ETHANOLIC PRECIPITATION - A SOURCE OF ERROR IN DIETARY FIBER DETERMINATION

The ethanol precipitation of soluble dietary fibre (SDF) constituents is a common step among modem analytical methods. This precipitation ma y involve two possible error sources: non-fibre component co-precipita tion and incomplete precipitation of SDF components such as pectins. B oth possibilities are studied in the present work. Citrus pectins and pectin-organic acid (citric and oxalic) mixtures were dissolved in pho sphate buffer and precipitated in 78% ethanol at different pH values. Incomplete recovery of pectins (84-89%, depending upon the pH of the s olution) and significant co-precipitation of organic acids were observ ed. Dialysis has been studied as an alternative method for obtaining S DF. Both dialysis and precipitation techniques were carried out to det ermine SDF in fruits, legumes and cereals. The SDF values (neutral sug ars plus uronic acids) obtained by dialysis were higher in all the sam ples than the values obtained by precipitation. These results show the incomplete determination of SDF with the present analytical methods. In addition, gravimetric methods can include a fraction of non-fibre c onstituents, such as organic acids, as fibre.