Tannins and phenols in black sorghum

Abstract

Shawaya, an uniquely black pigmented sorghum found in the western Sudan, was investigated for the possible commercial application of the phenolic compounds, in particular the flavonoid pigments. Selections from the segregating population of converted Shawaya grain, containing different levels of black pericarp pigmentation, were used to study the relationship between black pigmentation and the different phenolic compounds. Grain from different stages of maturity was also analyzed. The black Shawaya grain contained about 10 times more anthocyanidin pigments than the control cultivars used in this study, and also significantly more flavan-4-ols and phenolic acids. However, due to the absence of condensed tannins, the black grain contained less total phenols than the Type II and Type III control cultivars, and also lower levels of flavan-3-ols. Microscopy of the pericarp revealed the location of the pigments to be mainly the epicarp, and also the cross and tube cell area of the more intensely pigmented lines. In contrast to the original grain, the converted black Shawaya grain did not contain a pigmented testa, while the red pigmented lines did retain the testa. The location of the pigments in the outer layers of the kernel, as well as the absence of a pigmented testa, made removal of the pigments by abrasive decortication efficient. The anthocyanidin pigments, total phenolic compounds and flavan-4-ols increased through maturity, reaching a maximum at 35 days after anthesis (DAA). Ferulic and p-coumaric acid were the predominant phenolic acids at all stages of maturity. Maximum levels of phenolic acids were also observed at 35 DAA. The major pigments were identified as the 3-deoxyanthocyanidins luteolinidin and apigeninidin. Smaller amounts of a third, unidentified anthocyanidin as well as aurone and flavonol compounds were also present. The pigments showed improved thermal, light and pH stability compared to the common anthocyanins, and have potential as food colorants. In contrast to most other natural sources of anthocyanin pigments, sorghum bran is a waste product, it can also be stored for long periods of time, offering a much longer processing window than perishable products such as cranberries or grape skins...