Occurrence And Contents

The occurrence, content, and type of phenolics in oilseeds is dictated by the species involved; phenolic acids and phenylpropanoids are often prevalent in many oilseeds. Canola and other cruciferae seeds contain considerable amounts of phenolic compounds compared to other oilseeds. The total content of phenolic acids, including phenylpropanoids, in defatted rapeseed flour has been reported to be approximately 623-1281 mg/100g (Krygier et al., 1982; Kozlowska et al., 1983a,b; Naczk and Shahidi, 1989). Sinapic acid was recognized as a predominant phenolic compound of canola while p-hydroxybenzoic, vanillic, genistic, protocatechuic, syringic, p-coumaric, ferulic, and caffeic acids were among the minor phenolics present (Krygier et al.,1982; Dabrowski and Sosulski, 1984). Recently, Wanasundara et al. (1994) reported that the most active phenolic antioxidant in canola meal was 1-O-6-D-glucopyranosyl sinapate. Quercetin and isorhamnetin were isolated in less than 1 ppm from canola/rapeseed, while rapeseed contained a number of condensed tannins composed of cyanidin, pelargonidin, kaempferol, and their derivatives. While canola hulls serve as a rich source of condensed tannins, cotyledons possess little tannins (Blair and Reichart, 1984; Naczk et al., 1994). The acetone extractable condensed tannins of rapeseed hulls were composed of leucocyanidin units (Leung et al., 1979). The hulls may be separated by dehulling and the isolated tannins may potentially be used as a source of raw material for functional food applications (Naczk et al., 2000).

Mustard seeds also contain a considerable amount of phenolic acids; the levels being similar, to those in rapeseed and canola. The most abundant phenolics in mustard were trans-sinapic and p-hydroxybenzoic acids. In addition, cis-sinapic and trans-ferulic acids were present in relatively high amounts (Dabrowski and Sosulski, 1984). The antioxidant activity of ethanolic extracts of mustard flour were attributed to the presence of polyhydroxyphenols, including flavones and flavonols (Shahidi et al., 1994).

The phenolics of importance to soybeans are the isoflavones, and their content in raw soy is 0.1-0.5%. However, during processing, a considerable loss of isoflavones may occur. Soybean germ is a rich source of isoflavones, 2.5-3.5%. These include genistein, daidzein, and glycitin, all present mainly as glycosides, and minor amounts as aglycones. A very small amount of coumesterol (6.05 ppm) was also present. Meanwhile, presence of prunetin, formononetin, and 4',6',7'-trihydroxy-isoflavone (Foote et al., 1970) has been reported in soybean. Soybean flour also contained chlorogenic, isochlorogenic, caffeic, ferulic, p-coumaric, syringic, vanillic, p-hydroxybenzoic, salicylic, and sinapic acids (White and Xing, 1997). It should be noted that the distribution of isoflavones in the germ is different from that of the whole seed. Thus, the biological activity of phytochemicals may depend on their distribution in different seed parts. Soybean isoflavones have been reported to have both estrogenic and antiestrogenic properties. Several epidemiological studies have shown excellent correlation between soy protein consumption and reduced risk of cancer as well as inhibition of tumor formation (Boyd, 2001). Other beneficial health effects have also been attributed to the phytochemicals present in soybean products (Hirota et al., 2000). Thus, expanding utilization of soybean and its products in foods has been encouraged (Wolf, 1983; Liu, 2000).

The presence of chlorogenic, quinic, and caffeic acids as major phenolics in sunflower meal has been reported; chlorogenic acid being most predominant at 2.7% (Cater et al., 1972). Quinic acid was present at 0.38% and caffeic acid at 0.2% (Cater et al., 1972). In addition, p-hydroxybenzoic, syringic, trans-p--coumaric, trans-fer-ulic, and vanillic acids were present in small amounts in sunflower flour (Dabrowski and Sosulski, 1984). Rutin and quercetin glycosides have been identified in cottonseed; however, the main phenolic in glanded cottonseed is gossypol which is present at 1.1-1.3% (Lawhon et al., 1977). This polyphenolic compound has antifertility properties. Cottonseed also contained trans-ferulic, trans-p-coumaric, trans-caffeic, and p-hydroxybenzic acids in minute amounts. Peanut extracts contained dihydro-quercetin and taxifolin, as well as trans-p-coumaric acid and minor amounts of trans-ferulic, trans-sinapic, p-hydroxybenzoic, and trans-caffeic acids (Pratt and Miller, 1984).

The phenolic acid content of flaxseed was considerably lower than that of other oilseeds (Kozlowska et al., 1983a; Dabrowski and Sosulski, 1984; Wanasundara and Shahidi, 1994). Trans-ferulic acid was the predominant phenolic acid present, but trans-sinapic, trans-p-coumaric, trans-caffeic, and p-hydroxybenzoic acids were found in smaller amounts. In addition, presence of 0-coumaric, genistic, and vanillic acids was reported (Kozlowska et al., 1983a,b; Dabrowski and Sosulski, 1984). Flaxseed also contained a considerable amount of secoisolericresorsinol diglucoside (SDG; Amarowicz et al., 1994), a lignan with anticarcinogenic and chemopreventive activity (Westcott and Muir, 2000). Furthermore, the presence of matairesinol, another lignan, has been reported in smaller amounts in flaxseed (Adlercreutz and Mazur, 1997; Mazur and Adlercreutz, 1998), together with a number of polymeric phenolics (Bakke and Klostermann, 1956; Westcott and Muir, 1996, 2000). SDG and its corresponding aglycone are converted to enterodiol and enterolactone, once consumed. These latter metabolites have been postulated to possess antiestrogenic properties (Adlercreutz, 1997; Adlercreutz and Mazur, 1997).

The phenolic acids in sesame include trans-caffeic, trans-p-coumaric, and trans-ferulic acids in the decreasing order of abundance (Dabrowski and Sosulski, 1984). However, vanillic, syringic, sinapic, and 0-coumaric acids were also present (Kozlowska et al., 1983a). Two lignans, sesamin and sesamolin, were found in crude sesame oil at 0.44 and 0.25%, respectively. However, during the refining process these lignans may be converted to their corresponding alcohols. Thus, following bleaching of sesame oil, a drastic change in the amounts of sesamin, episesamin, and sesamolin occurred. The compounds sesamol, sesamolinol, sesaminol, pinoresinol, and a sesamol dimer were present in the processed oils (Fukuda et al., 1985). In addition, the sesame cake contained a number of glucosides of pinoresinol as well as those of sesaminol. These latter compounds, considered as pro-antioxidants, are converted to potent antioxidants once the meal is treated with 6-glucosidase to release the corresponding aglycones (Namiki, 1995).

Recently, phenolic components of borage and evening primrose meal were reported by Wettasinghe and Shahidi (1999a,b). Borage meal contained rosmarinic acid, syringic acid, and sinapic acid, while evening primrose contained gallic acid, (+) catechin and (-) epicatechin, and a high-molecular-weight polyphenol known as oenothin B (Shahidi, 2000a). The latter compound has also been isolated from leaves and branches of evening primrose as well as other plants (Hatano et al., 1990). This compound was found to serve as an effective antitumorigenic and anticarcinogenic agent (Miyamoto et al., 1993); thus its potential health-promoting activity requires further attention. The reactive oxygen species - and 2,2-diphenyl-1-picrylhydrazyl radical (DPPHO - scavenging activity of borage and evening primrose extracts have been documented (Shahidi et al., 2000, Wettasinghe and Shahidi, 2000). Dietary evening primrose was also found to have a hypocholesterolemic effect in rats (Balasinska, 1998).

In general, the beneficial health effects of seed phenolics, in part, arise from their antioxidant activity. Although antioxidant activity may arise from contributions of different bioactives, phenolic compounds are among the most powerful antioxidants known in food sources. In general, the type of phenolics and their substitution patterns determine the antioxidant activity of molecules involved. Thus, structure-antioxidant activity relationship of phenolics in oilseeds is of interest. Presence of a second hydroxyl or a methoxy group in the ortho- or para-position of benzoic acid derivatives, phenylpropanoids, or flavonoid/isoflavones is known to enhance the antioxidant activity of compounds involved. While the presence of two hydroxyl groups in the ortho- or para-position may lead to production of stable quinoid-type structures, a second methoxy group in the ortho- or para-position is an effective electron donor that stabilizes the free radicals formed, thus enhancing even further the activity of compounds involved. Phenylpropanoids are more effective antioxidants when compared with their phenolic acid counterparts. This is due to further stabilization of the free radicals formed via the extended conjugation arising from the propene moiety of the molecules.

In flavonoids, structural parameters play a prominent role in the antioxidant efficacy of the compounds involved. Therefore, presence of an ortho-diphenolic group in the B ring, a 2-3 double bond conjugated with the 4-oxo function, and occurrence of hydroxyl groups in positions 3 and 5 are most important structural features dictating the efficacy of flavonoids as antioxidants (Bors et al., 1990; Ratty and Das, 1988).

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