Finger millet polyphenols: Optimization of extraction and the effect of pH on their stability

Finger millet, one of the minor cereals, is known for several health benefits and some of the health benefits are attributed to its polyphenol contents. Investigations of suitable solvents for extraction of polyphenols and their stability, during changes of pH and temperature, were carried out. Histochemical examination of the millet kernels, and also analysis of the seed coat and the endosperm fractions of the millet for the polyphenol contents, revealed that nearly 90% of the polyphenols were concentrated in the seed coat tissue. In view of that, the polyphenol contents of the seed coat fraction of the millet were extracted with different polar and non-polar solvents, and it was observed that 1% HCl–methanol was very effective for extraction of the millet polyphenols. Accordingly, the polyphenols were extracted with acidic methanol and the polyphenols obtained were examined for pH and temperature stability. The phenolic contents (6.4 ± 1.0%) of the extract remained constant at highly acidic to near neutral pH (6.5) but decreased gradually to 2.5 ± 0.3% as the alkalinity increased to pH 10. The increase in pH resulted in precipitation of some of the extracted matter, and this increased from 4 ± 0.5% to 40 ± 3% of the extracted matter, as the pH increased from 1 to 10. But, the polyphenol contents of the extract were stable to the changes in the temperature of the extract. Fractionation of the polyphenols extracted by high performance liquid chromatography (HPLC) showed that the analytes were derivatives of benzoic acid (gallic acid, proto-catechuic acid, and p-hydroxy benzoic acid) and cinnamic acid (p-coumaric acid, syringic acid, ferulic acid and trans-cinnamic acid). However, in a highly alkaline condition (pH 10) of the extract, only gallic acid and proto-catechuic acid were detected.