Beer is a very popular alcoholic beverage, selling more than 133 billion liters per year. It is produced through the fermentation of starch-based material, commonly barley (Hordeum vulgare L.). The beer brewing process comprises several steps. Malting is a process applied to cereal grains, in which the grains are made to germinate and then are quickly dried before the plant develops. Malting grains develops enzymes that are required to modify the grains starches into sugars. Barley is the most common malt because of its high amylase content. Mashing is the first phase of brewing, in which the malted grains are crushed and soaked in warm water in order to create a malt extract. During sparging, water is filtered through the mash to dissolve the sugars. The darker, sugar-heavy liquid is called the wort. The wort is boiled along with any remaining ingredients (excluding yeast), to remove excess water and kill microorganisms. The hops (Humulus lupulus L.) are added during boiling. The yeast is added and the beer is left for fermentation. Then, the beer is bottled and left for CO2 levels to increase. Because the ingredients and procedures used to make beer can differ, characteristics such as taste and colour may vary between different beers.

 A common method of categorizing beer is by the type of yeast used in the fermentation process. In this method of categorizing, those beers with fast acting (top-fermenting) yeast, which leaves behind residual sugars, are termed ‘ales’. Beers with a slower and longer acting (bottom-fermenting) yeast, which removes most of the sugars leaving a clean and dry beer, are termed ‘lagers’. For Lambic beers wild yeasts are used, rather than cultivated ones. In terms of volume, most of today's beer is based on the pale-coloured lager first brewed in 1842 in the town of Pilsen (lager pilsner beer). Furthermore, dark beers, also called ‘stout’, exist in a sweet and in a bitter variant. Another more recent type of beer is alcohol-free beer. Alcohol is removed by (vacuum) distillation, vacuum evaporation, dialysis and reverse osmosis (37). In the food composition table, the beer types distinguished are regular beer (various lager and lager pilsner types of beer), dark beer, ale beer and alcohol free beer. Beer contains a wide variety of polyphenols and classes. However, content values for each of the single polyphenols are rather low. Nevertheless polyphenols play an important role in beer flavour (bitterness, astringency, harshness), colour and beer stability. 

Total polyphenol content in beer, as measured by the Folin-method, varies between 12 and 52 mg/100 ml, depending on the beer type. Ale beer and dark beer are richer in polyphenols (52 and 42 mg/100 ml respectively). Regular beer contains about 28 mg/100 ml total polyphenols. Alcohol free beer contains about 12 mg/100 ml polyphenols.

 However, when comparing the total polyphenol value measured by Folin (28 mg/100 ml for regular beer) with the total polyphenol value calculated from the sum of the individual polyphenols measured by chromatographic methods (4.04 mg/100 ml for regular beer), the latter appears to be inferior. This could be explained in one part by the lack of data on beer polyphenols in the literature, and thus in the food composition table. It is difficult to measure proanthocyanidins by chromatographic methods, since higher oligomers (>trimer) tend to coelute as large unresolved peaks. Therefore proanthocyanidin contents measured by chromatography can be underestimated. Another explanation is the reactivity of the Folin assay with non-polyphenol reducing compounds, like Maillard products. The interference of Maillard reaction products present in beer can lead to an overestimation of total polyphenol values measured by Folin. Beer contains a variety of polyphenols belonging to the following classes: prenylated flavonoids, phenolic acids, simple phenols, flavanols, hydroxycoumarins, flavonols and flavones. Brewing materials such as barley and hop deliver polyphenols to the beer in the brewing process. Furthermore, polymerization and formation of polyphenols can occur during further processing and storage of beer.

Prenylflavonoids (alkylchalcones, alkylflavanones and alkylmethoxyflavanones) in beer are derived from hops. Prenylflavonoids are phytoestrogens, and contribute to the estrogenicity of beer. Content values of prenylflavonoids in 13 different beer types have been found to range from 0 to 0.4 mg/100 ml, depending on beer type and brewing processes (38). The prenylflavonoid patterns are similar in most beers, with isoxanthohumol, xanthohumol and 6-prenylnaringenin being most abundant (>95% of total prenylflavonoids). Minor amounts of 8-prenylnaringenin and 6-geranylnaringenin were also detected. Regular type beers have much lower contents of prenylflavonoids (0.05 mg/100 ml) than the more bitter tasting ale and dark beers (0.26 and 0.20 mg/100 ml respectively). Alcohol free beer contains only minor amounts of prenylflavonoids (0.01 mg/100 ml). Beer is one of the most important dietary sources of prenylflavonoids, since these are rare in other plant foods often consumed. Phenolic acids are present in beer. Ferulic acid (0.84 mg/ 100 ml), gallic acid (0.57 mg/100 ml), vanillic acid (0.15 mg/ 100 ml), p-coumaric acid (0.14 mg/ 100 ml) and sinapic acid (0.03 mg/ 100 ml) are most abundant, as measured after hydrolysis in regular beer. Ferulic acid, caffeic acid and sinapic acid are present in beer mainly as bound forms. 4-Hydroxyphenylacetic acid and p-coumaric acid are present mainly as free forms. Vanillic acid and syringic acid are present almost equally in the free and bound form (39, 40). During brewing, the cinnamic acids, ferulic acid and p-coumaric acid, are transformed into the flavour active volatile phenols 4-vinylphenol and 4-vinylguaiacol, either by thermal fragmentation or enzymatic activity of the yeast strains. Their ethyl-derivatives 4-ethylphenol and 4-ethylguaiacol can also be formed after refermentation (41). Contents of both 4-vinylphenol and 4-vinylguaiacol in dark beer are both 0.03 mg/100 ml. Tyrosol is present in beer (0.32 mg/100 ml in regular beer). Flavanol monomers and oligo- and polymers (proanthocyanidins) are of interest to brewers as they play a role in non-biological haze formation. Beer haze may be defined as an insoluble or semi soluble particulate matter formed by complexation of polyphenols and proteins, which gives a colloidal suspension in beer reducing the transparency of the beer (42, 43). Stabilization of beer to protect against haze formation can be achieved by decreasing the flavanol content and thereby limiting flavanol complexation with proteins. Often, this stabilization is achieved by treatment with polyvinylpolypyrrolidone (PVPP) which forms insoluble complexes with polyphenols and more particularly proanthocyanidins. Comparison of beers with and without PVPP treatment showed indeed much lower flavanol monomer and dimer concentrations in stabilized beer, and a delay of haze formation during storage (44, 45). Contents of (+)-catechin and (-)-epicatechin in regular beer are 0.11 and 0.06 mg/100 ml. Ale beer contains 0.33 mg/100 ml (+)-catechin and 0.05 (-)-epicatechin. Proanthocyanidins are more abundant than catechins. Procyanidin dimer B3 and prodelphinidin dimer B3 contents are 0.15 and 0.18 mg/100 ml in regular beer. Low contents of hydroxycoumarins have been described in beer. 4-Hydroxycoumarin and esculin are present in regular beer with respectively 0.11 mg/100 ml and 0.02 mg/100 ml. Umbelliferone is only found in traces. Some flavonols are also present in beer. Quercetin 3-O-rutinoside content is 0.09 mg/100 ml in regular beer. After hydrolysis, however, no quercetin has been detected. 

Next to the type of beer, and the treatment with PVPP, other factors responsible for the variability of polyphenol contents in beer are the ingredients and the processing, as well as aging or storage. Polyphenol contents in barley vary with variety and environmental conditions, and thus influence polyphenol contents in corresponding malts. Polyphenol contents in barleys are higher than in their corresponding malts (46). Flavanols are largely degraded during malting. The content of polyphenols in beer also depends on the extent of their extraction during mashing. Mashing of malt releases bound cinnamic acids. Furthermore, the extraction of polyphenols from malt into the wort depends on the malt-to-water ratios, where low ratios yield higher levels of polyphenols (7). 

During the boiling step of beer brewing, hop is added to the wort. The contribution of hop polyphenols to boiled wort depends on the time of addition. Late addition allows the extraction, but limits their subsequent degradation upon oxidation (47). Ale beers are more hopped than lagers. A large proportion of flavonols is extracted from hop: 88% of the quercetin glycosides and 91% of the kaempferol glycosides present in the hop are extracted in the wort during the boiling process (48). During the fermentation step the content of flavonol glycosides is decreased, but storage of beer for up to two months produces no further losses in flavonols. This could mean that the content of flavonols in beer largely depends on the rates of hop addition to worts during the beer brewing process. 

The prenylated flavonoids in hops are predominantly of the chalcone type. Hops only contain minor quantities of prenylated flavanones. In contrast, beers contain much higher levels of prenylflavanones than prenylchalcones. During the boiling of hops with wort, prenylated chalcones undergo cyclization into their isomeric flavanones. Losses of prenylflavonoids during wort boiling occur due to incomplete extraction from the hop into the wort, adsorption to insoluble malt proteins, and adsorption to yeast cells during fermentation (49). 

Concentrations of esterified ferulic acids were reported to be about 2 to 3-fold higher than the corresponding free ferulic acids in beer (50). After one month of storage, the concentrations of free and bound ferulic acids decreased with 10-18% and 2-10% respectively. Polyphenols from hop and malt may slow down flavour deterioration during storage, especially during the first four months of storage (51). 

Beer consumption in Europe varies between countries, with intakes of 25 l/capita/year in Italy to 120 l/capita/year in Denmark. One regular beer of 33 cl provides 92 mg polyphenols (Folin), and one unit (33 cl) of ale beer provides 173 mg polyphenols. Alcohol free beer provides 40 mg polyphenols per 33 cl. According to USDA statistics for 2000, the average person in the United States consumes 225 ml of beer per day. Assuming a regular beer is consumed, the average polyphenol intake (Folin) from beer is 63 mg per day.



Several Vitis species provide edible fruits. The most common is Vitis vinifera. According to cultivars, its fruits can be directly consumed as dessert grape or used to produce wine. Other grapes native from North America are Vitis labrusca (Fox grape), Vitis aestivalis, or Vitis rotundifolia (Muscadine grape). They are usually grown to be consumed as fruits or transformed into juice, jam, or jelly, and sometimes into wine. Grape cultivars differ by their color which is related to the nature of the polyphenols present in the fruit. Polyphenols from grape are mainly concentrated in the seeds (60%) and the skin (30%), and to a lower extent in the pulp and stems (less than 10%). Polyphenols are extracted from grape during vinification. They contribute to the sensory characteristics of wine (color, flavor, astringency). Red, rosé, and white wines are obtained from black or green grapes by different vinification methods, and the quantity of polyphenols in wine largely depends on the vinification process. During red vinification, the juice is fermented during 3 to 21 days in contact with the solid parts of the grape and polyphenols diffuse in the juice, whereas during white vinification, the juice is separated from the solid parts immediately after crushing the grape. The process for rosé wine is intermediate. Polyphenol content is therefore highest in red wine, and lowest in white wine.

The main polyphenols present in wine are phenolic acids, stilbenes, flavonols, dihydroflavonols, anthocyanins, flavanol monomers (catechins) and flavanol polymers (proanthocyanidins). They are not evenly distributed in the fruit. Phenolic acids are largely present in the pulp, anthocyanins and stilbenes in the skin, and other polyphenols (catechins, proanthocyanidins and flavonols) in the skin and the seeds. The proportion of the different polyphenols in wines will therefore vary according to the type of vinification. Red grape will be richer in polyphenols abundant in the skin and seeds, and polyphenols in white wine will essentially originate from the pulp. The main polyphenols present in white wine are therefore phenolic acids together with lower amounts of catechins and stilbenes. Red wine also contains anthocyanin pigments, proanthocyanidins as major polyphenols, together with flavonols and other polyphenols as present in white wine. Wine polyphenols are further transformed during wine aging into complex molecules formed notably by the condensation of proanthocyanidins and anthocyanins which explain the modification of the colour. Anthocyanins react with catechins, proanthocyanidins and other wine components during wine ageing to form new polymeric pigments resulting in a modification of the wine colour and a low astringency (52, 53). Average total polyphenol content measured by the Folin method is 216 mg/100 ml for red wine and 32 mg/100 ml for white wine. The content of polyphenols in rosé wine (82 mg/100 ml) is intermediate between red and white wines. 

Over 60 anthocyanins have been detected in red wines (54, 55). Content values in wine could be found in the literature for only 16 of them. Anthocyanins are detected at high levels in red wines (22.7 mg/100 ml). They are absent or present as traces in white wines (0.04 mg/100 ml). Anthocyanins in wine are either free (aglycones) or glucosylated. They derive from five anthocyanidins: malvidin (70% of total anthocyanidins), petunidin, peonidin, delphinidin, and cyanidin which are glucosylated in position 3. The glucose residue is itself often acylated on the carbon 6 with acetic acid and to a lower extent by caffeic and p-coumaric acid. The two major anthocyanins are malvidin 3-O-glucoside (45%) and malvidin 3-O-(6’’-acetyl-glucoside) (15%). Anthocyanins can also react in wine with flavanols, proanthocyanidins, pyruvic acid, caffeic acid, acetaldehyde, or vinylphenol derivatives to form complex polymeric pigments. These complex pigments are highly diverse in structure. Each of them is present in wine in very low quantity. Therefore, they cannot be easily quantified and only few content values are included in the database. Content values are given for vitisin A, vitisin B, pinotin A, and pigment A which correspond respectively to the C4 cyclo-addition of pyruvic acid, acetaldehyde, caffeic acid, and 4-vinylphenol to malvidin 3-O-glucoside. 

Wine also contains flavanols, including proanthocyanidins (polymers) and catechins (monomers). Proanthocyanidins (29.4 mg/100 ml as estimated by direct phase HPLC) and flavanol monomers (11.5 mg/100 ml) are particularly abundant in red wine. They are present in lower amount in rosé and white wine (respectively 1.7 and 2.0 mg/100 ml). The main flavanol monomers are (+)-catechin (6.8 mg/100 ml in red wine and 1.0 mg/100 ml in white wine) and (-)-epicatechin (3.8 mg/100 ml in red wine and 0.9 mg/100 ml in white wine). (+)-Catechin was more abundant than (-)-epicatechin in most of the samples analysed in the different literature sources (epicatechin:catechin ratio <1 in 80% of the samples). Wine proanthocyanidins are procyanidins, prodelphinidins, and mixed procyanidin-prodelphinidins, with mean degree of polymerisation (mDP) varying from 6.9 to 13.0 (56, 57). (-)-Epicatechin is the main constitutive unit of grape proanthocyanidins (56). Galloylated forms are less abundant than the non-galloylated forms (58, 59). A few individual proanthocyanidin molecules have been estimated by reverse phase HPLC. Procyanidin dimers B1, B2, B3 and B4 were quantified; their average total concentration is 25.9 mg/100 ml in red wine. Other dimers such as gallocatechin-4,8-gallocatechin, gallocatechin-4,8-catechin and catechin-4,8-gallocatechin are also present (60). Proanthocyanidins are particularly abundant in the seeds, which contain 40 to 80% of the total proanthocyanidins of the fruit. Proanthocyanidins are less abundant in skin but more easily transferred into the wine (56). Proanthocyanidin structures differ depending on their localisation in the grape. A higher mDP is observed in the skin (range 2 to 80) as compared to the seeds (range 2 to 20) (58). 

Flavonols are the third most abundant flavonoids in wine. Red and white wine contain respectively 6.98 and 0.48 mg/100 ml flavonols. The main flavonols are in decreasing order of abundance, quercetin, myricetin, isorhamnetin and kaempferol. Comparatively to other food products, red wine contains high proportions of free aglycones (20-50% of total flavonols) (61, 62). Only 3-O-glycosides of flavonols have been described in wine. Bound sugars are glucose, rhamnose or rutinose. The major flavonols in red wine are quercetin 3-O-rhamnoside (1.16 mg/100 ml) and quercetin 3-O-glucoside (1.14 mg/100 ml). 

Three dihydroflavonols have been identified in grape or wine. Dihydroquercetin 3-O-rhamnoside (astilbin) and dihydrokaempferol 3-O-rhamnoside (engeletin) were identified for the first time in the skin and in the wine from white grapes (63). Astilbin was also identified in the stems of red and white grapes (64). Two dihydroflavonols were quantified in wine. Astilbin contents are respectively 0.97, 0.38 and 0.27 mg/100 ml in red, rosé and white wine. Dihydromyricetin 3-O-rhamnoside contents are respectively 4.47 and 0.30 mg/100 ml in red and white wine. Engeletin and some other additional compounds (dihydrokaempferol, dihydroquercetin and four glycosylated derivatives) were identified but not quantified in a Riesling wine (65). Astilbin and engeletin have also been found in low amounts in Champagne wine (66). 

Both hydroxycinnamic and hydroxybenzoic acids are present in wine. Red wine contains 9.63 mg/100 ml hydroxycinnamic acids and 7.02 mg/100 ml hydroxybenzoic acids, and white wine 2.84 mg/100 ml hydroxycinnamic acids and 2.49 mg/100 ml hydroxybenzoic acids. The main hydroxycinnamic acids are caffeic, p-coumaric and ferulic acids, largely esterified to tartaric acid esters. Their respective esters are called caffeoyl tartaric (caftaric), coumaroyl tartaric (coutaric), and feruloyl tartaric (fertaric) acids. In contrast to other berries, no 5-caffeoylquinic acid has been described in grape. Caftaric acid is the main hydroxycinnamic acid (35% of hydroxycinnamic acids in red wine). Caftaric acid also reacts in wine with glutathione to form the 2,5-di-S-glutathionyl caftaric acid. It is found in red wine in significant concentration (2.48 mg/100 ml). Free caffeic acid is present in red and white wines at respectively 1.88 and 0.25 mg/100 ml. Gallic acid is the main hydroxybenzoic acid described in wine. It is present as esters with (-)-epicatechin and proanthocyanidins, but the major part is present as free gallic acid (3.59 mg/100 ml) or as an ester with ethanol. Other benzoic acids such as vanillic, syringic and protocatechuic acids together with their respective aldehydes are also present in wine. 

Resveratrol has received much attention for its particular biological properties and potential therapeutic effects (67). Wine is its main dietary source. This compound is produced in the leaves and in the skin of berries in response to an infection or to a stress induced by herbicides, fungicides or UV light. Resveratrol is found as aglycones (trans- and cis- resveratrol), glucosides (trans- and cis-piceid), and dimeric forms (viniferin and pallidol). In wine, resveratrol is only present at trace levels. Content in red wines is about 10 times lower than in white wines. Content values of resveratrol and its 3-O-glucoside are respectively 0.28 and 0.62 mg/100 ml in red wine, 0.12 and 0.20 mg/100 ml in rosé wine, and 0.04 mg/100 and 0.25 mg/100 ml in white wine. The content of trans- and cis-resverastrol were compared in a large number of commercial wines. The highest levels of trans-resveratrol were found in wines from Pinot Noir (up to 1.34 mg/100 ml) (68, 69). According to the type of wine, the trans form of resveratrol (aglycone or glucoside) is usually but not always slightly more abundant than the cis form. No cis-resveratrol was detected in grapes.