Tag Archives: proanthocyanidins or condensed tannins

Polyphenols in grapes and wine: chemical composition and biological activities

Polyphenols in Grapes
Fig. 1 – Red Grapes

The consumption of grapes and grape-derived products, particularly red wine but only at meals, has been associated with numerous health benefits, which include, in addition to the antioxidant/antiradical effect, also anti-inflammatory, cardioprotective, anticancer, antimicrobial, and neuroprotective activities.
Grapes contain many nutrients such as sugars, vitamins, minerals, fiber and phytochemicals. Among the latter, polyphenols are the most important compounds in determining the health effects of the fruit and derived products.
Indeed, grapes are among the fruits with highest content in polyphenols, whose composition is strongly influenced by several factors such as:

  • cultivar;
  • climate;
  • exposure to disease;
  • processing

Nowadays, the main species of grapes cultivated worldwide are: European grapes, Vitis vinifera, North American grapes, Vitis rotundifolia and Vitis labrusca, and French hybrids.
Note: grapes are not a fruit but an infructescence, that is, an ensemble of fruits (berries): the bunch of grapes. In turn, it consists of a peduncle, a rachis, cap stems or pedicels, and berries.

What are polyphenols in grapes and wine?

Polyphenols in red grapes and wine are significantly higher, both in quantity and variety, than in white ones. This, according to many researchers, would be the basis of the more health benefits related to the consumption of red grapes and wine than white grapes and derived products.
Polyphenols in grapes and wine are a complex mixture of flavonoid compounds, the most abundant group, and non-flavonoid compounds.
Among flavonoids, they are found:

Among non-flavonoid polyphenols:

Most of the flavonoids present in wine derive from the epidermal layer of the berry skin, while 60-70% of the total polyphenols are present in the grape seeds. It should be noted that more than 70% of grape polyphenols are not extracted and remain in the pomace.
The complex chemical interactions that occur between these compounds, and between them and the other compounds of different nature present in grapes and wine, are probably essential in determining both the quality of the grapes and wine and the broad spectrum of therapeutic effects of these foods.
In wine, the mixture of polyphenols play important functions being able to influence:

  • bitterness;
  • astringency;
  • red color, of which they are among the main responsible;
  • sensitivity to oxidation, being molecules easily oxidizable by atmospheric oxygen.

Finally, they act as preservatives and are the basis of long aging.

Polyphenols in grapes and wine: anthocyanins

They are flavonoids widely distributed in fruits and vegetables.
They are primarily located in the berry skin (in the outer layers of the hypodermal tissue), to which they confer color, having a hue that varies from red to blue. In some varieties, called “teinturier”, they also accumulate in the flesh of the berry.
There is a close relationship between berry development and the biosynthesis of anthocyanins. The synthesis starts at veraison (when the berry stops growing and changes its color), causes a color change of the berry that turns purple, and reaches the maximum levels at complete ripening.
Among wine flavonoids, they are one of the most potent antioxidants.
Each grape species and cultivars has a unique composition of anthocyanins. Moreover, in grapes of Vitis vinifera, due to a mutation in the gene coding for 5-O-glucosyltransferase, mutation that determines the synthesis of an inactive enzyme, only 3-monoglucoside derivatives are synthesized, while in other species  the glycosylation at position 5 also occurs. Interestingly, 3-monoglucoside derivatives are more intensely colored than 3,5-diglucoside derivatives.

Polyphenols in Grapes
Fig. 2 – Malvidin-3-glucoside

In red grapes and wine, the most abundant anthocyanins are the 3-monoglucosides of malvidin (the most abundant one both in grapes and wine), petunidin, delphinidin, peonidin, and cyanidin. In turn, the hydroxyl group at position 6 of the glucose can be acylated with an acetyl, caffeic or coumaric group, acylation that further enhances the stability.
Anthocyanidins, namely the non-conjugated molecules, are not present in grapes and in wine, except as traces.
Anthocyanins are scarcely present in white grapes and wine.
The composition of anthocyanins in wine is highly influenced both by the type of cultivar and by processing techniques, since they are present in wine as a result of extraction by maceration/fermentation processes. For this reason, wines deriving from similar varieties of grapes can have very different anthocyanin compositions.
Together with proanthocyanidins, they are the most important polyphenols in contributing to some organoleptic properties of red wine, as they are primarily responsible for astringency, bitterness, chemical stability against oxidation, as well as of the color of the young wine. In this regard, it should be underscored that with time their concentration decreases, while the color is due more and more to the formation of polymeric pigments produced by condensation of anthocyanins both among themselves and with other molecules.
During wine aging, proanthocyanidins and anthocyanins react to produce more complex molecules that can  partially precipitate.

Polyphenols in grapes and wine: flavanols or catechins

Polyphenols in Grapes
Fig. 3 – Catechin

They are, together with condensed tannins, the most abundant flavonoids, representing up to 50% of the total polyphenols in white grapes and between 13% and 30% in red ones.
Their levels in wine depend on the type of cultivar.
Typically, the most abundant flavanol in wine is catechin, but epicatechin and epicatechin-3-gallate are also present.

Polyphenols of grapes and wine: proanthocyanidins or condensed tannins

Polyphenols in Grapes
Fig. 4 – Procyanidin C1

Composed of catechin monomers, they are present in the berry skin, seeds and rachis of the bunch of grapes as:

  • dimers: the most common are procyanidins B1-B4, but also procyanidins B5-B8 can be present;
  • trimers: procyanidin C1 is the most abundant;
  • tetramers;
  • polymers, containing up to 8 monomers.

Their levels in wine depend on the type of grape varieties and wine-making technology, and, like anthocyanins, are much more abundant in red wines, in particular in aged wines, compared to white ones.
In addition, as previously said, together with anthocyanins, condensed tannins are important in determining some organoleptic properties of the wine.

Polyphenols of grapes and wine: flavonols

They are present in a large variety of fruit and vegetables, even if in low concentrations.
They are the third most abundant group of flavonoids in grapes, after proanthocyanidins and catechins.
They are mainly present in the outer epidermis of the berry skin, where they play a role both in providing protection against UV-A and UV-B radiations and in copigmentation together with anthocyanins.
Flavanol synthesis begins in the sprout; the highest concentration is reached a few weeks after veraison, then it decreases as the berry increases in size.
Their total amount is very variable, with the red varieties often richer than the white ones.
In grapes, they are present as 3-glucosides and their composition depends on the type of grapes and cultivar:

  • the derivatives of quercetin, kaempferol and isorhamnetin are found in white grapes;
  • the derivatives of myricetin, laricitrin and syringetin are found, together with the previous ones, only in red grapes, due to the lack of expression in white grapes of the gene coding for flavonoid-3′,5′-hydroxylase.
Polyphenols in Grapes
Fig. 5 – Quercetin-3-glucoside

In general, the 3-glucosides and 3-glucuronides of quercetin are the major flavonols in most of the grape varieties. Conversely, quercetin-3-rhamnoside and quercetin aglycone are the major flavonols in muscadine grapes.
In wine and grape juice, unlike grapes, they are also found as aglycones, as a result of the acid hydrolysis that occurs during processing and storage. They are present in wine in a variable amount, and the major molecules are the glycosides of quercetin and myricetin, which alone represent 20-50% of the total flavonols in red wine.

Polyphenols in grapes and wine: hydroxycinnamates

Polyphenols in Grapes
Fig. 6 – Ferulic Acid

Hydroxycinnamic acids are the main class of non-flavonoid polyphenols in grapes and the major polyphenols in white wine.
The most important are p-coumaric, caffeic, sinapic, and ferulic acids, present in wine as esters with tartaric acid.
They have antioxidant activity and in some white varieties of Vitis vinifera, together with flavonols, are the polyphenols mainly responsible for absorbing UV radiation in the berry.

Polyphenols in grapes and wine: stilbenes

Polyphenols in Grapes
Fig. 7 – trans-Resveratrol

They are phytoalexins which are produced in low concentrations only by a few edible species, including grapevine (on the contrary, flavonoids are present in all higher plants).
Together with the other polyphenols in grapes and wine, also stilbenes, particularly resveratrol, have been associated with health benefits resulting from the consumption of wine.
Their content increases from the veraison to the ripening of the berry, and is influenced by the type of cultivar, climate, wine-making technology, and fungal pressure.
The main stilbenes present in grapes and wine are:

  • cis- and trans-resveratrol (3,5,4′-trihydroxystilbene);
  • piceid or resveratrol-3-glucopyranoside and astringin or 3′-hydroxy-trans-piceid;
  • piceatannol;
  • dimers and oligomers of resveratrol, called viniferins, of which the most important are:

α-viniferin, a trimer;
β-viniferin, a cyclic tetramer;
γ-viniferin, a highly polymerized oligomer;
ε-viniferin, a cyclic dimer.

In grapes, other glycosylated and isomeric forms of resveratrol and piceatannol, such as resveratroloside, hopeaphenol, or resveratrol di- and tri-glucoside derivatives, have been found in trace amounts.
Glycosylation of stilbenes is important for the modulation of antifungal activity, protection from oxidative degradation, and storage of the wine.
The synthesis of dimers and oligomers of resveratrol, both in grapes and wine, represents a defense mechanism against exogenous attacks or, on the contrary, the result of the action of extracellular enzymes released from pathogens in an attempt to eliminate undesirable compounds.

Polyphenols in grapes and wine: hydroxybenzoates

Polyphenols in Grapes
Fig. 8 – Gallic Acid

The hydroxybenzoic acid derivatives are a minor component in grapes and wine.
In grapes, gentisic, gallic, p-hydroxybenzoic and protocatechuic acids are the main ones.
Unlike hydroxycinnamates, which are present in wine as esters with tartaric acid, they are found in their free form.
Together with flavonols, proanthocyanidins, catechins, and hydroxycinnamates they are among the responsible of astringency of wine.

References

Andersen Ø.M., Markham K.R. Flavonoids: chemistry, biochemistry, and applications. CRC Press Taylor & Francis Group, 2006

Basli A, Soulet S., Chaher N., Mérillon J.M., Chibane M., Monti J.P.,1 and Richard T. Wine polyphenols: potential agents in neuroprotection. Oxid Med Cell Longev 2012 [Abstract]

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

Flamini R., Mattivi F.,  De Rosso M., Arapitsas P. and Bavaresco L. Advanced knowledge of three important classes of grape phenolics: anthocyanins, stilbenes and flavonols. Int J Mol Sci 2013;14:19651-19669 [Abstract]

Georgiev V., Ananga A. and Tsolova V. Recent advances and uses of grape flavonoids as nutraceuticals. Nutrients 2014;6: 391-415 [Abstract]

Guilford J.M. and Pezzuto J.M. Wine and health: a review. Am J Enol Vitic 2011;62(4):471-486 [Abstract]

He S., Sun C. and Pan Y. Red wine polyphenols for cancer prevention. Int J Mol Sci 2008;9:842-853 [Abstract]

Xia E-Q., Deng G-F., Guo Y-J. and Li H-B. Biological activities of polyphenols from grapes. Int J Mol Sci 2010;11-622-646
[Abstract]

Waterhouse A.L. Wine phenolics. Ann N Y Acad Sci 2002;957:21-36 [Abstract]

Anthocyanins: foods, absorption, metabolism

Anthocyanin rich foods

Anthocyanin
Fig. 1 – Red Cherries

Together with catechins and proanthocyanidins, anthocyanins and their oxidation products are the most abundant flavonoids in the human diet.
They are found in:

  • certain varieties of grains, such as some types of pigmented rice (e.g. black rice) and maize (purple corn);
  • in certain varieties of root and leafy vegetables such as aubergine, red cabbage, red onions and radishes, beans;
  • but especially in red fruits.

They are also present in red wine; as the wine ages, they are transformed into various complex molecules.
Anthocyanin content in vegetables and fruits is generally proportional to their color: it increases during maturation, and it reaches values up to 4 g/kg fresh weight (FW) in cranberries and black currants.
These polyphenols are found primarily in the skin, except for some red fruits, such as cherries and red berries (e.g. strawberries), in which they are present both in the skin and flesh.
Glycosides of cyanidin are the most common anthocyanins in foods.

Anthocyanins in fruits

  • Berries are the main source of anthocyanins, with values ranging between 67 and 950 mg/100 g FW.
  • Other fruits, such as red grapes, cherries and plums, have content ranging between 2 and 150 mg/100 g FW.
  • Finally, in fruits such as nectarines, peaches, and some types of apples and pears, anthocyanins are poorly present, with a content of less than 10 mg/100 g FW.

Cranberries, besides their very high content of anthocyanins, are one of the rare food that contain glycosides of the six most commonly anthocyanidins present in foods: pelargonidin, delphinidin, cyanidin, petunidin, peonidin, and malvidin. The main anthocyanins are the 3-O-arabinosides and 3-O-galactosides of peonidin and cyanidin. A total of 13 anthocyanins have been detected, mainly 3-O-monoglycosides.

Intestinal absorption of anthocyanins

Until recently, it was believed that anthocyanins, together with proanthocyanidins and gallic acid ester derivatives of catechins, were the least well-absorbed polyphenols, with a time of appearance in the plasma consistent with the absorption in the stomach and small intestine. Indeed, some studies have shown that their bioavailability has been underestimated since, probably, all of their metabolites have not been yet identified.
In this regard, it should be underlined that only a small part of the food anthocyanins is absorbed in their glycated forms or as hydrolysis products in which the sugar moiety has been removed. Therefore, a large amount of these ingested polyphenols enters the colon, where they can also suffer methylation, sulphatation, glucuronidation and oxidation reactions.

Anthocyanins and colonic microbiota

Few studies have examined the metabolism of anthocyanins by the colonic microbiota.
Within two hours, it seems that all the anthocyanins lose their sugar moieties, thus producing anthocyanidins.
Anthocyanidins are chemically unstable in the neutral pH of the colon. They can be metabolized by colonic microbiota or chemically degraded producing a set of new molecules that have not yet fully identified, but which include phenolic acids such as gallic acid, syringic acid, protocatechuic acid, vanillic acid and phloroglucinol (1,3,5-trihydroxybenzene). These molecules, thanks to their higher microbial and chemical stability, might be the main responsible for the antioxidant activities and the other physiological effects that have been observed in vivo and attributed to anthocyanins.

References

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

de Pascual-Teresa S., Moreno D.A. and García-Viguera C. Flavanols and anthocyanins in cardiovascular health: a review of current evidence. Int J Mol Sci 2010;11:1679-1703 [Abstract]

Escribano-Bailòn M.T., Santos-Buelga C., Rivas-Gonzalo J.C. Anthocyanins in cereals. J Chromatogr A 2004:1054;129-141 [Abstract]

Han X., Shen T. and Lou H. Dietary polyphenols and their biological significance. Int J Mol Sci 2007;9:950-988 [Abstract]

Manach C., Scalbert A., Morand C., Rémésy C., and Jime´nez L. Polyphenols: food sources and bioavailability. Am J Clin Nutr 2004;79(5):727-47 [Abstract]

Tsao R. Chemistry and biochemistry of dietary polyphenols. Nutrients 2010;2:1231-1246 [Abstract]

Proanthocyanidins and procyanidins in foods

Proanthocyanidins in foods

The interest on proanthocyanidins, and their content in foods, has increased as a result of the discovery, due to clinical and laboratory studies, of their anti-infectious, anti-inflammatory, cardioprotective and anticarcinogenic properties. These protective effects have been attributed to their ability to:

  • act as free radical scavenger;
  • inhibit lipid peroxidation;
  • act on various protein targets within the cell, modulating their activity.

Proanthocyanidins in different foods vary greatly in terms of:

  • total content;
  • distribution of oligomers and polymers;
  • constituent catechin units and bonds between units.

In some foods, such as black beans and cashew nuts, only dimers are present, whereas in most of the foods proanthocyanidins are found in a wide range of polymerizations, from 2 to 10 units or more.

Foods with the highest proanthocyanidin content are cinnamon and sorghum, which contain respectively about 8000 and up to 4000 mg/100 g of fresh weight (FW); grape seeds (Vitis vinifera) are another rich source, with a content of about 3500 mg/100 g dry weight.
Other important sources are fruits and berries, some legumes (peas and beans), red wine and to a less extent beer, hazelnuts, pistachios, almonds, walnuts and cocoa.
The coffee is not a good source.
Proanthocyanidins are not detectable in the majority of vegetables; they have been found in small concentrations in Indian pumpkin. They are not detectable also in maize, rice and wheat, while there are present in barley.

A-type procyanidins in foods

A-type Procyanidins
Fig. 1 – Procyanidin A2

Although many food plants contain high amounts of proanthocyanidins, only a few, such as plums, avocados, peanuts or cinnamon, contain A-type procyanidins, and none in amounts equal to cranberries (Vacciniun macrocarpon).
Note: A-type procyanidins exhibit, in vitro, a capacity of inhibition of P-fimbriated Escherichia coli adhesion to uroepithelial cells greater than B-type procyanidins (adhesion represents the initial step of urogenital infections).

B-type procyanidins in foods

B-Type Procyanidins
Fig. 2 – Procyanidins B1-B4

B-type procyanidins, consisting of catechin and/or epicatechin as constituent units, are the exclusive proanthocyanidins in at least 20 kinds of foods including blueberries (Vaccinium myrtillus), blackberries, marion berries, choke berries, grape seeds, apples, peaches, pears, nectarines, kiwi, mango, dates, bananas, Indian pumpkin, sorghum, barley, black eye peas, beans blacks, walnuts and cashews.

Proanthocyanidins in fruits

In the Western diet, fruit is the most important source of proanthocyanidins.

  • The major sources are some berries (blueberries, cranberries, and black currant) and plums (prunes), with a content of about 200 mg/100 g FW.
  • Intermediate sources are apples, chokeberries, strawberries, and green and red grapes (60-90 mg/100 g FW).
  • In other fruits the content is less than 40 mg/100 g FW.

In fruit, the most common proanthocyanidins are tetramers, hexamers, and polymers.
Good sources of proanthocyanidins are also some fruit juices.

Proanthocyanidins in grape seeds

A particularly rich source of proanthocyanidins is the seeds of grape.
Proanthocyanidins in grape seeds are only B-type procyanidins, for the most part present in the form of dimers, trimers and highly polymerized oligomers.
Grape seed proanthocyanidins are potent antioxidants and free radical scavenger, being the more effective either than vitamin E and vitamin C (ascorbic acid).
In vivo and in vitro experiments support the idea that proanthocyanidins, and in particular those from grape seeds, can act as anti-carcinogenic agents; it seems that they are involved, in cancer cells, in:

  • reduction of cell proliferation;
  • increase of apoptosis;
  • cell cycle arrest;
  • modulation of the expression and activity of NF-kB and NF-kB target genes.
References

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

Gu L., Kelm M.A., Hammerstone J.F., Beecher G., Holden J., Haytowitz D., Gebhardt S., and Prior R.L. Concentrations of proanthocyanidins in common foods and estimations of normal consumption. J Nutr 2004;134(3):613-617 [Abstract]

Han X., Shen T. and Lou H. Dietary polyphenols and their biological significance. Int J Mol Sci 2007;9:950-988 [Abstract]

Manach C., Scalbert A., Morand C., Rémésy C., and Jime´nez L. Polyphenols: food sources and bioavailability. Am J Clin Nutr 2004;79(5):727-47 [Abstract]

Nandakumar V., Singh T., and Katiyar S.K. Multi-targeted prevention and therapy of cancer by proanthocyanidins. Cancer Lett 2008;269(2):378-387 [Abstract]

Ottaviani J.I., Kwik-Uribe C., Keen C.L., and Schroeter H. Intake of dietary procyanidins does not contribute to the pool of circulating flavanols in humans. Am J Clin Nutr 2012;95:851-8 [Abstract]

Santos-Buelga C. and Scalbert A. Proanthocyanidins and tannin-like compounds: nature, occurrence, dietary intake and effects on nutrition and health. J Sci Food Agr 2000;80(7):1094-1117 [Abstract]

Tsao R. Chemistry and biochemistry of dietary polyphenols. Nutrients 2010;2:1231-1246 [Abstract]

Wang Y.,Chung S., Song W.O., and Chun O.K. Estimation of daily proanthocyanidin intake and major food sources in the U.S. diet. J Nutr 2011;141(3):447-452 [Abstract]

 

Flavonols: definition, structure, food sources

What are flavonols?

Flavonols are polyphenols belonging to the class of flavonoids.
They are colorless molecules that accumulate mainly in the outer and aerial tissues, therefore skin and leaves, of fruit and vegetables, since their biosynthesis is stimulated by light. They are virtually absent in the flesh.

They are the most common flavonoids in fruit and vegetables, where they are generally present in relatively low concentrations.
Due to their widespread in nature and human diet, they should be taken into consideration when the positive effect on health associated with fruit and vegetable consumption is examined. Their effect is probably related to their ability to:

  • act as antioxidants;
  • act as anti-inflammatory agents;
  • act as anticancer factors;
  • regulate different cellular signaling pathways; an example is the action of quercetin, the most widespread flavonols, on the oxidative stress-induced MAPK activities.

Chemical structure of flavonols

Basic Skeleton of Flavanols
Fig. 1 – 3-hydroxyflavone

Chemically, these molecules differ from many other flavonoids since they have a double bond between positions 2 and 3 and an oxygen (a ketone group) in position 4 of the C ring, like flavones from which, however, they differ in the presence of a hydroxyl group at the position 3. Therefore, flavonol skeleton is a 3-hydroxyflavone.
The 3-hydroxyl group can link a sugar, that is, it can be glycosylated.
Like many other flavonoids, most of them is found in fruit and vegetables, and in plant-derived foods, in glycosylated form. The sugar associated with flavonols is often glucose or rhamnose, but other sugars may also be involved, such as:

  • galactose;
  • arabinose;
  • xylose;
  • glucuronic acid.
Flavonols
Fig. 2 – Flavonols

Flavonol are mainly represented by glycosides of:

  • quercetin;
  • kaempferol;
  • myricetin;
  • isorhamnetin.

The most ubiquitous compounds are glycosylated derivatives of quercetin and kaempferol; in nature, these two molecules have respectively about 280 and 350 different glycosidic combinations.
Finally, it should be underlined that sugar moiety influences flavonol bioavailability.

Foods rich in flavonols

The major sources in human diet are:

  • fruit;
  • vegetables;
  • beverages such as red wine and tea.

In human diet, the richest source are capers, which contain up to 490 mg/100 g fresh weight (FW), but they are also abundant in onions, leeks, broccoli, curly kale, berries (e.g. blueberries), grapes and some herbs and spices, for example dill weed (Anethum graveolens). In these sources, their content ranges between 10 and 100 mg/100 g FW.
Even cocoa, tea, both green and black ones, and red wine are good sources of flavonols. In wine, together with other polyphenols such as catechins, proanthocyanidins and low molecular weight polyphenols, they contribute to the astringency of the beverage.

The main flavonols in foods

The main flavonols in foods, listed in decreasing order of abundance, are quercetin, kaempferol, myricetin and ishoramnetin.

Quercetin

The richest sources of quercetin are capers, followed by onions, asparagus, lettuce and berries; in many other fruit and vegetables, it is present in smaller amounts, between 0.1 and 5 mg/100 g FW.
This flavonol is also present in cocoa and it could be one of its main protective agents against LDL oxidation.
Together with isoflavones, quercetin glycosides are the most well-absorbed polyphenols, followed by flavanones and catechins (on the contrary, gallic acid derivatives of catechins are among the least well absorbed polyphenols, together with anthocyanins and proanthocyanidins).

Kaempferol

Typical dietary sources of kaempferol include vegetables, such as spinach, kale and endive, with concentrations between 0.1 and 27 mg/100 g FW, and some spices such as chives, fennel and tarragon, with concentrations between 6.5 and 19 mg/100 g FW.
Fruit is a poor source of the molecule, with content down to 0.1 mg/100 g FW.

Myricetin

Myricetin is the third most abundant flavonol. It is found in some spices, such as oregano, parsley, and fennel, with concentrations between 2 and 20 mg/100 g FW, but also in tea, 0.5-1.6 mg/100 ml, and red wine, 0-9.7 mg/100 ml.
In fruit, it is only found in high concentrations in berries, while in most other fruit and vegetables it is present in a content of less than 0.2 mg/100 g FW.

Isorhamnetin

A fourth flavonol, less abundant than the previous ones, is isorhamnetin. It is only present in some foods such as some spices: chives, 5.0-8.5 mg/100 g FW, fennel, 9.3 mg/100 g FW, tarragon, 5 mg/100 g FW.
In fruit and vegetables it is only present in almonds, with a concentration between 1.2 and 10.3 mg/100 g FW, pears and onions.

References

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

Han X., Shen T. and Lou H. Dietary polyphenols and their biological significance. Int J Mol Sci 2007;9:950-988 [Abstract]

Manach C., Scalbert A., Morand C., Rémésy C., and Jime´nez L. Polyphenols: food sources and bioavailability. Am J Clin Nutr 2004;79(5):727-47 [Abstract]

Tsao R. Chemistry and biochemistry of dietary polyphenols. Nutrients 2010;2:1231-1246 [Abstract]

Anthocyanins: definition, structure and pH

What are anthocyanins?

Anthocyanins are a subgroup of flavonoids, therefore they are polyphenols, which give plants their distinctive colors.
They are water soluble pigments and are present in the vacuolar sap of the epidermal tissues of flowers and fruit.
They are responsible for the colors of the most of the petals, fruits and vegetables, and of some varieties of cereals such as black rice. In fact, they impart red, pink and purple to blue colors to berries, red apples, red grapes, cherries, and of many other fruits, red lettuce, red cabbage, onions or eggplant, but also red wine.
Together with carotenoids, they are responsible for autumn leaf color.
Finally, anthocyanins contribute to attract animals when a fruit is ready to eat or a flower is ready for pollination.

They are bioactive compounds found in plant foods that have a double interest for man:

  • the first one, a technological interest, due to their effects on the organoleptic characteristics of food products;
  • the other due to their healthy properties, being implicated in the protection against cardiovascular risk.
    In fact:

in vitro, they have an antioxidant activity, due to their ability to delocalize electrons and form resonance structures, and a protective role against oxidation of low density lipoproteins (LDL);

like other polyphenols, such as catechins, proanthocyanidins and other uncolored flavonoids, they can regulate different signaling pathways involved in cell growth, differentiation and survival.

Chemical structure of anthocyanins

Basic Skeleton of Anthocyanins
Fig. 1 – 2-phenylbenzopyrilium

The basic chemical structure is flavylium cation (2-phenylbenzopyrilium), which links hydroxyl (-OH) and/or methoxyl (-OCH3) groups, and one or more sugars.
The sugar-free molecule is called anthocyanidins.
Depending on the number and position of hydroxyl and methoxyl groups, various anthocyanidins have been described, and of these, six are commonly found in vegetables and fruits:

  • pelargonidin
  • cyaniding
  • delphinidin
  • petunidin
  • peonidin
  • malvidin

Anthocyanins, as most of the other flavonoids, are present in plants and plant foods in the form of glycosides, that is, linked to one or more sugar units.
The most common sugars present in these natural pigments are:

  • glucose, the most common
  • fructose
  • galactose
  • xylose
  • arabinose
  • rhamnose

The sugars are linked mainly to the C3 position as 3-monoglycosides, to the C3 and C5 positions as diglycosides (with the possible forms: 3-diglycosides, 3,5-diglycosides, and 3-diglycoside-5-monoglycosides). Glycosylations have been also found at C7, C3 ‘and C5’ positions.
The structure of these molecules is further complicated by the bond to the sugar unit of different acyl substituents such as:

  • aliphatic acids, such as acetic, malic, succinic and malonic acid;
  • cinnamic acids (aromatic substituents), such as sinapic, ferulic and p-coumaric acid;
  • finally, there are pigments with both aromatic and aliphatic substituents.
Antocyanins
Fig. 2 – Antocyanins

Furthermore, some anthocyanins have several acylated sugars in the molecule; these anthocyanins are sometimes called polyglycosides.

Depending on the type of hydroxylation, methoxylation and glycosylation patterns, and the different substituents linked to the sugar units, more than 500 different anthocyanins have been identified that are based on 31 anthocyanidins. Among these 31 monomers:

  • 30% are from cyanidin;
  • 22% are from delphinidin;
  • 18% are from pelargonidin.

Methylated derivatives of cyanidin, delphinidin and pelargonidin, namely peonidin, malvidin, and petunidin, all together represent 20% of the anthocyanins.
Therefore, up to 90% of the most frequently encountered anthocyanins are related to delphinidin, pelargonidin, cyanidin, and their methylated derivatives.

Anthocyanins and pH

The color of these molecules is influenced by the pH of the vacuole where they are stored, ranging in color from:

  • red, under very acidic conditions;
  • to purple-blue, in intermediate pH conditions;
  • until yellow-green, in alkaline conditions.

In addition to the pH, the color of these flavonoids can be affected by the degree of hydroxylation or methylation pattern of the A and B rings, and by glycosylation pattern.
Finally, the color of certain plant pigments result from complexes between anthocyanins, flavones and metal ions.
It should be noted that anthocyanins are often used as pH indicators thanks to the differences in chemical structure that occur in response to changes in pH.

References

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

de Pascual-Teresa S., Moreno D.A. and García-Viguera C. Flavanols and anthocyanins in cardiovascular health: a review of current evidence. Int J Mol Sci 2010;11:1679-1703 [Abstract]

Escribano-Bailòn M.T., Santos-Buelga C., Rivas-Gonzalo J.C. Anthocyanins in cereals. J Chromatogr A 2004:1054;129-141 [Abstract]

Han X., Shen T. and Lou H. Dietary polyphenols and their biological significance. Int J Mol Sci 2007;9:950-988 [Abstract]

Manach C., Scalbert A., Morand C., Rémésy C., and Jime´nez L. Polyphenols: food sources and bioavailability. Am J Clin Nutr 2004;79(5):727-47 [Abstract]

Ottaviani J.I., Kwik-Uribe C., Keen C.L., and Schroeter H. Intake of dietary procyanidins does not contribute to the pool of circulating flavanols in humans. Am J Clin Nutr 2012;95:851-8 [Abstract]

Tsao R. Chemistry and biochemistry of dietary polyphenols. Nutrients 2010;2:1231-1246 [Abstract]

Proanthocyanidins: definition, structure and absorption

What are proanthocyanidins?

Proanthocyanidins or condensed tannins, also calledpycno-genols and leukocyanidins, are polyphenolic compounds (in particular they are a flavonoid subgroup) widely distributed in the plant kingdom, second only to lignin as the most abundant phenol in nature.
They are present in high concentrations in various parts of the plants such as flowers, fruits, berries, seeds (e.g. in grape seeds), and bark (e.g. pine bark).

Together with anthocyanins and their oxidation products, and catechins, they are the most abundant flavonoids in human diet and it has been suggested that they constitute a significant fraction of the polyphenols ingested in the Western diet.
Therefore, condensed tannins should be taken into consideration when the epidemiological association between the intake of polyphenols, especially flavonoids, and chronic diseases are examined.

Chemical structure of proanthocyanidins

Proanthocyanidins
Fig. 1 – Procyanidin Skeleton

Condensed tannins have a complex chemical structure being oligomers (dimers to pentamers) or polymers (six or more units, up to 60) of catechins or flavanols, which are joined by carbon-carbon bonds.
They may consist exclusively of:

  • (epi)catechin, and they are named procyanidins;
  • (epi)afzelechin, and they are named propelargonidins;
  • (epi)gallocatechin, and they are named prodelphinidins.

Propelargonidins and prodelphinidins are less common in nature and in foods than procyanidins.

Depending on the bonds between monomers, proanthocyanidins have a:

  • B-type structure, if the polymerization occurs via carbon-carbon bond between the position 8 of the terminal unit and the 4 of the extender (or C4-C6);
  • A-type structure, less frequent, if monomers are doubly linked via an ether bond C2-O-C7 or C2-O-C5 plus a B-type bond.

Procyanidins

Proanthocyanidins
Fig. 2 – Procyanidins B1-B4

The most common dimers are B-type procyanidins, B1 to B8, formed by catechin or epicatechin; in B1, B2, B3 and B-4 dimers, the two flavanol units are joined by a C4-C8 bond; in B5, B6, B7 and B8 dimers the two units are joined by C4-C6 bond.

Procyanidin C1 is a B-type trimer.

Procyanidin A-2 is an example of A-type procyanidin.

Intestinal absorption of proanthocyanidins

Condensed tannins are poorly absorbed from the intestine; together with anthocyanins and gallic acid ester derivatives of tea catechins, they are the least well-absorbed polyphenols.
It seems that low molecular weight oligomers (2-3 monomers) may be absorbed as such while polymers are not.
In the systemic circulation, dimers reach concentrations of two orders of magnitude lower than those of catechins.
It seems that condensed tannins with a degree of polymerization greater than three transit into the stomach and small intestine without significant modifications, and then, into the large intestine, they are catabolized by colonic microflora, with production of phenylpropionic, phenilvaleric and phenylacetic acids. These degradation products have been suggested to be the major metabolites of proanthocyanidins in healthy humans.

Procyanidins and catechins

It had been proposed that the catabolism of procyanidins in the gastrointestinal tract lead to the release of monomeric catechins, thus indirectly contributing to their systemic pool in humans. In recent years, it has been shown that this does not happen because procyanidins do not significantly contribute to:

  • the concentration of catechin metabolites in the systemic circulation;
  • the total catechin metabolites excreted in the urine;
  • finally, they do not significantly affect plasma metabolite profile derived from catechol-O-methyltransferase activity.

Therefore, analyzing the potential health benefits associated with the intake of foods containing these phytochemicals, catechins and procyanidins should be considered distinct classes of related compounds.

References

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

Gu L., Kelm M.A., Hammerstone J.F., Beecher G., Holden J., Haytowitz D., Gebhardt S., and Prior R.L. Concentrations of proanthocyanidins in common foods and estimations of normal consumption. J Nutr 2004;134(3):613-617 [Abstract]

Han X., Shen T. and Lou H. Dietary polyphenols and their biological significance. Int J Mol Sci 2007;9:950-988 [Abstract]

Manach C., Scalbert A., Morand C., Rémésy C., and Jime´nez L. Polyphenols: food sources and bioavailability. Am J Clin Nutr 2004;79(5):727-47 [Abstract]

Nandakumar V., Singh T., and Katiyar S.K. Multi-targeted prevention and therapy of cancer by proanthocyanidins. Cancer Lett 2008;269(2):378-387 [Abstract]

Ottaviani J.I., Kwik-Uribe C., Keen C.L., and Schroeter H. Intake of dietary procyanidins does not contribute to the pool of circulating flavanols in humans. Am J Clin Nutr 2012;95:851-8 [Abstract]

Santos-Buelga C. and Scalbert A. Proanthocyanidins and tannin-like compounds: nature, occurrence, dietary intake and effects on nutrition and health. J Sci Food Agr 2000;80(7):1094-1117 [Abstract]

Tsao R. Chemistry and biochemistry of dietary polyphenols. Nutrients 2010;2:1231-1246 [Abstract]

Wang Y.,Chung S., Song W.O., and Chun O.K. Estimation of daily proanthocyanidin intake and major food sources in the U.S. diet. J Nutr 2011;141(3):447-452 [Abstract]

Catechins: definition, structure, green tea, black tea, cocoa

What are catechins?

Catechins or flavanols, with flavonols such as quercetin, and flavones such as luteolin, are a subgroup of flavonoids among the most widespread in nature.
Flavanols and proanthocyanidins, together with anthocyanins and their oxidation products, are the most abundant flavonoids in human diet.

Chemical structure of catechins

Basic Skeleton of Catechins
Fig. 1 – Basic Flavanol Skeleton

Chemically they differ from many other flavonoids as:

  • they lack the double bond between positions 2 and 3 of the C ring;
  • they not have a keto group at position 4;
  • they have a hydroxyl group in position 3, and for this reason they are also called flavan-3-ols.

Another distinctive feature of flavan-3-ols is their ability to form oligomers (two to ten units) or polymers (eleven or more units, up to 60 units) called proanthocyanidins or condensed tannins.

Catechins in foods

Catechins
Fig. 2 – Flavanols

Flavanols commonly found in plant-derived food products are catechin, epicatechin, gallocatechin, epigallocatechin, and their gallic acid ester derivatives: catechin gallate, gallocatechin gallate, epicatechin gallate, and epigallocatechin gallate (EGCG).
Flavanols present with higher frequency are catechin and epicatechin, which are also among the most common known flavonoids, and almost as popular as the related flavonol quercetin.
Cocoa and green tea are by far the richest sources in flavanols. In these foods the main flavonoids are catechin and epicatechin (cocoa is also a good source of epigallocatechin), but also their gallic acid ester derivatives, the gallocatechins.
However, they are also present in many fruits, especially in the skins of apples, blueberries (Vaccinium myrtillus) and grapes, in vegetables, red wine and beer, and peanuts.
As in many cases flavanols are present in the seeds or peels of fruits and vegetables, their intake may be limited by the fact that these parts are discarded during processing or while eaten.
Furthermore, in contrast to other flavonoids, catechins are not glycosylated in foods.
Proanthocyanidins, that is polymeric flavan-3-ols, are also commonly found in plant-derived food products. Their presence has been reported in the skin of peanuts and almonds, as in the berries.

Catechins in green tea and black tea

Green tea is an excellent source of flavonoids. The main flavonoids present in the leaves of the tea (as in cocoa beans) are catechin and epicatechin, monomeric flavanols, together with their gallate derivatives such as epigallocatechin gallate (EGCG).
Epigallocatechin gallate (EGCG) is the most abundant catechin in green tea and it seems to have an important role in determining green tea benefits, as the reduction of:

  • vascular inflammation;
  • blood pressure;
  • concentration of oxidized LDL.

Black tea (fermented tea) contains fewer monomeric flavanols, as they are oxidized during fermentation of the leaves to more complex polyphenols such as theaflavins (theaflavin digallate, theaflavin-3-gallate, and theaflavin-3′-gallate, all dimers) and thearubigins (polymers).
Theaflavins and thearubigins are present only in the tea; their concentrations in brewed tea are between 50- and 100-folds lesser than in tea leaves.

It should be noted that tea epicatechins are remarkably stable to heat in acidic environment: at pH 5, only about 15% is degraded after seven hours in boiling water (therefore, adding lemon juice to brewed tea does not cause any reduction in their content).

Catechins in cocoa and cocoa products

Cocoa has the highest content of polyphenols and flavanols per serving, a concentration greater than those found in green tea and red wine. Most of the flavonoids present in cocoa beans and derived products, such as black chocolate, are catechin and epicatechin, monomeric flavanols, but also epigallocatechin, and their derivatives such as the gallocatechins; among polymers, proanthocyanidins are also important.

Catechins in fruits, vegetables, and legumes

Catechin and epicatechin are the main flavanols in fruits. They are found in many fruits in different concentrations, respectively, between 5-3 and 0.5-6 mg/100 g fresh weight.
On the contrary, gallocatechin, epicatechin gallate, epigallocatechin, and epigallocatechin gallate (EGCG) are present in various fruits such as red grapes, berries, apples, peaches and plums, but in very low concentrations, less than 1mg/100 g fresh weight.
Except for lentils and broad beans, few legumes and vegetables contain catechins, and in very low concentrations, less than 1.5 mg/100 g fresh weight.

References

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

de Pascual-Teresa S., Moreno D.A. and García-Viguera C. Flavanols and anthocyanins in cardiovascular health: a review of current evidence. Int J Mol Sci 2010;11:1679-1703 [Abstract]

Han X., Shen T. and Lou H. Dietary polyphenols and their biological significance. Int J Mol Sci 2007;9:950-988 [Abstract]

Manach C., Scalbert A., Morand C., Rémésy C., and Jime´nez L. Polyphenols: food sources and bioavailability. Am J Clin Nutr 2004;79(5):727-47 [Abstract]

Tsao R. Chemistry and biochemistry of dietary polyphenols. Nutrients 2010;2:1231-1246 [Abstract]