from which they were first isolated, cannot be extended to the hosts of new compounds which, we can anticipate, will be isolated and characterized with the help of chromato- graphic methods. Finally, it remains to be seen how much of the work on the characterization or identification of flavonoid compounds recorded in the literature is reliable. </p><p>So far as is known, the catechins are never glycosylated, and the leuco-anthocyanins seem also to occur, as a rule, uncombined with sugars. Johnson et al. (1951) report, however, that the main component of peach tannin (which from the description they give resembles in every respect a leuco-anthocyanin) is associated with carbohydrate, since glucose appears on hydrolysis with HC1. </p></li><li><p>268 E. C. BATE-SMITH </p><p>The catechins in tea occur mainly in the form of 3-galloyl esters such as catechin-3-gallate (XVI). </p><p>XVI </p><p>Many anthocyanins are acylated with aliphatic or aromatic hydroxy- acids, attached as a rule to the glycosyl sugar residues. </p><p>11. PROPERTIES OF FLAVONOID COMPOUNDS SIGNIFICANT IN FOODS 1 . Properties Depending upon Their General Phenolic Character </p><p>I n spite of their frequently brilliant color, i t is not so much this property which is important in foods as their tendency to undergo dis- coloration. This is due to their general phenolic character which allows them to serve as effective substrates for oxidase action. They are, in fact, of all classes of phenolic substances, those most universally present in the plant kingdom. Seldom does the analysis of a plant extract fail to reveal one or more substances of flavonoid character, and if these are absent, chlorogenic acid or one of the closely related coumarins is almost certain to be present. Thus the flavonoid compounds and coumarins are the most commonly available substrates, actual or potential, for polyphenol- oxidase or peroxidase activity. </p><p>Little work has been done with the specific aim of showing that flavonoid substances are competent to act as substrates for phenolase, but what has been done allows no room for doubt that they are. The most extensive work in this connection, that of Roberts and Wood (1951a, b, 1953) with tea oxidases and polyphenols, will be discussed in detail later on (p. 292). Having established that the mixed polyphenolic substances were acted upon by the purified phenolase from tea, they proceeded to show that individual catechins, flavonols, chalcones, etc., were similarly attacked. They showed, moreover, that certain glycosides of flavonols were highly resistant to attack by the enzyme, although the correspond- ing aglycones were themselves readily acted upon. In the authors own laboratory (Baruah and Swain, 1952) it has, similarly, been shown that the glycosides quercitrin and rutin are not acted upon by potato poly- phenolase, whereas the corresponding aglycone, quercetin, is rapidly oxidized. </p><p>It is evidently of great importance whether the phenolic compounds are present in the free state, or as glycosides, in the tissues of the plant. </p></li><li><p>FLAVONOID COMPOUNDS I N FOODS 269 </p><p>Where both active polyphenoloxidases and competent phenolic sub- strates are present, the result of the action of the one upon the other is to produce a discoloration of the tissues, usually brown and therefore known as enzymic browning. A review of this subject has appeared so recently (Joslyn and Ponting, 1951) that there is no need to embark on any detailed description of i t here. Browning ensues when tissues are damaged by cutting or bruising, by physiological injury such as storing fruits and vegetables in inappropriate atmospheres, and by freezing and thawing. It is, in fact, an indication of post-mortem change, and is, in almost all circumstances connected with food, undesirable. What interests us here is whether, by attention to the particular nature of the flavonoid sub- strates, any means of control of enzymic browning suggests itself. </p><p>Occasionally, but only very occasionally, a proper development of browning is a desired step in the preparation of a food product. This is so, for instance, in the manufacture of tea and cider. It is necessary in these few instances t o ensure that the right phenolic substrates are present in the raw material and that the conditions are right for the enzymes to act upon them. </p><p>a. Destruction of Ascorbic Acid. An indirect outcome of the un- restrained action of polyphenoloxidase in plant tissues is the total and rapid destruction of the ascorbic acid in the tissue. This might, in certain cir- cumstances, be a more serious disadvantage from the viewpoint of the use of the material as food (for instance in citrus products, and vitamin concen- trates) than the discoloration itself. Enzymic browning does not, in fact, begin until all, or almost all, of the ascorbic acid has been destroyed (Reid (1952) in apple juice; Miller and Heilmann (1952) in pineapple). </p><p>The reason for this is that the polyphenol, in the course of oxidation, can reversibly transfer oxygen to ascorbic acid, being itself reduced to its original state. The sequence in such a cycle of changes might be indicated as follows: </p><p>0 OH It 0 OH </p><p>where R is a substituent group such as </p><p>OH CO acid, and DHA dehydroascorbic acid (cf. Bate-Smith and Morris, 1952). </p></li><li><p>270 E. C. BATE-SMITH </p><p>b. Reactions with Metals. In common with other phenols, the fla...</p></li></ul>

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