Soy isoflavone analysis: quality control and a new internal standard

Development of a database of the soy isoflavone content of foods requires a ccurate and precise evaluation of different food matrixes. To evaluate accu racy, we estimated recoveries of both internal and external standards in 5 different soyfoods weekly. Standards were evaluated daily for system qualit y assurance. To evaluate sample precision, we analyzed soybeans and soymilk bimonthly for within-day precision and over 4 d for day-to-day precision. CVs should be less than or equal to 8%. We validated our methods for single and multiple recovery concentrations by using our new internal standard, 2 ,4,4'-trihydroxydeoxybenzoin, and the external standards daidzein, genistei n, and genistin. Concentrations of 12 isoflavone isomers, 3 aglycones (daid zein, genistein, and glycitein), and 9 glucosides (daidzin, genistin, glyci tin, acetyldaidzin, acetylgenistin, acetylglycitin, malonyldaidzin, malonyl genistin, and malonylglycitin) were measured in a variety of soybeans and s oyfoods. The extraction methods used depended on soyfood type. The HPLC con ditions for soy isoflavone analysis were improved, leading to good separati on with a short analysis time (60 min/sample). A data bank of concentration and distribution of isoflavones in different soybean products was assemble d, a wide range of isoflavone concentrations, from <50 mu g/g to >20 000 mu g/g, was found in different soy products. The glucoside forms are almost t wice the molecular weight of the aglycones; reported isoflavone concentrati ons should be normalized to the aglycone mass (or an isoflavonoid equivalen t) rather than a simple sum of all isomers.