A collaborative study was conducted to determine the soluble dietary fiber (SDF) content of foods and food products by using a combination of enzymatic and gravimetric procedures. The method was basically the same as that employed for determining total dietary fiber (TDF), 985.29, and the method for insoluble dietary fiber (IDF), 991.42. Ten laboratories were each sent 13 test samples (6 blind duplicates and 1 standard [green beans] containing 29-33% TDF, 19-23% IDF, and 8-13% SDF) and were instructed to assay for IDF, SDF, and TDF independently. Included in the package were the 3 enzymes, namely alpha-amylase, amyloglucosidase, and protease, and the filter aid Celite, which was thought to be the major cause of high reproducibility relative standard deviation (RSDR) values for SDF obtained in a previous collaborative study. The foods to be analyzed were apricots, carrots, chickpeas, onions, raisins, and the sugar beet fiber Fibrex™. IDF, TDF, and SDF were calculated as the weight of residue minus the weight of protein and ash on a dry weight basis. RSDR values of the IDF results averaged 8.02%, with only 1 food having an RSDR >10%. The RSDR values for the TDF results averaged 4.97%, and all foods had an RSDR <7%. Although the RSDR values for SDF averaged 14.17%, 4 of the 6 foods had an RSDR <10%, and 1 of the 2 remaining foods that had a high RSDR had an SDF content of only 1.2%. In all cases, the RSDR values of the SDF content of the foods were less than the values for the same foods analyzed in a previous collaborative trial. The enzymatic-gravimetric method for the determination of SDF was adopted first action by AOAC INTERNATIONAL.
An international survey was conducted to get the views of 147 professionals in the field on the definition of dietary fiber. The survey also solicited opinions on analytical methods for nutrition labeling, quality control, and nutrition research. The survey finds that dietary fiber is generally defined as polysaccharides and lignin that are not hydrolyzed by human alimentary enzymes. Support is strong for expansion of the definition to include oligosaccharides that are resistant to hydrolysis by human alimentary enzymes. Among techniques for nutrition labeling and quality control, enzymatic-gravimetric methods get the highest support. For nutrition research, more detailed methods such as gas-liquid chromatography and liquid chromatography were considered more appropriate. Respondents support labeling of total, soluble, and insoluble dietary fiber or total dietary fiber alone as sufficient for nutrition labeling of food packages. However, for nutrition research, detailed analytical methods, improvements in accuracy (i.e., closer simulation of in vitro techniques to conditions of human gastrointestinal tract), and improvements in precision and simplicity are suggested. Less than 20% of the participants use reference materials for dietary fiber analysis.
A method for soluble and insoluble dietary fiber determinations was developed for psyllium-containing food products, which are highly viscous in aqueous solutions. The assay is based on a modification of the AOAC soluble and insoluble dietary fiber method (991.43), which was recommended for nutrition labeling in the final U.S. food labeling regulations. We found that method 991.43 and other existing dietary fiber methods could not be applied to psyllium food products, which exhibit high viscosity in aqueous solutions, because highly viscous solutions could not be filtered easily. In this study, we modified AOAC method 991.43 to accommodate the filtration process of viscous sample solutions. Sonication followed by highspeed centrifugation was used before filtration. The principles of the method are similar to those for AOAC method 991.43, including the use of the same 3 enzymes (heat-stable α-amylase, protease, and amyloglucosidase) as well as similar enzyme incubation conditions. The modification using sonication and high-speed centrifugation did not alter the method performance for analytically normal products such as wheat bran, oat bran, and soy fiber. Yet, the modification allowed the separation of soluble dietary fiber fractions from insoluble fractions for psyllium products with satisfactory precision. This method for psyllium dietary fiber determinations may be applied to other food products that exhibit high viscosity in aqueous solutions.
An enzyme-linked immunosorbent assay (ELISA) was developed for the quantitation of ochratoxin A amended to wheat. Ochratoxin A conjugated to horseradish peroxidase (HRP) was used as an enzyme marker in the assay. At toxin levels below 30 ppb, a cleanup treatment was necessary for ELISA. Among 3 cleanup methods tested (solvent partition, Sep-Pak cartridge treatment, solvent partition and cartridge treatment), reverse phase cartridge treatment was the most simple and effective. In the analysis, ochratoxin A was extracted from wheat with methanol. The methanolic extract was diluted with water to a final 10–15% methanol content, and then passed through a cartridge. Ochratoxin A was eluted from the cartridge with 85% methanol which was then concentrated. The final solution, in 0.1M, pH 7.5 sodium phosphate–Tween 20 buffer and 5% methanol, was then subjected to ELISA. ELISA allowed minimal detection of the toxin in wheat at the 1–2 ppb level after cleanup. Recoveries of toxin added to wheat samples in the 1.0–30 ppb range were 85–90% with standard deviations of 10–15%.
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