Effect of tannins on microwave-assisted extractability and color properties of sorghum 3-deoxyanthocyanins.

Sorghum derived 3-deoxyanthocyanins (3-DXA) are of growing interest as natural food colors due to their unique stability compared to anthocyanins, but are generally difficult to extract. Microwave-assisted extraction (MAE) can dramatically improve extraction efficiency of 3-DXA from sorghum tissue. However, condensed tannins common in some sorghums could impact MAE extractability and color properties of 3-DXA. The objective of this work was to determine how presence of condensed tannins affect MAE extractability, stability, and color properties of sorghum 3-DXA. Sorghums of varying 3-DXA profile and tannin content, as well as purified tannins, were subjected to MAE and pigment yield and profile, aqueous color properties and stability at pH 1 - 5 monitored over time using, UV-vis spectroscopy, colorimetry, and UPLC-MS. The relative yield of 3-DXA from tannin sorghums was higher (3 - 10-fold) after MAE than from non-tannin sorghum (2-fold). During MAE, condensed tannins underwent extensive oxidative depolymerization to anthocyanidins (cyanidin and 7-O-methylcyanidin), which caused the tannin-sorghum pigment extracts to have a redder hue (12-43H°) compared to the non-tannin pigment extract (58H°). The tannin-derived anthocyanidins transformed over time into xanthylium pigments, resulting in increased extract H°. Tannins enhanced both color intensity (pH 1) and stability (pH 3-5) of the 3-DXA over 14 days, indicating they acted as copigments. The presence of tannins in sorghum enhances MAE extractability of 3-DXA from sorghum tissue, and could also potentially enhance their functionality in aqueous food systems. However, the initial changes in extract hue properties due to tannin-derived anthocyanidins should be considered.

Changes in extractable phenolic profile during natural fermentation of wheat, sorghum and teff.

Whole grain polyphenols are associated with structure-specific bioactive properties. However, the phenolic profile of grain ingredients can be significantly altered by processes like fermentation. This study investigated how polyphenol profiles in different cereal grains respond to microbial metabolism during sourdough fermentation. Whole grain wheat (white and red), sorghum (white and lemon-yellow), and teff (white and brown) flours were subjected to natural sourdough fermentation for 48-96 h, and phenolic profiles and their metabolites monitored using UPLC-tandem quadrupole MS. Flavonoid O-glycosides (dominant in sorghum) were rapidly metabolized (66% reduction in 48 h) to release aglycones (2.5 fold increase). O-Glycoside groups in mixed O/C-glycosides (dominant in teff) were selectively hydrolyzed, but more slowly (11-32% reduction in 48 h) than homo-O-glycosides, suggesting steric hindrance from the C-glycoside groups. Flavonoid C-glycosides (dominant in wheat) and aglycones (white sorghum) were generally stable to microbial degradation. Extractable phenolic acids and their esters (most abundant in white sorghum) were extensively degraded (80% reduction in 48 h) with few metabolites detected at the end of fermentation. Thus, extractable phenolics in sorghum were generally most extensively metabolized, whereas those in wheat were the least impacted by sourdough fermentation. New microbial metabolites, putatively identified as O-methylcatechol-vinyl-isoflavans, were detected in all fermented samples, with levels increasing with fermentation time. Based on structure, these compounds were likely derived from cell wall C-C linked diferulic acid metabolism. As expected, Folin reactive phenols and antioxidant capacity increased in fermented samples, but the extent was distinctly smaller in sorghums (1.3-1.9 fold) vs teff (2.4-3.2 fold) and wheat (2.0-6.1 fold), likely due to higher presence of easily metabolizable phenolics in sorghum. The phenolic profile of a cereal grain affects the products of microbial metabolism during fermentation, and may thus alter phenolic-dependent bioactive properties associated with a specific grain.

Stability of 3-deoxyanthocyanin pigment structure relative to anthocyanins from grains under microwave assisted extraction.

Sorghum derived 3-deoxyanthocyanin (DXA) pigments are stable relative to their anthocyanin analogs, and are of growing interest in food applications. However, the 3DXA are poorly extractable from grain tissue. This work aimed to determine the relative stability and extractability of sorghum 3-DXA vs anthocyanins from maize and cowpea under microwave-assisted extraction (MAE). UV-Vis and UPLC-MS/MS spectrometry were used to characterize the properties. The 3-DXA remained structurally stable to MAE conditions up to 1200 W/100 °C/30 min. MAE increased sorghum 3-DXA yield 100% versus control (3100 vs 1520 mg/g). On the other hand, both maize and cowpea anthocyanins were unstable and rapidly degraded under MAE. Cell wall-derived ferulate esters were detected in sorghum and maize MAE extracts, indicating cell wall degradation occurred during MAE. Thus the enhanced extraction of 3-DXA under MAE was due to their structural stability, along with improved diffusion from cell matrix due to microwave-induced sorghum cell wall disruption.

Rye flavonoids - Structural profile of the flavones in diverse varieties and effect of fermentation and heat on their structure and antioxidant properties.

Flavones are important bioactive flavonoids in cereal grains, but are poorly characterized. This study investigated rye flavone profile as influenced by phenotype (grain color), sourdough fermentation, and subsequent heat processing. Twelve rye varieties belonging to 4 phenotypes were characterized using UPLC-tandem quadrupole MS before and after fermentation and baking. Antioxidant properties were also assessed. Rye flavones (range 57-137 µg/g) were dominated by O-glycosides (50-68%), present as derivatives of tricin (exclusively O-glycosides), chrysoeriol (O-/C-glycosides), and apigenin (exclusively C-glycosides). Phenotype did not influence flavone content. Fermentation partially hydrolyzed O-glycosides to their aglycones, but did not affect C-glycosides. Extractable phenols and antioxidants increased 1.9-3.6X after 96-h fermentation; baking further increased these components by 36-96% in fermented samples, likely via enhanced cell wall disruption. The high proportion of flavone-O-glycosides in rye is of interest due to their known higher bioavailability relative to typical cereal grain C-glycosides.

Combined cereal and pulse flavonoids show enhanced bioavailability by downregulating phase II metabolism and ABC membrane transporter function in Caco-2 model.

Predominant flavonoids in cereals and pulses are structurally different and may positively interact to enhance bioactivity in combined diet. This work investigated the effects of combined cereal 3-deoxyflavonoids (apigenin, naringenin) and pulse flavonols (quercetin), along with natural extracts, on their bioavailability and underlying mechanisms using Caco-2 monolayer model. Membrane permeability, phase II metabolism, and ATP binding cassette (ABC) membrane transporter expression and function were measured. Apparent absorption of quercetin and apigenin increased (p < 0.05) 3.3× and 1.5×, respectively, while both compounds were significantly less metabolized in combined treatments. Combinations with naringenin had insignificant effect, suggesting a role for flavonoid C2C3 conjugation. Both natural extracts and apigenin-quercetin combinations synergistically (3-40 fold) downregulated ABC transporter expression, and inhibited P-glycoprotein activity, suggesting direct binding and inhibition of ATPase. Combination of conjugated cereal and pulse flavonoids enhances their potential bioavailability through synergistic inhibition of membrane transporter and phase II enzyme function.

Structural profile of soluble and bound phenolic compounds in teff (Eragrostis tef) reveals abundance of distinctly different flavones in white and brown varieties.

Reliable data on polyphenol in teff, an increasingly important food crop, is currently lacking. This study investigated the structural and quantitative profile of soluble and bound polyphenols in white and brown teff grown in Ethiopia and USA using LC-tandem quadrupole mass spectrometry. Thirty-four phenolic compounds (32 newly identified in teff), mostly flavones and phenolic acid derivatives, were characterized. Unusually high levels of flavones were present in both white (1398-2049 µg/g), and brown (1720-1847 µg/g) teff soluble fractions. Interestingly, white teff exclusively contained apigenin derivatives, whereas brown teff contained mainly luteolin derivatives, mostly di-C-linked-glycosides. Additionally, non-extractable procyanidins (condensed tannins) were detected in brown teff only. Phenolic acids (600-728 µg/g) were mostly present in bound fractions, dominated by ferulic acid. Polyphenol profiles of Ethiopian and USA grown grains were similar. The high levels of the relatively rare flavones in teff may have important implications in chronic disease prevention.