ISOFLAVONES AND THEIR CONJUGATES IN SOY FOODS - EXTRACTION CONDITIONS AND ANALYSIS BY HPLC MASS-SPECTROMETRY

Identification of isoflavone glycosidic conjugates from several soy products was carried out by HPLC-mass spectrometry. Positive ion mass spectra obtained using the heated nebulizer-atmospheric pressure chemical ionization interface gave the most sensitive and structurally useful information about each isoflavone conjugate. Although extraction of isoflavones from soy products with 80% aqueous methanol at room temperature was just as efficient as at 60-80 degrees C, extraction at higher temperatures caused changes in isoflavone composition and should be avoided. Soybeans and defatted soy flour (which had been minimally heated during their preparation) contained mostly isoflavone 6''-O-malonylglucoside conjugates, with lesser quantities of the beta-glucosides and only trace amounts of 6''-O-acetylglucoside conjugates. Soy milk, tofu, and soy molasses, each of which involves heating to 100 degrees C during their manufacture, contained mostly isoflavone beta-glucosides. Toasted soy flour and an isolated soy protein had moderate amounts of each of the isoflavone conjugates.