Phenolic Compounds Contribute to Dark Bran Pigmentation in Hard White Wheat

Abstract

Unacceptably dark bran color has prevented the white-kernelled variety Argent from meeting grain color marketing standards for hard white wheats (Triticum aestivum L.). The objective of this research was to identify phenolic compounds that negatively affect bran color in white wheat using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and vanillin-HCl and NaOH staining methods. In mature bran, FT-ICR-MS detected derivatives of the flavonol quercetin in varieties Argent and RL4137 (red-kernelled wheat) but not in W98616, a white wheat variety with acceptable grain color. Derivatives of the isoflavone formononetin were more abundant in W98616 relative to RL4137 and Argent. Vanillin-HCl staining indicated that RL4137 sequestered high levels of proanthocyanidin (PA) throughout its entire seed coat, whereas white wheats sequestered PAs as discrete speckles. Argent possessed abundant speckles over its entire seed coat, whereas speckles were almost undetectable in W98616. In mature kernels, flavonoids throughout the seed coat of RL4137 reacted with NaOH, but only the speckles appeared to react in white wheats. W98616 consistently had lighter grain than Argent before and after NaOH treatment. Free and bound phenolic differences in bran samples confirmed that the darker seed coat color of Argent, relative to W98616, was likely due to higher total phenolic acid content. Although isoflavones accumulated in Argent and RL4137, it appears that the majority of the flux through the flavonoid pathway ultimately accumulates quercetin derivatives and PAs. In W98616, PAs accumulate, but it appears that flavonoid biosynthesis ultimately accumulates isoflavones. Argent, compared to W98616, generally accumulated higher levels of total phenolics (flavonols, stilbenes, and PAs) within its darker pigmented bran.

Profiles of phenolic compounds in modern and old common wheat varieties determined by liquid chromatography coupled with time-of-flight mass spectrometry

“…In addition, numerous studies investigated the pharmacological activity of stilbenes, in particular the antioxidant, anti-inflammatory and anticarcinogenic effects of resveratrol and pinosylvin [47][48][49][50][51]. Although very few studies reported on stilbene occurrence in wheat, two pinosylvin derivatives were described by Matus-Cádiz et al [17]. Proanthocyanidins are a class of colorless phenolics characterized by an oligomeric or polymeric structure, based on flavanol units.…”

“…The interpretation of mass spectra allowed the identification of 34 phenolic compounds (104 including isomer forms) in the 22 investigated genotypes. Most of the detected polyphenols were previously described in wheat in other reports [13,14,16,17,31,32]. As outlined below, compounds were grouped into chemical classes and structural formulae of representative compounds are shown in Fig.…”

“…In our study, flavones resulted the most representative flavonoid group of the investigated wheat grains, comprising of 2 aglyconic forms, 9 C-glycosylated and 2 O-glycosylated compounds. In wheat, the glycosidic forms are mainly apigenin (5,7,4 -trihydroxyflavone) and luteolin (5,7,3 ,4 -tetrahydroxyflavone) derivatives and accumulate as their respective 6-C and/or 8-C-glycosidic conjugates [13,17]. The interpretation of the mass spectra allowed the detection of two aglyconic flavones, identified in the free and bound extracts of the wheat genotypes as apigenin (isomer compounds 47,52,63,65,94) In common with previous studies, lucenin-1/3 (luteolin-6/8-Anthocyanidins are secondary metabolites belonging to the flavonoid class and playing an important physiological role in UV protection of plant tissues.…”

“…Previous studies investigated the phenolic content of whole grains and their different fractions (endosperm, aleurone layer, bran) both free and bound forms, mainly using colorimetric methods as spectrophotometrical analyses [6][7][8]. Recently traditional methods have been replaced by various separation techniques (HPLC, GC, CE) coupled with mass spectrometry (HPLC-MS, GC-MS, CE-MS) as more sensitive tools for the characterization and identification of individual polyphenols in wheat [10][11][12][13][14][15][16][17]. Phenolic acids are the most representative and investigated class of wheat phenolics and their profile has been thoroughly described in recent years [9,12,[18][19][20][21].…”

Analysis of Kernel Colour, Flour and Whole Wheat End-product Quality of Commercially Grown Canada Hard White Spring Wheat, Snowbird

“…Grain colour is of a primary concern as the presence of darker kernels may be considered a disadvantage particularly in markets where Australian white wheat is considered the standard (Bason, Zounis, Ronalds, & Wrigley, 1995;Reifschneider, 2003). Several researchers (Bequette & Herrman, 1994;Bason et al, 1995;Wu, Carver, & Goad, 1999;Peterson et al, 2001;Matus-Cádiz, Hucl, Perron, & Tyler, 2003;Matus-Cádiz et al, 2008;Lukow, Adams, Suchy, DePauw, & Humphreys, "in press") have reported that there is variability in the white-seeded varieties with seed coats ranging from very white and pasty to reddish in colour.…”