Determination of Insoluble and Soluble Dietary Fiber in Foods and Food Products: Collaborative Study

Abstract A collaborative study was conducted to validate a method to determine the Insoluble dietary fiber (IDF) and soluble dietary fiber (SDF) contents of foods and food products by using a combination of enzymatic and gravimetric procedures. The method was basically the same as that for determining total dietary fiber, which was adopted as final action by AOAC and further modified to Include changes in the concentration of buffer and base and substitution of hydrochloric acid for phosphoric acid. Thirty-nine collaborators were each sent 7 test samples In a staggered design for duplicate blind analysis. They were also sent a standard containing 4.3-5.4% IDF and 1.5-2.7% SDF. The 22 foods that were analyzed for IDF and SDF were cabbage, carrots, French beans, kidney beans, butter beans, okra, onions, parsley, chick peas, brussels sprouts, barley, rye flour, turnips, soy bran, wheat germ, raisins, Callmyrna figs, prune powder, Black Mission figs, apple powder, peach powder, and apricot powder. Both IDF and SDF values were calculated as the weight of residue minus the weight of protein and ash reported on a dry weight basis. The reproducibility relative standard deviation (RSDR) of the IDF results ranged from 3.68 to 19.44% for the foods analyzed; almost half the test samples had an RSDR <10%. The RSDR values for the SDF results were somewhat higher. Approximately 50% of the foods analyzed had an RSDR >20%, and 45% had an RSDR between 10 and 20%. An RSDR approaching 45% was calculated for the 2 test samples with the lowest SDF content, 1.35 and 1.90%. Raisins and prune powder had high RSDR values for both SDF and IDF. A major reason for high RSDR values seems to be filtration problems, which are avoidable by analyzing 0.5-0.25 g test samples. The method for the determination of SDF requires further study, but the method for the determination of IDF was adopted first action by AOAC International.