Résumé
The comprehensive phenolic profile, antioxidant activity and color of sorghum (Sorghum bicolor L., caudatum) grains were investigated in five different stages of growth (from cellular division to maturity). Grains were harvested (2018, Montpellier-France), freeze-dried and cryogenically ground. Colorimetric parameters (L*, a*, b*) were determined and free (FPC) and bound (BPC) phenolic compounds were extracted from wholegrain flours. Antioxidant capacity was determined by DPPH and Folin-Ciocalteu methods in microplates. Phenolic profile was analyzed by UPLC-ESI-QTOF-MSE in negative mode. Data were processed with Progenesis QI using a customized database (PubChem and Phenol explorer) applying: mass error (<10ppm), isotope similarity (>80%) and reproducibility (3/3). Sorghum grain flours became significantly darker and redness throughout maturation. FPC and BPC ranged from 207.19 to 1052.77 and 268.71 to 624.75 mg GAE/100g (db), respectively; and antioxidant activity between 10.84 to 108.68 and 29.92 to 67.50 mg TE/100g (db). The most immature stage (S1) had the highest FPC content and antioxidant activity in free extracts, while the highest values of BPC were found in mature grains (S4, mature). S2 represents the grain filling stage and showed the same values in both extracts. Globally, 69 FPC and 120 BPC were tentatively identified with 18 compounds present in both extracts. The first stages (S1-S3) showed the highest number and abundance of phenolic compounds. Bound phenolics were the most abundant (72%), mainly in S2, while free soluble phenolics were higher in immature stages, decreasing along grain maturation. PCA allowed a distinguished phenolic profile in each stage, where S1, S2 and mature presented more than 10 unique PC. This work seems to be the first to reveal the phenolic profiling based on metabolomics during sorghum grain growth. These data can be hereafter correlated with proteomic analysis, in order to better understand the digestion problematic involving tannins and kafirins in this cereal matrix.