Cereals are important food crops in the world. They supply most of their food energy as starch. They are also a significant source of protein. The most important cereals based on annual production are maize, wheat and rice, and also barley, sorghum, oat, rye and buckwheat. All these crops with the exception of buckwheat belong the Poaceae (or Gramineae) family. Buckwheat (Fagopyrum esculentum) belongs to the Polygonaceae family. It is categorized as a pseudo-cereal: Indeed it shows several similarities with the true cereals including its way of cultivation and utilization, and the presence of a starchy endosperm. Whole grains are good sources of dietary fiber; they also contain minerals, vitamins, essential fatty acids, and phytochemicals such as polyphenols. Polyphenol classes present in the different cereal types are phenolic acids, flavonoids and resorcinols. Anthocyanins are present in coloured cereal varieties, like red maize. Most polyphenols occur in the outer layers of the grain. They are largely lost during refining and refined flour contains very low amounts of polyphenols.
Phenolic acids in cereal grains are distributed as free, soluble-esterified, and insoluble-bound forms either esterified or etherified to the cell wall constituents. Free phenolic acids are determined in aqueous methanol, ethanol or acetone extracts. The soluble-esterified phenolic acids are estimated in the same extracts after alkali or acid hydrolysis. The insoluble-bound phenolic acids remaining in the extraction residue are quantified after a similar hydrolysis of the solids (250, 251, 252). In the present composition table, two different content values are given: phenolic acids determined by chromatography methods without hydrolysis correspond to the free phenolic acids, while phenolic acids determined by chromatography after hydrolysis correspond to the total free and bound (both soluble and insoluble esters) phenolic acids.
In the present composition table for cereals, polyphenol contents are reported as fresh weight values. When original values were reported as dry weight, values were calculated using the moisture content of the cereal flour samples. When the moisture content was not given in the original publication, values were calculated using an average moisture content of 10 % for wheat, rye, barley, rice and oat, 12% for maize and buckwheat, and 9% for sorghum.
Barley (Hordeum vulgare) is a major food and animal feed crop. Barley ranks fourth on annual world cereal production, and covers 6% of total cereal production (http://faostat.fao.org). Barley types can be distinguished by the number of kernel rows in the head. Three forms have been cultivated; two-row barley (traditionally known as Hordeum distichum), four-row (Hordeum tetrastichum) and six-row barley (Hordeum vulgare). Two-row barley has a lower protein content than six-row barley. High protein barley is best suited for animal feed or beer malts.
The hulls of the barley grain have to be removed before consumption. Barley grains with hulls are called covered barley. Once the grain has the inedible hull removed, it is called hulled barley. At this stage, the grain still contains its bran and germ, and is considered a whole grain. Pearled barley is barley which has been processed further to remove the bran. Hulled and pearled barley can be processed into a variety of barley products including flour, flakes, and grits. It can be malted and used in the production of alcoholic beverages. Malting barley is a key ingredient in beer and whiskey production. Two-row barley is traditionally used in German and English beers, and six-row barley in American beers.
Little data on polyphenols in barley have been published. Although values may present good averages, attention has to be paid to variability due to analytical and sampling methods, and variability between barley species analysed. Total polyphenol contents (Folin values) in whole grain barley flour are 73 mg/100 g. Phenolic acids are abundant in barley, especially 4-hydroxybenzoic acid and ferulic acid: about respectively 100 and 30 mg/100g in whole grain barley flour (these data could not be aggregated due to some ambiguity on the moisture content of the samples) (252). Barley also contains proanthocyanidins. In an analysis of nine varieties of barley by HPLC, the procyanidin dimer B3 and prodelphinidin B3 (dimer GC-C) were found to be most abundant (46). Of the procyanidin trimers, trimer C2 was present in highest amounts. Coloured barley varieties contain anthocyanins. Blue grain barley, which has a blue aleurone layer, contains about 3 mg/100 g anthocyanins (253). Furthermore, 5-pentacosylresorcinol (1.7 mg/100 g) and 5-heneicosylresorcinol (1.2 mg/100 g) are the main alkylresorcinols in whole grain barley flour. Alkylresorcinol contents are lower in barley than in wheat and rye whole grain flours.
Most phenolic acids present in barley are esterified to hemicelluloses in cell walls. Their estimation is generally carried out after pre-treatment with acid or alkali to hydrolyse the ester bound. A large variety of pre-treatment protocols, combining enzymatic (a-amylase, cellulose) and chemical treatments have been applied and this may partly explain the variability of content values. The variety of barley does influence polyphenol content although the relative proportions of the major polyphenol classes remain unchanged (254). Processing also influences polyphenol contents. Barley varieties appear to be richer in polyphenols than their corresponding malts. After malting the polyphenol contents are decreased, especially for flavanols (46).
Several content values on polyphenols in different forms of barley are lacking, due to the paucity of data in the literature. More specifically, more data for flavanols, phenolic acids and flavonols are needed to compare pearled and hulled barley, as well as for barley products such as breakfast cereals, breads and biscuits.
To obtain buckwheat flour, the buckwheat grains need to be dehulled. Dehulling can be obtained by thermal treatment of the grains. Groats, the part of the grain left after the hulls are removed from the seeds, and flour made from the groats, are used for breakfast food, porridge (called ‘kasha’ in Eastern Europe), and as thickening material in soups and dressings. Buckwheat pancakes are eaten in different countries. Buckwheat flour is noticeably darker than wheat flour, hence the name black wheat given in some countries (not to be confused with black varieties of wheat).
Buckwheat seeds are rich in flavonoids, mainly flavonols. The most important polyphenol in buckwheat is quercetin-3-O-rutinoside, also called rutin (37 and 6 mg/100 g in whole grain buckwheat flour and refined flour respectively). Quercetin and apigenin 6-O-glucoside (isovitexin) are present in minor amounts (0.1 and 0.9 mg/100 g respectively in whole grain flour). In buckwheat hulls three other flavonoids have been isolated, namely luteolin 8-C-glucoside (orientin), apigenin 8-O-glucoside (vitexin) and isoorientin. Furthermore, four flavanols were isolated in buckwheat groats, namely (-)-epicatechin, (+)-catechin 7-O-beta-D-glucopyranoside, (-)-epicatechin 3-O-p-hydroxybenzoate and (-)-epicatechin 3-O-(3,4-di-O-methyl)gallate (255).
Phenolic acids are present in buckwheat in low amounts. The most abundant is p-coumaric acid, especially present in the bran fraction (9 mg/100 g, as measured after hydrolysis)(256). The ratio of total phenolic acids to esterified phenolic acids equals 4:1 (257). Lignans are present in buckwheat whole grain flour, lariciresinol (0.36 mg/100 g) and syringaresinol (0.25 mg/100 g) being most abundant.
Higher levels of flavonoids are observed in tartary buckwheat (Fagopyrum tartaricum) compared to common buckwheat. Rutin contents were found to be about five times higher in tartary buckwheat hulls, and up to 180 times higher in tartary whole grain buckwheat flour. However, these content values were measured using the Prussian Blue method (258). Growth location is the main source of variation in flavonoid and rutin contents in buckwheat seeds, while growing season has also significant influence on the flavonoid content of the hulls (259). Variation in phenolic acids in buckwheat was found to be mainly due to cultivar and seasonal effects, and less to growing location (257).
Dehulled buckwheat groats can be boiled or roasted before consumption. Also, buckwheat can be extruded to produce cereal and snack products. Total polyphenol content (Folin) was not found to be affected by roasting processes (260). However, a decrease in total polyphenol content was found after extrusion cooking, as well as an increase in free and esterified phenolic acids (261). A reduction of the flavonoid concentration was found after heat treatments used to dehull the buckwheat grains (262).
Buckwheat flour or bran can be an important source of rutin in the diet. One pancake made from buckwheat contains about 40 gram of buckwheat flour. Thus, one pancake provides approximately 15 mg rutin if whole grain flour is used or 2.5 mg if the flour was refined.
Maize (Zea mays L.), also called corn, grows in ‘ears’. The kernels are tightly clustered around the core, or cob, protected by a hull. Different types of maize exist, like waxy maize, popcorn maize, flour maize, and amylomaize. White, red and yellow are the most common basic colours of maize, but other colour varieties exist, like blue, purple, brown and orange. Maize is the most important cereal crop in terms of production, covering 31% of total cereal production worldwide (http://faostat.fao.org). Maize is a staple crop in many parts of the world and is used as fodder for animals. It is also becoming more important as a natural fuel source. Maize meal is often consumed as porridge, tortillas (unleavened flat bread) and fried tortilla chips. Table tortillas are the staple food of Mexico and Central America, and corn and tortilla chips have become an important snack food in the world. When whole grain flour is produced from maize, it contains a lower bran fraction than wheat. However, it lacks the gluten, so it is more difficult to use for baking goods due to its poor rising capabilities.
To make tortillas and other maize products from raw kernels, an ancient process called nixtamalization is used. Nixtamalization consists of alkaline cooking of maize kernels in a calcium hydroxide solution. Hereafter, the maize is milled to make dough that is called masa, from which tortillas are formed. During nixtamalization the bran is partly removed, to leave semi-whole grain products. Furthermore, calcium is incorporated in the kernels, and colour and flavour compounds are formed. Total polyphenol values (Folin assay) in whole grain maize are 179 mg/100 g. Maize is particularly rich in phenolic acids. Anthocyanins are the primary pigments in coloured maize. Like other cereals, phenolic acids in maize exist in free forms (measured without hydrolysis) or esterified forms and insoluble-bound forms. Most phenolic acids in maize are in the insoluble bound forms, with percentages of more than 80% of total phenolic acids (251, 263). Maize flour contains about 3 times more phenolic acids than rice, wheat and oat flour (251). Ferulic acid is the most important phenolic acid with a total content of 171 mg/100g whole grain maize, as determined after hydrolysis. Other phenolic acids present in maize, though in lower amounts than ferulic acid, are sinapic acid, p-coumaric acid, 2-hydroxybenzoic acid, caffeic acid and syringic acid. Low amounts of protocatechuic acid, 4-hydroxybenzoic acid and vanillic acid, gallic acid and 4-hydroxyphenyl acetic acid are also detected. Bran is the richest fraction of the kernel with a total phenolic acid content of 4% (w/w) (264).
Like in rice, wheat and rye, sterol cinnamates have been detected in maize, especially in the bran part. Sitosterol-, sitostanol-, stigmasterol-, campesterol-, campestanol-, ∆-7-sitosterol- and ∆-7-campesterol-ferulates have been identified (265, 266). Sitostanyl- and campestanyl-p-coumarates have also been detected in maize (267). Maize bran oil is a rich source of sterol ferulates. It contains stigmastanol ferulate (360 mg/100 g), 24-methylcholestanol ferulate (100 mg/100g), 24-methylcholesterol ferulate (50 mg/100g), sitosterol ferulate (30 mg/100g), schottenol ferulate (10 mg/100g), 24-methylenecholestanol ferulate (5 mg/100g) and 24-methyllathosterol ferulate (2 mg/100g).
Anthocyanins in blue maize are present at about 30 mg/100g (268), with cyanidin 3-O-glucoside (11 mg/100g) as the major compound (253). Red maize and purple maize are somewhat richer in anthocyanins, with cyanidin 3-O-glucoside values of 28 and 30 mg/100 g respectively and pelargonidin 3-O-glucoside values of 2.8 and 5.5 mg/100 g resp. Other major anthocyanins present in red and purple maize are peonidin 3-O-glucoside (5.9 and 2.7 mg/100 g resp.), cyanidin 3-(6-succinyl)-O-glucoside (2.2 and 5.6 mg/100 g), cyanidin disuccinyl-O-glucoside (0.87 and 5.8 mg/100g), cyanidin 3-(6-malonyl)-O-glucoside (1.4 and 2.3 mg/100 g) and peonidin succinyl-O-glucoside (0.7 and 1.3 mg/100g)(253, 263, 269, 270).
Lignans have been detected in maize bran in minor amounts (271). Hydroxymatairesinol (0.41 mg/100g), syringaresinol (0.22 mg/100g) and oxomatairesinol (0.16 mg/100g) are mostly present (271). The volatile simple phenols 4-vinylguaiacol and 4-vinylphenol have been detected in maize tortilla chips, in amounts of 0.15 mg/100g and 0.006 mg/100g respectively (272).
During nixtamalization, polyphenol contents are greatly reduced due to the combined effect of alkaline and heat processing, and the diffusion of polyphenols into the lime solution. Further processing of the masa dough into tortillas or chips reduces only slightly the polyphenol contents in the final products. Anthocyanin contents decrease in the same way during processing. Furthermore, nixtamalization causes an increase in free and soluble-esterified ferulic acid, and a loss of bound ferulic acid (263, 273). Losses caused by nixtamalization were found to differ between maize types. Total polyphenol contents in Mexican white maize decreased by 90%, whereas total polyphenol contents in Mexican blue maize and American blue maize decreased by 61% and 78% (268). Cooking time and alkali concentration in the lime solution, influence the losses of the different polyphenols during nixtamalization (274).
Oat (Avena sativa L.) is mainly produced in Russia, Canada and the United States, and also thrives in the cooler, wetter climates of Northwest Europe. Oat is mainly used as feed and as food for human consumption, but it is consumed in much lower quantities worldwide than wheat and rice. However, oat products usually contain the whole grain, and therefore provide bioactive compounds present in the bran fraction.
Oat grains contain an inedible outer hull that has to be removed before further processing and consumption. The oat grains that have their hulls removed are called groats. Once dehulled, groats are stabilized by a heat and moisture treatment in order to prevent development of rancidity. Rolled oats are the groats that have been steamed and rolled into flakes. When groats are coarsely ground, they are called oatmeal. Groats can be debranned by a process called pearling. The bran fractions, called pearlings, are separated from the refined (white) flour. Oat is often processed into breakfast cereals, or added to bakery products like cookies and bread. Precooked or instant oat is also produced, in order to obtain a minimal preparation time.
Average total polyphenol contents (Folin assay) in different forms of oat are 26 mg/100g (rolled oats), 39 mg/100g (groats) (275, 276, 277), and 82 mg/100g (whole grain oat flour). The main polyphenols present in oat are phenolic acids, including avenanthramides. Oat also contains lignans.
Phenolic acids in oat are mainly present as insoluble-bound esters. Ferulic acid is the major phenolic acid, with content values of 36 mg/100g in whole grain oat flour, as measured after hydrolysis. Refined oat flour contains about 6 mg/100g ferulic acid, as measured after hydrolysis. Free ferulic acid content in whole grain flour is 0.19 mg/100g. Other phenolic acids present in oat are vanillic acid, 4-hydroxybenzoic acid, caffeic acid and vanillin. Minor amounts of syringic acid, p-coumaric acid, 4-hydroxybenzaldehyde and sinapic acid have also been detected. Oat bran is especially rich in phenolic acids (278, 279). In the insoluble fiber fraction of oat, dehydrodiferulic acids are abundant, with a total content value of 360 mg/100g. 5-5 Diferulic acid is a major diferulic acid (13.2 mg/100g), as well as 8-O-4 diferulic acid (10.7 mg/100g), 8-8’-aryl diferulic acid (9.6 mg/100g) and 8-5’ diferulic acid (8.4 mg/100g)(280).
Avenanthramides are a group of alkaloids that consist of an anthranilic acid derivative linked to a hydroxycinnamic acid derivative by a pseudo peptide bond. They have been classified according to nature of the alkaloid: anthranilic acid (type 1), 5-hydroxyanthranilic acid (type 2) and 5-hydroxy-4-methoxyanthranilic acid (type 3), and that of the linked cinnamic acids: p-coumaric acid (type p), caffeic acid (type c), ferulic acid (type f), sinapic acid (type s) (277). Oat is especially rich in the avenanthramide 2c (N-caffeoyl-5-hydroxyanthranilic acid), avenanthramide 2f (N-feruloyl-5-hydroxyanthranilic acid), and avenanthramide 2p (N-p-coumaroyl-5-hydroxyanthranilic acid), with respective contents values of 3.8, 2.6 and 2.7 mg/100g in whole grain oat flour. The content of avenanthramide K (N-caffeoyl-4-hydroxyanthranilic acid) in whole grain flour is 1.9 mg/100g. About 25 distinct avenanthramides have been identified in groats (281).
Three other phenolic acids have been detected in groats, namely avenalumic acid (4’-hydroxycinnamylideneacetic acid), 3’-hydroxyavenalumic acid and 3’-methoxyavenalumic acid. These phenolic acids occur in oat as conjugates that are covalently linked to the amine function of orthoaminobenzoic acids (282).
In oat bran, minor amounts of iso-hydroxymatairesinol are found (0.07 mg/100g) (271). Furthermore, after hydrolysis of the oat bran, other lignans were detected, of which lariciresinol (0.77 mg/100g), hydroxymatairesinol (0.71 mg/100g), pinoresinol (0.57 mg/100g), matairesinol (0.44 mg/100g), syringaresinol (0.3 mg/100g) are the major ones (271).
The quality of the oat end products depends on the oat variety, the quality of the raw material, storage conditions, and the processing methods used. Polyphenol contents differ with oat variety. The storage of raw oat increases the levels of the free phenolic acids: vanillic acid, caffeic acid, p-coumaric acid, ferulic acid, vanillin, 4-hydroxybenzoic acid. Avenanthramide contents remain unchanged (283). Pearling of oat groats causes a decrease in the concentration of total polyphenols, and of several phenolic acids, in the remaining flour fraction. As pearling time increases, more endosperm is present in the bran fraction, and a decrease in total polyphenol content and of several phenolic acids in the bran fraction is noticed. In contrast to phenolic acids, avenanthramides are more evenly distributed in the groats, although still being 58-83% lower in the endosperm than in the bran fractions (279).
The processing (steaming and drying) of hulled oat leads to an increase of the free phenolic acids: p-coumaric acid, 4-hydroxybenzoic acid, ferulic acid, vanillic acid and vanillin, as compared to unprocessed samples. This increase is not seen in oat processed without hulls, except for ferulic acid content, which increases in dehulled oat after processing. Caffeic acid and avenanthramide contents decrease after processing compared to unprocessed oat. This decrease is identical for hulled and dehulled processed groats (283).
Rice is represented by two species of grass (Oryza sativa and Oryza glaberrima). Rice is the third most important cereal crop worldwide after maize and wheat, covering 28% of total global cereal production (http://faostat.fao.org). The seeds of the rice plant are first milled using a rice huller to remove the chaff, which is the outer hull of the grain. This creates whole grain rice which includes the bran. This process can be continued, removing the germ and the bran, to create polished rice or refined rice. The refined rice may also be parboiled or processed into flour. Rice bran is used in Asia to produce oils or pickled vegetables. Black rice, red rice and wild rice (with colours varying from green-brown to blackish) are coloured rice species.
Most often, polyphenol values for rice come from single sources, so values can be less reliable because of variability due to variety, analytical methodology, or milling practices. The data show that no alkylresorcinols are present in rice (278, 284), which distinguishes it from wheat, rye and barley. Rice contains phenolic acids, and like most other cereal grains, the major phenolic acid is ferulic acid (about 30 mg/100 g total ferulic acid in whole grain rice, as measured after hydrolysis). Because phenolic acids are mostly present in the bran fractions of rice, refined rice contains smaller amounts of phenolic acids. About 60% of the phenolic acids in rice are present in the bound form (250). Comparison of uncooked and cooked brown rice showed that composition and contents of phenolic acids does not change during cooking (278).
Black rice contains high amounts of anthocyanins (325 mg/100 g), an amount that is high compared to coloured wheat and barley species (253). However, anthocyanin contents in black rice vary greatly according to cultivar, so lower amounts can be measured (285). In wild rice and red rice anthocyanin contents of 3 and 9 mg/100 g respectively are found (253). Cyanidin 3-glucoside is the main anthocyanin in coloured rice species, with values of 1 mg/100 g in red rice to 200 mg/100 g in black rice. A portion of red rice of 75 gram (dry weight) contains about 7 mg of anthocyanins (253).
Rice bran oil is rich in steryl ferulates. The mixture of these steryl ferulates is named γ-oryzanol. It contains feruloyl esters of the following sterols: cycloartenyl, 24-methylenecycloartanyl, ∆-campestenyl, sitosteryl, ∆-sitostenyl, stigmasteryl, ∆-stigmasteryl, and sitostanyl and campestanyl ferulates (266).
Rye (Secale cereale) is a grass grown extensively as a grain and forage crop. It is a member of the wheat tribe (Triticeae) and is closely related to wheat and barley. Rye production covers about 1% of total cereal production worldwide (http://faostat.fao.org). Rye grain is used to make flour, rye bread, rye beer, certain whiskies and vodkas, and animal fodder. It can also be consumed as whole boiled grains, or as rolled grains. Rye flour is used to bake the traditional German or Scandinavian sourdough breads, pumpernickel and crisp breads. Rye flour has a lower gluten content than wheat flour, and contains a higher proportion of soluble fiber. Often, breads are a mixture of rye and wheat flours to add gluten.
Phenolic acids are the most abundant polyphenols in rye. Ferulic acid is present in highest amounts, average values ranging from 23 mg/100 g in refined rye flour to 91 mg/100 g in whole grain rye flour (total ferulic acid contents measured after hydrolysis). Other phenolic acids detected in rye are sinapic acid and p-coumaric acid, and in minor amounts vanillic acid, syringic acid, hydroxybenzoic acid, caffeic acid, protocatechuic acid and gentisic acid. Dimerization of ferulates forms ferulic acid dehydrodimers present in lower amounts than ferulic acid. The most abundant ferulic acid dehydrodimer is 8-O-4-diferulic acid, followed by 5-8-benzofuran diferulic acid, 5-5-diferulic acid and 5-8-diferulic acid. Resorcinols are another important polyphenols present in rye. Major resorcinols in rye are, 5-nonadecylresorcinol, 5-heneicosylresorcinol and 5-heptadecylresorcinol (20, 16 and 16 mg/100 g in whole grain rye flour respectively). 5-Tricosylresorcinol, pentacosylresorcinol and 5-nonadecenylresorcinol are present in minor amounts. Lignans are present in low amounts in whole grain rye flour. The total polyphenol content in rye bran (130 mg/100 g) (286) is higher than the content in refined flour (45 mg/100 g). Phenolic acid contents in bran fractions were estimated to be 10 to 20-fold higher than in the endosperm fraction (287).
Variability of the content of alkylresorcinols has been more particularly studied as these compounds have been used as urinary markers of rye consumption. Baking of whole grain flour into bread results in some losses of alkylresorcinols (288). Small variations are seen in the relative content of the different alkylresorcinols in different rye cultivars, and wide variations in their total contents (289).
Other names for sorghum are Durra, Egyptian or great millet, Guinea Corn, Kafir corn, Jowar, Feterita and Milo, amongst others. Numerous varieties of sorghum exist, of which grain sorghum (Sorghum bicolor ssp. bicolor) is the most important for human consumption. Depending on the variety, sorghum grains are white, yellow, red or brown.
The five largest producers of sorghum in the world are the United States, India, Mexico, China and Nigeria. Sorghum production covers about 3% of the total global cereal production, and ranks fifth after maize, wheat, rice and barley (http://faostat.fao.org). The consumption of sorghum is highest in countries where other cereals do not thrive, and where incomes are relatively low, including countries in Asia and Africa. The national average per capita consumption of sorghum can reach up to 100 kg per year in sub-Saharan Africa (290). In most other countries sorghum consumption is relatively small or negligible, compared to that of other cereals. Uses of sorghum in human consumption are as unleavened flat bread, porridge (fermented or not), pancakes, and the steamed, fried or popped whole grain or flour. Sorghum is also used to make malted and distilled beverages.
Not much data are available on polyphenol contents in sorghum. Values are often derived from single data sources. Total polyphenol contents of whole grain sorghum, measured by the Folin method, are 413 mg/100 g, a value about 6 times higher than that measured for rye or barley. Sorghum contains large amounts of proanthocyanidins and minor amounts of phenolic acids and lignans.
Monomeric and oligomeric proanthocyanidins have been estimated in whole grain sorghum. Monomers (9.5 mg/100g), dimers (22 mg/100g) and trimers (28 mg/100g) are less abundant than oligomers and polymers of higher polymerization degree (proanthocyanidins ³4 units present in amounts of 1927 mg/100 g). The bran fraction of sorghum is the richest in proanthocyanidins (291).
When sorghum whole grains are processed into flakes, the proanthocyanidin oligomer and polymer contents decrease greatly. The 7-10mers are decreased by 65%, and the 4-6mers are decreased by 50%. Polymer (>10) contents are reduced by 90% after processing of grains into flakes. However, contents of 1-4 mers are increased after extrusion. This could possibly be due to the breakdown of higher oligomers into smaller molecules (291).
Phenolic acids have been identified in sorghum in the free and bound form. Ferulic acid, p-coumaric acid, 4-hydroxybenzoic acid, protocatechuic acid and caffeic acid were found to be most abundant (292). Small amounts of the lignans syringaresinol (0.09mg/100g) and pinoresinol (0.03 mg/100g), and very small amounts of hydroxymatairesinol (0.02 mg/100g), cyclolariciresinol and secoisolariciresinol (both 0.01 mg/100g) have been detected in sorghum bran (271) .
The polyphenol content varies among sorghum varieties (293). Sorghum colour is not an indicator of the tannin content. Polyphenols in sorghum contribute to bitterness and astringency. Tannin-containing sorghums are more bitter and astringent than tannin-free sorghum. However, both types are bitter to some extent (294). Polyphenols in sorghum can have detrimental effects on the digestibility of proteins, and play a role in enzyme inhibition. Attempts have been made to decrease the polyphenol content in sorghum in order to increase the nutritive value of sorghum. Polyphenol contents can be reduced by soaking the grains in water or an alkaline (NaOH) or acid (HCl) solution. Of these treatments, soaking in 0.3% NaOH solution for 8 hours at 25°C was found to be most effective in reducing polyphenol contents (295).
Different wheat varieties have specific characteristics that are suitable for certain products. The two most important wheat species are bread wheat (Triticum aestivum) and durum wheat (Triticum durum). Bread wheat flour consists of two types: hard wheat flour is characterized by a vitreous endosperm and high levels of gluten, and therefore is used for breads; soft wheat flour has a mealy endosperm and is lower in protein. It is primarily used for cakes, cookies, biscuits, and breakfast foods. Durum wheat flour is used for pasta, noodles and couscous. On average, it has a higher protein content than bread wheat flour. Coloured wheat species exist: red, white, black, blue, purple and yellow types are distinguished.
Phenolic acids are the most abundant polyphenols in wheat. Of the phenolic acids, ferulic acid is present in highest amounts (47 mg/100 g in whole grain wheat flour, measured after hydrolysis), principally in insoluble-bound form. Free, soluble-conjugated, and insoluble-bound forms of ferulic acid are estimated in a ratio of 0.1:1:100 (250). Other phenolic acids detected in wheat are sinapic acid, p-coumaric acid, vanillic acid, caffeic acid, and diferulic acids, and in minor amounts syringic acid, o-coumaric acid, gentisic acid and 2-hydroxybenzoic acid.
Further polyphenol classes present in wheat are resorcinols, lignans, anthocyanins and flavones. The major resorcinols in wheat are 5-heneicosylresorcinol, 5-heneicosenylresorcinol and 5-nonadecylresorcinol (19, 17 and 14 mg/100 g in whole grain wheat flour respectively). In coloured wheat species, anthocyanins are the natural pigments responsible for the red, blue, or purple colours (296). Average anthocyanin contents range from 7 mg/100 g in purple wheat, to 21 mg/100 g in blue wheat (253). Lignans have been detected in low amounts (297, 298).
Cereal grains are divided into the endosperm (starchy part), the germ and the bran (outer part rich in fiber). Polyphenol content vary greatly according to the part considered. In wheat, the total polyphenol content of bran fractions is about 15-fold higher than that of endosperm fractions. Wheat bran contains high amounts of phenolic acids ( 390-452 mg/100 g, measured after hydrolysis) (256, 278) and alkylresorcinols (221-322 mg/100 g) (278, 284, 299). Milling processes will affect the polyphenol content in the flour, depending on the extraction rate. Refining whole grain wheat flour into white flour thus reduces polyphenol contents (256, 300, 301). Bread made of whole grain flour contains much higher amounts of polyphenols than bread made of refined flour. In the same way, pasta made from durum whole grain is richer in polyphenols than refined flour pasta.
Polyphenol content in wheat varies according to cultivars and growing conditions. Environmental factors, such as soil pH, rainfall and temperature, have a greater influence on total polyphenol content than genotype (302, 303). The production of bread from wheat flour influences the concentration of polyphenols. Alkylresorcinol contents can decrease during fermentation and baking of bread (288, 304).
Combining composition data with average portion sizes provides an estimate of the polyphenol content in a portion of wheat product. One slice of whole grain bread (35 g; total polyphenols 223 mg/100 g (Folin)) thus provides about 80 mg of total polyphenols, and 9 mg of alkylresorcinols.