Polyphenols: food sources, properties and applications – a review
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There is currently much interest in phytochemicals as bioactive compounds of food. The roles of fruit, vegetables and red wine in disease prevention have been attributed, in part, to the antioxidant properties of their constituent polyphenols (vitamins E and C, and the carotenoids). Recent studies have shown that many dietary polyphenolic constituents derived from plants are more effective antioxidants in vitro than vitamins E or C, and thus might contribute significantly to the protective effects in vivo. Polyphenols are abundant micronutrients in our diet, and evidence for their role in the prevention of degenerative diseases is emerging. Dietary polyphenols show a great diversity of structures, ranging from rather simple molecules (monomers and oligomers) to polymers. Higher-molecular-weight structures (with molecular weights of ∼500) are usually designated as tannins, which refers to their ability to interact with proteins. Among them, condensed tannins (proanthocyanidins) are particularly important because of their wide distribution in plants and their contributions to major food qualities. This paper focuses on polyphenols; we illustrate their sources from food, properties and their beneficial uses.
Phytochemicals can be defined, in the strictest sense, as chemicals produced by plants. However, the term is generally used to describe chemicals from plants that may affect health, but are not essential nutrients. While there is ample evidence to support the health benefits of diets rich in fruits, vegetables, legumes, whole grains and nuts, evidence that these effects are due to specific nutrients or phytochemicals is limited. Because plant-based foods are complex mixtures of bioactive compounds, information on the potential health effects of individual phytochemicals is linked to information on the health effects of foods that contain those phytochemicals.
Polyphenols are secondary compounds widely distributed in the plant kingdom. They are divided into several classes, i.e. phenolic acids (hydroxybenzoic acids and hydroxycinnamic acids), flavonoids (flavonols, flavones, flavanols, flavanones, isoflavones, proanthocyanidins) stilbenes, and lignans, which are distributed in plants and food of plant origin (Manach et al., 2004, 2005). Phenolics are an important constituent of fruit quality because of their contribution to the taste, colour and nutritional properties of fruit (Cheynier, 2005).
There is evidence that phenolic substances act as antioxidants by preventing the oxidation of LDL-lipoprotein, platelet aggregation, and damage of red blood cells (Cheynier, 2005). Additionally, phenolics act as: (i) metal chelators, (ii) antimutagens and anticarcinogens, (iii) antimicrobial agents and (iv) clarifying agents (Proestos et al., 2005). They are responsible for red wine colour, astringency and bitterness and contribute to its sensory profile (Lesschaeve & Noble, 2005). They are derived from the fruit and vine stems, or by the yeast metabolism. In addition, phenolics serve as important oxygen reservoirs and substrates for browning reactions.
The types and distribution of polyphenols in foods and also their properties and applications are the topics of the present review.
Polyphenols in plants
Phenolic compounds are commonly found in both edible and non edible plants, and they have been reported to have multiple biological effects, including antioxidant activity. Crude extracts of fruits, herbs, vegetables, cereals, and other plant materials rich in phenolics (Table 1) (Manach et al., 2004) are increasingly of interest in the food industry because they retard oxidative degradation of lipids and thereby improve the quality and nutritional value of food (Kähkönen et al., 1999). These phenolic compounds may be classified into different groups as a function of the number of phenol rings that they contain and of the structural elements that bind these rings to one another. Distinctions are thus made between the phenolic acids, flavonoids, stilbenes, and lignans (Fig. S1).
Table 1. Polyphenol content in foodsa
| Quercetin||Curly kale||300–600|
| Myricetin||Cherry tomato||15–200|
| ||Black currant||30–70|
| ||Beans, green or white||10–50|
| ||Black grape||15–40|
| ||Black tea infusion||30–45|
| ||Green tea infusion||20–35|
| ||Red wine||2–30|
| Luteolin||Capsicum pepper||5–10|
| Hesperetin||Grapefruit juice||100–650|
| Daidzein||Soybeans, boiled||200–900|
| ||Soy milk||30–175|
| ||Green tea||100–800|
| ||Black tea||60–500|
| ||Red wine||80–300|
| Pelargonidin||Black currant||1300–4000|
| Delphinidin||Black grape||300–7500|
| ||Red wine||200–350|
| ||Red cabbage||250|
| Protocatechuic acid||Raspberry||60–100|
| Gallic acid||Black currant||40–130|
| p-Hydroxybenzoic acid||Strawberry||20–90|
| Caffeic acid||Kiwi||600–1000|
| Chlorogenic acid||Cherry||180–1150|
| Coumaric acid||Plum||140–1150|
| Ferulic acid||Aubergine||600–660|
| Sinapic acid||Apple||50–600|
| ||Corn flour||310|
| ||Flour: wheat, rice, oat||70–90|
Flavonoids are molecules with a phenolic benzopyran structure and occur only in plants where they are present predominantly as glycosides. The flavonoids may themselves be divided into six subclasses as a function of the type of heterocycle involved: flavonols, flavones, isoflavones, flavanones, anthocyanidins, and flavanols (Fig. S2). Flavonoids and other plant phenolics, such as phenolic acids, stilbenes, tannins and lignans are especially common in leaves, flowering tissues, and woody parts such as stems and barks (Kähkönen et al., 1999). They are important in the plant for normal growth development and defense against infection and injury. Flavonoids also partly provide plant colours present in flowers, fruits, and leaves. Plant polyphenols comprise a great diversity of compounds, among which flavonoids and several classes of nonflavonoids are usually distinguished (Harborne, 1989). The latter are mostly rather simple molecules, such as phenolic acids and stilbenes, but also include complex molecules derived from them (e.g. stilbene oligomers, gallotannins, ellagitannins, and lignins). The former of flavonoids (Fig. S2) share a common nucleus consisting of two phenolic rings and an oxygenated heterocycle. More than 4000 flavonoids have been identified in plants, and the list is constantly growing (Harborne & Williams, 2000). Recently, a study has been performed on determination of oil palm fruit phenolic compounds and their antioxidant activities using spectrophotometric methods (Neo et al., 2008). Their analyses of the total phenolic (TPC), total flavonoid (TFC), o-diphenols index, hydroxycinnamic acid index contents, showed ranges between 5.64 and 83.97 g L−1 gallic acid equivalent (GAE), 0.31–7.53 g L−1 catechin equivalent, 4.90–93.20 g L−1 GAE, 23.74–77.46 g L−1 ferulic acid equivalent, respectively) (Neo et al., 2008). While in Turkish plants, the total phenol content of plant extracts ranged between 117.20 and 1.27 mg of gallic acid equivalents per g dw (Kırca & Arslan, 2008). There was a positive linear correlation between the TEAC and total phenols of plant materials (r = 0.916). Authors reported also that the extracts of Hypericum perforatum, Arbutus andrachne and Paliurus spina-christii showed higher antioxidant activities (both TEAC and DPPH assays) (Kırca & Arslan, 2008).
Flavonols are the most ubiquitous flavonoids in foods, and the main representatives are kaempferol and quercetin (Manach et al., 2005). Considered as the most abundant dietary flavonol, quercetin is a potent antioxidant because it has all the right structural features for free radical scavenging activity. Flavonols are generally present at relatively low concentrations of ∼15–30 mg kg−1 fresh weight. The richest sources are onions (up to 1.2 g kg−1 fresh weight), curly kale, leeks, broccoli, and blueberries (Manach et al., 2004). Red wine and tea also contain up to 45 mg flavonols L−1. These compounds are present in glycosylated forms. The associated sugar moiety is very often glucose or rhamnose, but other sugars may also be involved (e.g. galactose, arabinose, xylose, glucuronic acid). In oil palm fruit, analyses of flavonols and phenol index showed ranges between 3.62–95.33 g L−1 rutin equivalent and 15.90–247.22 g L−1 GAE, respectively (Neo et al., 2008).
In fruit and vegetables flavones are much less common than flavonols. Flavones consist chiefly of glycosides of luteolin and apigenin. The only important edible sources of flavones identified to date are parsley and celery (Manach et al., 2005). Cereals such as millet and wheat contain C-glycosides of flavones (Boyle et al., 2000; Erlund et al., 2000; Graefe et al., 2001). Large quantities of polymethoxylated flavones: tangeretin, nobiletin, and sinensetin (up to 6.5 g L−1 of essential oil of mandarin) have been identified on the skin of citrus fruit (Nielsen et al., 2003). These polymethoxylated flavones are the most hydrophobic flavonoids.
In human foods, flavanones are found in tomatoes and certain aromatic plants such as mint, but they are present in high concentrations only in citrus fruit. The main aglycones (the non sugar component that results from hydrolysis of glycoside flavanones) are naringenin in grapefruit, hesperetin in oranges, and eriodictyol in lemons. Flavanones are generally glycosylated by a disaccharide at position seven: either a neohesperidose, which imparts a bitter taste (such as to naringin in grapefruit), or a rutinose, which is flavourless. Orange juice contains between 200 and 600 mg hesperidin L−1 and 15–85 mg narirutin L−1, and a single glass of orange juice may contain between 40 and 140 mg flavanone glycosides (Hertog et al., 1993). Because the solid parts of citrus fruit, particularly the albedo (the white spongy portion) and the membranes separating the segments, have a very high flavanone content, the whole fruit may contain up to five times as much as a glass of orange juice.
Isoflavones are provided only by soybean-derived products. They can be present as aglycones or glycosides, depending on the soy preparation. Isoflavones are found almost exclusively in leguminous plants. Soya and its processed products are the main source of isoflavones in the human diet. The isoflavone content of soya and its manufactured products varies greatly as a function of geographic zone, growing conditions, and processing. Soybeans contain between 580 and 3 800 mg isoflavones kg−1 fresh wt, and soymilk contains between 30 and 175 mg L−1 (Hollman et al., 1996; Moon et al., 2000).
Flavanols exist in both the monomer form (catechins) and the polymer form (proanthocyanidins). Catechins are found in many types of fruit (apricots, which contain 250 mg kg−1 fresh weight, are the richest source) (Manach et al., 2005). They are also present in red wine (up to 300 mg L−1), but green tea and chocolate are by far the richest sources. An infusion of green tea contains up to 200 mg catechins (Day et al., 2001). Black tea contains fewer monomer flavanols, which are oxidised during ‘fermentation’ (heating) of tea leaves to more complex condensed polyphenols known as theaflavins (dimers) and thearubigins (polymers).
Catechin and epicatechin are the main flavanols in fruit, whereas gallocatechin, epigallocatechin, and epigallocatechin gallate are found in certain seeds of leguminous plants, in grapes, and more importantly in tea (Manach et al., 1998; Wittig et al., 2001). Referring to recent studies, flavanols are the major polyphenols in apples, accounting for 65–85% of total polyphenol contents in the dessert (Guyot et al., 2002) and cider (Guyot et al., 2003) varieties analysed.
Proanthocyanidins, which are also known as condensed tannins, are dimers, oligomers, and polymers of catechins that are bound together by links between C4 and C8 (or C6). Proanthocyanidins are the major polyphenols in grapes, where they are localised mostly in skins and seeds. Seed proanthocyanidins are partly galloylated procyanidins, with degrees of polymerisation in the range of 1 (monomers) to ∼20 (Prieur et al., 1994). Skin proanthocyanidins contain both procyanidin and prodelphinidin units and are much larger than skin tannins (∼30 units, on average) (Souquet et al., 1996).
Through the formation of complexes with salivary proteins, condensed tannins are responsible for the astringent character of fruit (grapes, peaches, kakis, apples, pears, berries, etc.) and beverages (wine, cider, tea, beer, etc.) and for the bitterness of chocolate (Baba et al., 1981). This astringency changes over the course of maturation and often disappears when the fruit reaches ripeness (Aura et al., 2002).
Two classes of phenolic acids can be distinguished: derivatives of benzoic acid and derivatives of cinnamic acid (Fig. S1). Hydroxybenzoic acids are components of complex structures such as hydrolyzable tannins (gallotannins in mangoes and ellagitannins in red fruit such as strawberries, raspberries, and blackberries) (Manach et al., 2004). The hydroxycinnamic acids are more common than are the hydroxybenzoic acids and consist chiefly of p-coumaric, caffeic, ferulic, and sinapic acids. These acids are rarely found in the free form, except in processed food that has undergone freezing, sterilisation, or fermentation. The bound forms are glycosylated derivatives or esters of quinic acid, shikimic acid, and tartaric acid. Caffeic and quinic acid combine to form chlorogenic acid, which is found in many types of fruit and in high concentrations in coffee: Authors reported that a single cup may contain 70–350 mg chlorogenic acid (Clifford, 1999). The types of fruit having the highest content (blueberries, kiwis, plums, cherries, apples) contain 0.5–2 g hydroxycinnamic acids kg−1 fresh weight (Macheix et al., 1990).
Caffeic acid, both free and esterified, is generally the most abundant phenolic acid and represents between 75% and 100% of the total hydroxycinnamic acid content of most fruit. Hydroxycinnamic acids are found in all parts of fruit, although the highest concentrations are seen in the outer parts of ripe fruit. Concentrations generally decrease during the course of ripening, but total quantities increase as the fruit increases in size.
Ferulic acid is the most abundant hydroxycinnamic acid found in cereal grains, which constitute its main dietary source. When present in free form in tomatoes or beer, it is efficiently absorbed (Bourne & Rice-Evans, 1998; Bourne et al., 2000). However, ferulic acid is also the main polyphenol present in cereals, in which it is esterified to the arabinoxylans of the grain cell walls. The ferulic acid content of wheat grain is ∼0.8–2 g kg−1 dry wt, which may represent up to 90% of total polyphenols (Sosulski et al., 1982; Lempereur et al., 1997). It is found chiefly in the outer parts of the grain.
Lignans are formed of two phenylpropane units (Fig. S1). The richest dietary source is linseed, which contains secoisolariciresinol (up to 3.7 g kg−1 dry wt) and low quantities of matairesinol. Other cereals, grains, fruit, and certain vegetables also contain traces of these same lignans, but concentrations in linseed are ∼1 000 times as high as concentrations in these other food sources (Adlercreutz & Mazur, 1997). Lignans are metabolised to enterodiol and enterolactone by the intestinal microflora. Thus, there are undoubtedly other lignans of plant origin that are precursors of enterodiol and enterolactone and that have not yet been identified (Heinonen et al., 2001). Thompson et al. (Thompson et al., 1991) used an in vitro technique involving the fermentation of foods by human colonic microflora to quantitatively evaluate precursors of enterodiol and enterolactone. They confirmed that oleaginous seeds (linseed) are the richest source and identified algae, leguminous plants (lentils), cereals (triticale and wheat), vegetables (garlic, asparagus, carrots), and fruit (pears, prunes) as minor sources.
Stilbenes are found in only low quantities in the human diet. One of these, resveratrol, for which anticarcinogenic effects have been shown during screening of medicinal plants and which has been extensively studied, is found in low quantities in wine (0.3–7 mg aglycones L−1 and 15 mg glycosides L−1 in red wine) (Bertelli et al., 1998; Bhat & Pezzuto, 2002; Vitrac et al., 2002). However, because resveratrol is found in such small quantities in the diet, any protective effect of this molecule is unlikely at normal nutritional intakes.
Properties of food polyphenols
The beneficial effects of polyphenols are mainly attributed to their antioxidant properties, since they can act as chain breakers or radical scavengers depending on their chemical structures (Rice-Evans, 2001). Polyphenols might also trigger changes in the signalling pathways and subsequent gene expression (Chen et al., 2002; Pfeilschifter et al., 2003). It is possible that the distinct chemical and receptor-mediated activities of polyphenols might result in similar outcomes via different pathways (Weiss & Landauer, 2003). Under some circumstances, polyphenols can exhibit pro-oxidative effects.
Landbo and Meyer reported that red grape juice concentrate inhibited lipid peroxidation of LDL by prolonging the lag phase by 2.7 times relative to a control when evaluated at a total phenolic concentration of 10 μm gallic acid equivalents (GAE) (Landbo & Meyer, 2001). They also confirmed that red grape juices tested blocked lipid peroxidation of LDL at 20 lMGAE. While white grape juice exerted pro-oxidant activity at 5 ± 20 μm GAE (Landbo & Meyer, 2001).
Depending on their particular structures, polyphenols exhibit a wide range of properties. They include yellow, orange, red, and blue pigments, as well as various compounds involved in food flavour. Some volatile polyphenols, such as vanillin and eugenol (which is responsible for the characteristic odour of cloves), are extremely potent odorants, but the major flavours associated with polyphenols are bitterness and astringency. Other major polyphenol characteristics include their radical-scavenging capacity, which is involved in antioxidant properties, and their ability to interact with proteins. The latter is responsible for astringency perception (resulting from interactions of tannins with salivary proteins), for formation of haze and precipitates in beverages, and for inhibition of enzymes and reduced digestibility of dietary proteins.
Catechin, epicatechin and gallates of epicatechin are major catechins with dietary importance for human health. In recent years, catechins have been used as natural antioxidant in oils and fats against lipid oxidation, supplement for animal feeds both to improve animal health and to protect animal products, as antimicrobial agent in foodstuffs and as health functional ingredient in various foods and dietary supplements (Yilmaz, 2006). In recent years it has been supposed that flavonoids are mainly responsible for antioxidant anticarcinogenic and antiarteriosclerotic actions of tea (Wang et al., 2000). It has been also demonstrated that the increased consumption of mint, tea or tea enriched with mint may contribute to the improvement in quality of healthy life by increasing the antioxidant defence and delaying the onset of various degenerative diseases caused by oxidative stress (Padmini et al., 2008)
Major polyphenol pigments in plants are anthocyanins, the yellow flavonols and flavones. Anthocyanins are highly reactive species. Conversion of genuine anthocyanins to other molecules during the course of food processing results in either loss or stabilisation of colour and increases the range of available hues.
Jakobek et al., 2009 have reported that anthocyanins contributed more to the antioxidant activity of all fruits (∼90%) than flavonols, flavan-3-ols and phenolic acids (∼10%) (Jakobek et al., 2009). In this study, authors reported also that sour cherries and blackberries which stand out with the highest total phenol content (1416 and 1040 mg kg−1) had also the strongest antioxidant activity (EC50 = 807 and 672 g of fruit per gram of 1,1 diphenyl-2-picrylhydrazyl (DPPH) and can be considered as good source of dietary phenols (Jakobek et al., 2009). Authors have revealed great differences in the phenol composition of the investigated fruits; they reported that this diversity in the phenol composition between the fruits may be related to different biological activities (Jakobek et al., 2009). In all fruits investigated in this study, the contribution of anthocyanins to antioxidant activity was higher than the contribution of flavonols, flavan-3-ols and phenolic acids (Jakobek et al., 2009). Authors concluded that the anthocyanins can be regarded as major phenolic antioxidants of these fruits. The antioxidant activity was influenced by a high phenol content, and by the presence of stronger phenolic antioxidants (catechins, ellagic acid, and cyanidin-3-glucoside) (Jakobek et al., 2009).
Other major polyphenol properties, such as the ability to complex with proteins and free radical-scavenging capacity, are primarily related to the number and accessibility of phenol (in particular, o-diphenol) moieties. The oxygen radical-scavenging capacity of procyanidin dimers and trimers was shown to increase with galloylation and to a lesser extent with longer chain length but was also influenced by the position of galloyl substituents (Ricardo da Silva et al., 1991). Similar results were obtained for scavenging of the radical cation 2,2′-azinobis (3-ethyl-benzothiazoline-6-sulfonate) in the aqueous phase, but antioxidant activity decreased from the trimer to the tetramer. These conflicting results suggest that antioxidant effects are exerted through different mechanisms in the different assays. The affinity of proanthocyanidins for proteins (Ricardo da Silva et al., 1991) and their astringency (Vidal et al., 2003) increase with both the degree of polymerisation and the extent of galloylation. Complex transformation products of plant polyphenols can be similarly expected to act as radical scavengers and bind to proteins.
This was classically ascribed to an increase in molecular weight, because larger tannins were thought to be insoluble and thus non-astringent. However, recent studies showed that higher molecular-weight proanthocyanidins are both soluble and more astringent than the oligomeric proanthocyanidins (Vidal et al., 2003). Consequently, the decrease in astringency observed during wine aging is likely to involve acid-catalysed processes leading to lower molecular-weight species, as described above, rather than polymerisation reactions. However, the taste of polyphenol reactions products and the effect on astringency of incorporating anthocyanin units into a tannin structure remain to be investigated.
Properties of polyphenols are also greatly affected by their interactions with other constituents of the food matrix. The astringency of tannins may also be altered by the presence of various molecules, including polysaccharides and proteins.
Finally, strong interactions with other constituents of the food matrix are likely to interfere with the metabolism of polyphenols and should be taken into account in bioavailability studies (Cheynier, 2005). Indeed, interactions of polyphenols with food proteins and digestive enzymes are well known to reduce protein digestibility and can be expected to alter polyphenol bioavailability similarly (Cheynier, 2005). Furthermore, as it was reported recently, domestic processing, such as cooking in boiling water, seems to have a dramatic effect on phenolic content on both kinds of food, and, as a consequence, on antioxidant activity (Lima et al., 2009).
The principal sources of polyphenols are fruits and beverages such as tea, red wine, and coffee, but vegetables, leguminous plants, and cereals are also good sources. The beneficial effects of polyphenols have been ascribed to their strong antioxidant activity that is, their ability to scavenge oxygen radicals and other reactive species. These features make phenols a potentially interesting material for the development of functional foods or possible therapy for the prevention of some diseases. The health effects of polyphenols depend on both their respective intakes and their bioavailability, which can vary greatly. Numerous genetic, environmental, and technologic factors may affect the polyphenol concentrations in food, some of which can be controlled to optimise the polyphenol content of foods. One of the possibilities for phenolic compounds increase in fruit juices may be the fruit cultivar selection, as cultivars differ greatly in their phenolics content. The strong potential may also be the processing technology itself which must be taken into consideration.
I’m grateful to Prof. Antonio Piga (Università degli Studi di Sassari, Italy) for his assistance and help; it was very helpful and encouraging. This article is dedicated to the memory of my dear father.