Vol. 7, Issue 1, Part B (2025)

This study evaluated quinoa-derived polyphenols as natural antioxidants by coupling detailed chemical profiling with multi-assay functional testing across varieties (white, red, black) and processing states (raw, toasted, germinated). Objectives were to: (i) quantify free, conjugated, and bound phenolic fractions; (ii) identify key phenolic drivers by UHPLC-MS/MS; (iii) benchmark antioxidant capacity using complementary assays (DPPH, ABTS, FRAP); (iv) examine processing effects; (v) estimate post-digestive availability; and (vi) model composition-function relationships. Milled, saponin-removed flours were extracted with 80% ethanol (± ultrasound), fractionated by acid/alkali hydrolysis, and analyzed by spectrophotometry and UHPLC-MS/MS. Antioxidant capacity was measured as Trolox equivalents; a static in vitro digestion generated “bio-accessible” fractions for re-assay. Two-way ANOVA, correlation tests, and chemometrics (PCA/PLS) supported inference. Results showed strong color and processing effects on totals and activity. Total phenolic content (TPC) spanned ~1.6-4.6 mg GAE/g DW, and total flavonoids (TFC) ~0.39-1.04 mg QE/g DW, with black > red > white in each state; germination increased TPC by ~21-33% (color-dependent), whereas toasting reduced TPC by ~5-12%. Bound phenolics comprised 36-43% of TPC, indicating substantial wall-associated pools. Antioxidant capacity rose with pigmentation and germination: DPPH, ABTS, and FRAP were highest in black-germinated samples (≈30.1, 34.3, and 21.4 µmol TE/g DW, respectively). TPC correlated strongly with DPPH (r = 0.89) and ABTS (r = 0.86) and moderately with FRAP (r = 0.71). UHPLC-MS/MS highlighted quercetin-, kaempferol-, and isorhamnetin-glycosides plus ferulic and p-coumaric acids as dominant constituents; PLS predicted DPPH with R² = 0.83, Q² = 0.76, and VIP > 1.0 for quercetin-3-O-glc, ferulic acid, and isorhamnetin-3-O-glc. In-vitro digestion decreased direct radical-scavenging (DPPH/ABTS ≈ −24%) but preserved more of the reducing capacity (FRAP ≈ −12%) and yielded appreciable bioaccessibility of phenolic acids (e.g., ferulic ≈ 65%, p-coumaric ≈ 58%; quercetin-3-O-glc ≈ 42%). Collectively, colored (especially black) and germinated quinoa deliver higher phenolic loads and stronger antioxidant performance; the sizeable bound fraction and digestion outcomes underscore the importance of processing and matrix effects. Practical recommendations include selecting colored cultivars, incorporating brief germination, minimizing harsh heat, standardizing saponin removal and milling, using ~80% ethanol (± ultrasound) extractions, and applying multi-assay QC with UHPLC markers to support clean-label food and nutraceutical applications.