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Occurrence and Location of Tannins in Finger Millet Grain and Antioxidant Activity of Different Grain Types

¹Discipline of Dietetics and Human Nutrition, University of KwaZulu-Natal, Private Bag X01 Scottsville 3209, Pietermaritzburg, South Africa. ²Department of Food Science, University of Pretoria, Pretoria 0002, South Africa. ³African Centre for Crop Improvement, University of KwaZulu-Natal, Private Bag X01 Scottsville 3209, Pietermaritzburg, South Africa. ⁴Present address: Nickerson-Zwaan b.v., Post Box 28, 4920 AA Made, The Netherlands. ⁵Corresponding author. E-mail: gyebi.duodu@up.ac.za

Grain of 22 finger millet types obtained from Southern and East Africa were analyzed to determine the influence of grain type on tannins, antioxidant properties, and tannin localization in the grain. Four grain types were creamy white and 18 were brown. A high proportion (80–100%) of kernels of all but two of the pigmented types stained black with the bleach test, while the light types did not stain black. There was a wide range of total phenolics, condensed tannin content, and antioxidant activity across the grain types. Light-colored grain types had much lower total phenolics and tannins relative to the pigmented types, and types that stained black with the bleach test had much higher tannin content and much higher antioxidant activity. Light microscopy revealed that kernels that stained black with the bleach test and had high tannin content had a dark-colored testa layer, indicating that the tannins were located in that layer as in sorghum. This is the first report that the bleach test can be used to detect tannin in finger millet types. The work demonstrates that occurrence of tannins in finger millet grain is a varietal property, as in sorghum.