[Evaluation of two methods to determine the polyphenol content in raw and cooked beans and its effect on protein...

The polyphenolic compounds present in raw and cooked, and dried, with and without the cooking broth of common white, black and red beans (Phaseolus vulgaris) were measured by the Folin-Denis method for total polyphenols, and by the protein precipitation method of Hagerman-Butler, which measures their biological activity. The polyphenol content was measured during 20 consecutive days on the same sample, using three different extracts of volume from each sample. Statistical analysis of the results by the Folin-Denis method indicated that variability among the three aliquots was different for each bean color. A non-parametric analysis, however, indicated that the average in the three levels of concentration for beans of all colors, was the same. A similar analysis of the results by the Hagerman-Butler method demonstrated that variability and average values for the three aliquots were equal for black and red beans but not for white beans. The coefficient of variation was lower for the higher aliquot of the extract. A significant correlation (r = 0.72, P less than 0.05, n = 60) was found between the two methods for all beans using the larger aliquot of the extract. The correlation was highly significant (r = 0.84, P less than 0.05) when white bean values were eliminated. The polyphenolic content varied with seed color and the thermic process reduced their content, as measured by the two methods. The losses in polyphenolics as measured by the Folin-Denis in the cooked beans dried with broth, varied from 31.4 to 36.3%, and from 39.8 to 51.1% for the cooked bean flour dried without broth. The losses by the Hagerman-Butler method were from 25.0 to 93.5% in the cooked bean flours dried with cooking broth, and from 33.3 to 95.7% when dried without the broth. The higher losses were recorded for red beans. In vivo digestibility for cooked bean flours, dried and without broth, were 73.2, 69.6 and 64.5%, and 71.9, 71.9 and 68.8% for white, red and black beans, respectively. A negative correlation (r = -0.39) and significant (p less than 0.05) was found between polyphenolic content in the diet and in vivo protein digestibility.