Quanti-qualitative evaluation of pectins in the dietary fibre of 24 foods

The dietary fibre of 24 foods was analysed for its proportions of insoluble and soluble (SDF) fractions and for its content of high methoxyl pectins (HM), low methoxyl pectins (LM) and protopectin. The fractional extraction and quantitative determination of pectins were performed on the total dietary fibre residue, following the procedure suggested by Robertson (1979). Total pectin content (TP), calculated as sum of the three fractions, ranged from 2.4 to 49.8 g/kg of dry matter. The variation coefficient of TP measurements repeated on the same foods were, on average, 2%. Total pectin content was 49.8 g/kg DM in dried beet pulp and averaged 33.8 +/- 0.3 g/kg DM in fruits and vegetables, 13.2 +/- 8.4 g/kg DM in legumes and tubers and only 2.8 +/- 0.5 g/kg DM in cereals. HM fraction prevailed in apple and pear samples (> 40% of TP), while LM and protopectin largely prevailed in legumes and vegetables. A strong variability among foods was found for the TP/SDF ratio. In general, within each food category, increasing levels of SDF were associated with decreasing values of TP/SDF ratio. Since many foods contain low amounts of pectins, care in the development of calibration curves for spectrophotometric reading is required. Finally, the whole procedure for pectin extraction and quantification on dietary fibre of food is very complex and time-consuming.