Determination of Phenolic Compounds of Dry Beans Using Vanillin, Redox and Precipitation Assays

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Abstract
Phenolic compounds of ten genotypes of dry beans (Phaseolus vulgaris L.) were investigated using four different methods. Assayable tannins using the 0.5% vanillin assay ranged from 49–306 mg catechin equivalents/100 g beans. Significant differences were observed for tannins measured by the vanillin and the protein precipitable phenol tests. Of the two redox assays, the Prussian blue assay gave significantly higher total phenol values than the Folin‐Ciocalteu assay. Both these reagents were more sensitive to tannic acid than to catechin. Although tannin values varied 2–3 fold for a given variety, excellent correlation was observed between the different assays employed for bean tannin analysis.