Biochemistry of phenolic compounds in wheat grain (Triticum aestivum L.).
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Phenolic content, o-diphenol oxidase activity, flour colour and bread crumb colour have been examined for flour streams from a commercial flour mill and in flours and wholemeals of a number of New Zealand wheat cultivars. Phenolic content and o-diphenol oxidase activity varied significantly both between milling flour streams and between cultivars. Flour and bread crumb colour correlated significantly with phenolic content and o-diphenol oxidase activity in the milling flour streams. A similar relationship was also observed between o-diphenol oxidase activity and the colour of flour and bread prepared from several different cultivars. These observations are consistent with an involvement of o-diphenol oxidase and phenolics in the pigmentation of both wheat flour and bread. Commercial wheat bran has been found to contain low levels (<50 µg.g-1) of (+)catechin and soluble proanthocyanidins soluble in aqueous acetone. Detection of these compounds in crude extracts by conventional methods was complicated by the presence of methoxyhydroquinone glycosides and other interfering substances. Chromatographic studies indicated that, in addition to several dimeric proanthocyanidins, bran contains trimeric and/or other oligomeric proanthocyanidins. The oligomeric proanthocyanidins contained mostly prodelphinidin and some procyanidin units, whilst the dimeric proanthocyanidins may also contain some propelargonidin units. Two dimeric proanthocyanidins were isolated and tentatively characterized as catechin-(4α→)-catechin (procyanidin B3) and gallocatechin (4α→8)-catechin (prodelphinidin B3). These observations suggest that the flavanol content of wheat grain may be qualitatively similar to that of barley, although quantitatively much smaller. The identification of these compounds in mature wheat grain confirmed earlier reports of their presence in immature grain and supports the hypothesis that they contribute to seedcoat pigmentation. Changes in soluble phenolics and hydroxycinnamic acids and the activities of L-phenylalanine ammonia-lyase (PAL, EC No. 126.96.36.199) and chalcone-flavanone isomerase (CHI E.C. 188.8.131.52) were examined during the development and maturation of several spring bread wheat cultivars. Activities of both PAL and CHI were high in the early milk stage of grain development and this activity was located principally in the testa and pericarp tissues. Comparison of enzyme activities at this stage indicated significant differences between red- and white-grained cultivars. These different patterns of activity also correlated with significant differences in germinability at maturity. It is suggested that the common association between red seedcoat pigmentation and dormancy may be related to differences in the regulation of phenolic biosynthesis in the immature seedcoat. Studies of the tissue distribution of enzyme activities suggested that phenolic biosynthetic activities were higher in the embryo during the dough stage of grain development. Chromatographic studies suggested that there were significant qualitative and quantitative changes in phenolic content during grain development and maturation. Changes in ferulic and other hydroxycinnamic acids indicated that these were subject to considerable turnover and metabolism during grain development.