Determination of isoflavones in soy and selected foods containing soy by extraction, saponification, and liquid chromatography: Collaborative study

Isoflavones are biologically active compounds occurring naturally in a vari ety of plants, with relatively high levels found in soybeans. Twelve labora tories participated in a collaborative study to determine the aglycon isofl avone content of 8 test samples of soy and foods containing soy. The analyt ical method for the determination of isoflavones incorporates a mild saponi fication step that reduces the number of analytes measured and permits quan titation versus commercially available, stable reference standards. Test sa mples were extracted at 65 degreesC with methanol-water (80 + 20), saponifi ed with dilute sodium hydroxide solution, and analyzed by reversed-phase li quid chromatography with UV detection at 260 nm. Isoflavone results were re ported as mug/aglycon/g or mug aglycon equivalents/g. The 8 test samples in cluded 2 blind duplicates and 4 single test samples with total isoflavone c oncentrations ranging from approximately 50 to 3000 mug/g. Test samples of soy ingredients and products made with soy were distributed to collaborator s with appropriate reference standards. Collaborators were asked to analyze test samples in duplicate on 2 separate days. The data were analyzed for i ndividual Isoflavone components, subtotals of daidzin-daidzein, glycitin-gl ycitein, and genistin-genistein, and total isoflavones. The relative standa rd deviation (RSD) for repeatability was 1.8-7.1 %, and the RSD for reprodu cibility was 3.2-16.1 % for total isoflavone values of 47-3099 mug/g.