Onions, flavones

More nsnal vegetables were stndied by Nnntila et al. [358] who characterized onions and spinach for the phenolic composition, with and without previous hydrolysis. The authors performed the simultaneous determination of phenolic acids, flavonols, flavones glycosides, and cathechins. Fignre 19.9 reprodnces the separation obtained for the standard mixture in this study. 

FIGURE 19.9 HPLC chromatogram of standards of phenolic acids, flavonols, flavones, and glycosides isolated in onions and spinach. (From Nnntila, A.M. et al., Food Chem., 76, 519, 2002. Copyright 2002. With permission from Elsevier.)... 

Two classes of dimeric anthocyanins isolated from plants (section 10.2.6) have been identified in plants for the first time. One class includes pigments where an anthocyanin and a flavone or flavonol are linked to each end of a dicarboxylic acyl unit. The other class includes four different catechins linked covalently to pelargonidin 3-glucoside. During the last decade, seven new desoxyanthocyanidins and a novel type of anthocyanidin called P)Tanoanthocyanidins have been reported (Section 10.2.2). Toward the end of the 20th century, several color-stable 4-substituted anthocyanins, pyranoanthocyanins, were discovered in small amounts in red wine and grape pomace.Recently, similar compounds have been isolated from extracts of petals of Rosa hybrida cv. M me Violet, scales of red onion, and strawberries. About 94% of the new anthocyanins in the period of this review are based on only six anthocyanidins (Table 10.2). 

An HPLC separation method with diode array detector and mass spectrometric (MS) detection equipped with atmospheric pressure ionization (API) was developed to determine flavone, flavonol, and flavanone in various vegetables, including green bean, broccoli, brussels sprouts, celery, kale, leek, onion, parsley, pepper (green, yellow, and red), and tomato (118). The flavonoids were analyzed as aglycones after acid hydrolysis. The extraction and acid hydrolysis conditions are based on previous work by Hertog et al. (119). Quercetin is the overall major flavonol, followed by kaempferol. The flavones, apigenin and luteolin, were found only in limited foods,... 

In the study on colorectal cancer (Table 20.4), a reduced risk was found for increasing intake of anthocyanidins (OR, 0.67 for the highest versus the lowest quintile,p-trend, 0.001), flavonols (OR, 0.64,p-trend < 0.001), flavones (OR, 0.78, p-trend, 0.004), and isoflavones (OR, 0.76, p-trend, 0.001). [Rossi et al., 2006], The estimates did not substantially differ for colon and rectal cancers. After allowance for fruit and vegetable consumption, for dietary fiber, or for micronutrients previously associated to this tumor including vitamin C, the associations with flavonoids did not change by more than 10%. A recent case-control study of 1456 pairs of cases and controls conducted in Sweden confirmed a significant decrease in risk of colorectal cancer for intake of anthocyanidins and flavonols [Theodoratou et al., 2007], but there was no relation for isoflavones and flavones. However, the results on isoflavones and flavones are questionable due to the unusually and generally low intakes of the populations studied. In the Italian population, anthocyanidins were derived mainly from wine, red fruit, and onions, and flavonols from apples or pears, wine, and mixed salads. 

The naturally occurring llavones are yellow and are very widely distributed in plants. They accumulate in almost any part of a plant, from the roots to the flower petals. Unlike the anthocyanins, which are too reactive and short-lived, the much more stable llavones have, from time immemorial, been used as dyes, for they impart various shades of yellow to wool. As an example, the inner bark of one of the North American oaks, Quercus velutina, was a commercial material known as quercitron bark and much used in dyeing it contains quercetrin. The corresponding aglycone, quercetin, is one of the most widely occurring flavones, found, for example, in Chrysanthemum and Rhododendron species, horse chestnuts, lemons, onions and hops. 

The flavonoids are a group of secondary metabolites widely distributed in the plant kingdom [63]. In 1984, more than 4000 flavonoids were identified in plants [64]. The major dietary sources of flavones are spices and pot herbs, such as parsley, rosemary, and thyme [65], whereas flavonols are predominantly found in onions, kale, broccoli, apples, berries and cherries, and in tea and red wine [66]. The flavanones are mainly restricted to citrus fruits [67], and flavanols are found in considerable amounts in tea, apricots, apples, and cherries [66]. 

The corresponding aglycone, quercetin, is one of the most widely occurring flavones, found, for example, in Chrysanthemum and Rhododendron species, horse chestnuts, lemons, onions, and hops. 

An interesting reaction is that of the flavones with aluminum ions. The very faintly yellow-colored flavones can be made to dye tissues a bright yellow. An instance occurred within the author s experience, when a tripe-dresser complained that when boiled with onions in an aluminum kettle his tripe became bright yellow in color. This was due to the quercetin glycosides (rutin, etc.) present in the yellow-skinned onions he had happened to use. Had he chosen purple-skinned or white-skinned varieties this difficulty would probably not have been encountered, since these contain much smaller quantities of flavone (c/. p. 283). 

Flavonol and flavone intakes have been studied in the United States in health professionals (37 886 men and 78 886 women) using a semiquantitiative food frequency questionnaire. Of the flavonols and flavones investigated, quercetin contributed 76% in men and 73% in women. The mean flavonol and flavone intake was 20-22 mg/day, and onions, tea, and apples contributed the greatest amounts of flavonols and flavones. This information should prove useful in the investigation of the role of flavonoids in disease prevention. 

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